US Army Course (1986) Mechanical and Electromechanical Measurement Principles MM0486

8/3/2019 US Army Course (1986) Mechanical and Electromechanical Measurement Principles MM0486

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SUBCOURSE EDITION

MM0486 8

MECHANICAL AND

ELECTRO-MECHANICAL

MEASUREMENT PRINCIPLES


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U.S. ARMY SIGNAL CENTER AND FORT GORDON

Fort Gordon, GA

E R R A T A S H E E T

SIGNAL SUBCOURSE SM0486

EDITION 8

MECHANICAL AND ELECTRO-MECHANICAL PRINCIPLES

Effective: 6 Jun 86

IMPORTANT READ AND POST

ADMINISTRATIVE INSTRUCTIONS:

Make the following changes:

1. Examination, page 151, question 13

Delete "You apply 300 psig to a gage and it indicates 312 psig."

Add "A gage has a range of 0 to 300 psig and actually indicates 312 psig when

you apply a true pressure of 300 psig."

PLEASE NOTE

Proponency for this subcourse has changed from Signal

Center & School (SM) to Ordnance Missile and Munitions

Center & School (MM).

IMPORTANT READ AND POST


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READ THIS PAGE

GENERAL INFORMATION

This subcourse consists of one or more lessons and an examination. Each of thelessons is divided into two parts; the text and the lesson exercises. For one

lesson subcourse the lesson exercise serves as the examination. A heading at the

beginning of each lesson gives the title, the hours of credit, and the objectives

of the lesson. The final examination consists of questions covering the entire

subcourse.

If a change sheet is included, be sure to post the changes before starting the

subcourse.

THE TEXT

All the text material required for this subcourse is provided in the packet. The

text is the information you must study. Read this very carefully. You may keep

the text.

THE LESSON EXERCISES

Following the text of each lesson are the lesson exercises. After you hove studied

the text of each lesson, answer the lesson exercises. After you have answered all

the questions, go back to the text and check your answers. Remember your answers

should be based on what is in the text and not on your own experience or opinions.

If there is a conflict, use the text in answering the question.

When you are satisfied with your answers, check them against the approved solution

in back of this text. Re-study those areas where you have given an incorrect

answer by checking the reference given after each answer.

THE EXAMINATION

After you have completed all the lessons and exercises, select the correct answer

to all the examination questions. Carefully mark the correct answer on the exam

response sheet. Be sure to include your social security number, subcourse number,

and edition number are correct. Final exams should be mailed in the envelope

provided. The exam will be graded and you will be notified of the results. Your

final grade for the subcourse will be the same as your examination grade.

*** IMPORTANT NOTICE ***

THE PASSING SCORE FOR ALL ACCP MATERIAL IS NOW 70%.

PLEASE DISREGARD ALL REFERENCES TO THE 75% REQUIREMENT.

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ACKNOWLEDGEMENTS

This subcourse has been adapted from Air Force Career Development Course CDC

32470 for Army use.

Information and illustrations used to support the material in this subcourse

were adopted from manufacturer's instruction manuals published by the companieslisted below:

M. B. Electronics

Wm. Ainsworth and Sons, Inc.

SOLDIER'S TASK

This subcourse supports the following MOS 35H Tasks:

093-435-1270 Calibrate Torque Tester

093-435-1271 Calibrate Torque Wrench

093-435-1283 Calibrate Compound Gauge

093-435-1290 Calibrate Thermometer093-435-3261 Calibrate Gram Weight Set

093-435-3300 Calibrate Thermometer Set

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CORRESPONDENCE COURSE

OF THE

U.S. ARMY SIGNAL CENTER & SCHOOL

AIPD Subcourse Number SM0486

Mechanical and Electro-Mechanical

Measurement Principles

(8 credit hours)

INTRODUCTION

Mechanical and electro-mechanical devices play an important role in our

everyday life. Lesson one of this subcourse includes information which will help

you understand molecular activity changes and the relationship of these changes to

heat. It also explains the operating principles for related heat sensing and

measuring instruments. The second lesson explains the physical principles ofpressure measurements and also includes information on pressure devices which you

must use and calibrate. Lesson three contains information on rotary and torque

measurements. It also includes information which will help you understand the

operating principle of the proving rug. Lesson four discusses the principle which

you must apply to perform vibration measurements and tells you when you must

calibrate vibration equipment. Lesson five teaches operation of the analytical

balance and principles of mass measurement. This subcourse is organized as follows:

Lesson 1 Temperature and Humidity Measurements..............2 Hours

Lesson 2 Pressure Measurements and Devices..................2 Hours

Lesson 3 Rotary Torque Measurements and Equipment...........1 Hour

Lesson 4 Vibration Measurements and Equipment...............1 Hour

Lesson 5 Weights and Balance................................1 HourExamination.................................................1 Hour

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LESSON 1: TEMPERATURE AND HUMIDITY MEASUREMENT

AIPD Subcourse Number SM0486...........MECHANICAL AND ELECTRO-MECHANICAL


Lesson Objective.......................Given learning objectives and text, you

should be able to answer all exercisequestions pertaining to the nature of heat

and temperature, temperature scales and

thermometers, heat sensing and measuring

instruments, and humidity with no errors.

Credit Hours...........................Two

TEXT

1. INTRODUCTION

Your next assignment may be the calibration of a temperature measuring

instrument or the use of a temperature measuring instrument for calibrationpurposes. Your job will be much easier if you have a complete understanding of the

terms and principles associated with temperature measurements. The information

contained in this chapter concerns the nature of heat and temperature, the methods

by which heat is generated and transferred, the units used in temperature

measurements, and the principles applied in temperature measurements.

2. THE NATURE OF HEAT AND TEMPERATURE

a. Heat is considered to be a form of energy. The terms "thermal" and

"kinetic" are usually added as confusion factors. To make your day complete, some

authors use the terms "heat" and "temperature" as if they are the same. Let's see

if we can identify some of the terms associated with heat and temperature

measurements and establish practical definitions for these terms.

b. Heat. Most of us use the word "heat" without bothering to consider or

determine its true meaning. We usually have a general idea of what we mean when we

use the word, but for measurements in a laboratory, you must know precisely what

the word means and the conditions and limitations under which the meaning is true.

What answer would you give if a photographer, or an artist, or a common laborer

asked the question, "What is heat?" In your search for an answer, would you say

that heat:

Causes metals to expand?

Can be generated by rubbing two or more bodies together?

Is generated by compression?

Is an invisible weightless fluid called caloric?

Is electricity?

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Is a form of energy?

Is the total kinetic energy of moving molecules (a name applied to the kinetic

energy possessed by the moving molecules of a body)? Let's examine the list of

possible answers in sequence to see if any of them or a combination of them agrees

with your concepts of the nature of heat and what heat consists of.

(1) Heat and expansion. While the word "expansion" identifies one of the

effects heat produces in metals, it is not a satisfactory answer for the original

question of "What is heat?" We hope that you chose one or more of the other

answers. If you didn't, choose one before we proceed.

(2) Heat and friction. You know that the moving parts of the engine of

your car generate heat because of friction. In some instances, the intensity of

heat is such that the resulting expansion of metals prevents the movement of some

parts. Although the preceding statements are true, the original question has not

been answered; you have only chosen one method whereby heat is generated.

(3) Heat and compression. When a gas is compressed, the space between

individual molecules is decreased. The decrease in space between molecules results

in an increase in the activities of the molecules involved. The increase in theactivities of the molecules results in an increase in the kinetic energy of the gas

compressed. All of the statements concerning an increase in heat (kinetic energy)

by means of compression are true; but have we answered the original question on the

nature of heat? Partially, yes. We say partially because the use of the

expression "kinetic energy" in parentheses following heat indicates that heat is

kinetic energy.

(4) Heat is an invisible weightless fluid called caloric. At one time heat

was considered to be the caloric just described. With the development of the laws

of the conservation of energy, the idea that caloric (heat) could be increased or

decreased as a separate entity (a quantity existing independent of other

quantities) was disproved. The increase or decrease in the quantity of heat is

always accompanied by the transformation of one form of energy to another. Anotherfailure; we still haven't given a satisfactory answer to the original question.

This one isn't even partly true.

(5) Heat is electricity. If heat is electricity, then electricity is heat.

Well, technically no. You already know that electrons which constitute electrical

currents are forced through resistances by an EMF (electromotive force). The

movement of these electrical particles creates an increase in the activity (kinetic

energy) of the particles concerned and a subsequent increase in the quantity of

heat on the electrical conductor. This means that electricity can be used to

produce heat, but we can't say that they are the same.

(6) Heat as a form of energy. This statement is acceptable as a general

definition for heat, but it should be combined with the last state-

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ment in the list to give a better explanation of what heat is. Heat is a name

given to that form of energy which represents the total kinetic energy (force

created by molecular motion) possessed by the moving molecules of a body. There

are other definitions for heat, but this one contains the concept you need to fully

understand the nature of heat.

(7) Heat and Energy. You have already learned something of the statues ofmatter and energy, the basic relationship of energy to heat, and how energy is

transformed from one type to another (such as electrical energy to heat energy).

Our primary concern in this section of the lesson is to increase this knowledge to

the extent that you can:

(a) Visualize, understand, and describe the molecular theory of matter.

(b) Understand the relationship between the molecular structure of

matter and kinetic energy.

(c) Understand the relationship between kinetic energy and heat.

(d) Associate the forms of energy and the transformation of energy with

heat.

(e) Apply your knowledge of the molecular structure of matter, the

relationship of molecular activity to kinetic energy, and the relationship of

kinetic energy and heat to the heat measurements you make.

c. The molecular theory of matter. Let's assume that all matter is composed

of tiny particles called molecules and the molecules are arranged in a lattice

structure, as shown in figure 1. The individual molecules attract or repel their

neighbors in accordance with the separation between molecules. Generally speaking,

when the separation is large, the force between molecules is small and is one of

attraction. The molecules of the material represented in figure 1 are located at

separations such that the forces of attraction and repulsion are equal to support

our discussion.

Figure 1. Molecular lattice structure.

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(1) The letters R and A shown in figure 1 represent the forces of repulsion

(R) and attraction (A) between the molecules. The line drawn through the middle of

the lines between molecules is used to show that the forces of repulsion and

attraction are equal to that distance. When a fixed lattice as shown (figure 1),

the forces between molecules nearly cancel each other so that there is very little

vibratory motion.

(2) The lattice structure shown in figure 1 is similar to the symmetry of

structure in crystals. If the molecules are pressed closer together, the force of

repulsion increases. If the molecules are forced farther apart, the force of

attraction increases. External forces exerted on the molecules of the lattice in a

solid cause the molecules to vibrate about their center positions. This vibration

motion is relatively weak, and the centers of the molecules remain fixed.

(3) In liquids the molecules are free to move greater distances. Since the

vibratory motion in liquids is greater than in solids, the energy which the moving

molecules can transfer to other molecules (kinetic energy) is greater. The

relationships between molecular separation forces, molecular kinetic energy, and

resulting-temperature conditions are shown in Table 1.

TABLE 1

THERMAL PROPERTIES OF SOLIDS, LIQUIDS, AND GASES

d. From the preceding paragraphs and Table 1, you should conclude that:

(1) Heat is the kinetic energy which a body possesses.

(2) Heat can be generated by means of electricity, compression, or friction.

(3) Kinetic energy is the work potential which a body possesses because of

its motion.

(4) Normally, the forces of attraction between molecules in a solid are

strong, the molecular energy is small, and the temperature values are low.

e. Molecular kinetic-energy level changes. Let us restate our conclusions

from the preceding paragraphs in a simple statement. When we think of heat, we

should think of kinetic energy. Because molecular kinetic

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energy is the energy of molecules in motion, heat considerations must also include

the vibratory motion of molecules. Our primary point of concern is that heat

measurements are affected by the vibratory motion of molecules and the relative

changes in their motion. An increase in the heat that a body possesses is due to

an increase in its kinetic energy. In order to increase the molecular kinetic

energy in a body, you must increase the energy which produces the vibratory motion.

(1) The circuit in Figure 2 is an example of changing the kinetic energy

(molecular vibratory motion) level of components (resistors) in an electrical

circuit.

(2) From the circuit in Figure 2, you can see that the only factor,

affecting the power (kinetic energy) dissipated by the resistors are the voltage

(E) and the current (I). It is a simple series circuit in which the total current

flows through each resistor. Since the resistances are equal, the voltage drops

across the resistors are equal, and each resistor dissipates the same amount of

power. This means that the power values listed in figure 2 apply to R1 and R2.

The differences in power values listed in figure 2 represent the changes in applied

power (voltage and current) caused by changes in the power switch position. The

resulting changes in power values (kinetic energy) also represent changes in the

energy losses in the form of heat. Table 2 is included to help you understand howchanges in the values of power applied to a circuit (or body) produce changes in

the kinetic energy of that body.

Figure 2. Energy, heat, and power in an electrical circuit.

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TABLE 2

Applied Energy--Kinetic Energy--Heat

(3) Figure 2 and table 2 summarize the information which supports the

following conclusions:

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(a) When you increase the level of energy applied to the molecules of a

material (in this case R1 and R2), there is a proportional increase in the kinetic

energy in the material.

(b) The increase in the kinetic energy in a material represents an

increase in the heat generated in the body.

(4) The wattage values for R1 and R2 in figure 2 were chosen arbitrarily.

From the energy developed in R1 and R2 as the applied voltage is increased, you can

imagine how the heat generated in each of these increases. From you experience in

testing electronic equipment, you know how hot some resistors get when they draw

too much current. If you compare the wattage values of R1 with the wattage

developed when the switch is in the 5 position, you can see that R1 will probably

"burn out" and cause an open circuit.

(5) Electrical energy is just one of the basic forms of energy which can be

transformed into heat. We used electrical energy as an example because we know

that you are familiar with many transformations of electrical energy to heat energy

which occur in electronic stoves, blankets, irons, and many other heating devices.

f. Temperature.

(1) The word "temperature" becomes important when you need to know the

intensity of heat in a body. You have watched water boil when fire from a stove

heats the water, or you have felt heat if you touched an object which has been

exposed to the sun. You have probably been in a room where the heat was so intense

that you could feel it on parts of your body. Our point is this: Regardless of how

heat is generated (sun, fire, friction, or other means), its generation causes an

increase in the motion of molecules in the material to which the heat is

transferred. What you feel as "heat", however, depends on the intensity of the

heat at a particular spot, not on the total amount of heat. An all-metal poker,

for instance, may be too hot to touch at the tip, but perfectly comfortable to hold

by the handle.

(2) When you measure the temperature of a body, you are measuring the

intensity of heat rather than the amount of heat. The amount of heat possessed by

a body at a given temperature depends on its weight and its specific heat. The

specific heat of a given material is the amount of heat necessary to raise the

temperature of a specific number of grams of that material 1. Specific heat can be

expressed in calories-per-pound-per-degree Celsius or in BTU's-per-pound-per-degree

Fahrenheit. The relationship between the quantity of heat (in BTU's) and specific

heat is involved in the problem which follows: PROBLEM: How many BTU's are

necessary to heat 5 pounds of iron from 80 to 100F.? Solution: The specific heat

of iron is 0.11 BTU/1 F. Therefore,

5 x 0.11 x (100 - 80)

= 0.55 x 20= 11.00 BTU's

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3. TEMPERATURE SCALES AND THERMOMETERS

a. Temperature is a measure of the intensity of heat. Temperature indicates

the thermal state of a body and its ability to transfer heat to other bodies or

objects. We know that heat is present, or transferred, if we can measure it. The

measurement of heat requires the use of scales and measurement devices.

b. Temperature Scales. Temperature measurements have been made possible with

the use of scales such as those in figure 3. Figure 3 shows the relationships of

some of the temperature scales used in heat intensity measurements. Note the

difference in value for the freezing and boiling points of water on the different

scales.

Figure 3. Common thermometer scales.

(1) Temperatures for the most common of the scales shown in figure 3

(centigrade and Fahrenheit) can be converted from one scale to another by means of

a simple proportion:

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(2) You can make conversions to the Kelvin and Rankin scales using the

relations:

K = 273.16 + C.

R = 459.69 + F.

(3) Table 3 shows the differences in steam point, freezing point, andabsolute zero for the four scales shown in figure 3.

TABLE 3

Common Thermometer Scales

Mercury-in-Glass Thermometer

(4) You have been told that temperature is a measure of the intensity of

heat and that one of several scales used to indicate the intensity of heat is

usually a part of a temperature measuring instrument. Some of the more common

practical measuring instruments are listed in table 4, with their usable ranges.

c. Mercury-in-Glass Thermometer.

(1) Table 4 includes the mercury-in-glass thermometer in the list of common

temperature measuring instruments. In its simplest form, the mercury-in-glass

thermometer is a hollow glass tube, hermetically scaled at both ends, and expandedinto a bulb at its lower end. The bulb is filled with mercury, and most of the air

is evacuated from the tube before it is sealed. This partial vacuum permits the

free expansion of the mercury to the top of the tube. When the mercury is heated,

it expands and rises in the tube. When the mercury is cooled, it contracts and its

level in the tube is lowered. A typical mercury-in-glass thermometer is shown in

figure 4.

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Figure 4. Mercury-in-Glass Thermometer

(2) You can see that the temperature has caused an expansion of the mercuryin the tube. The temperature indicated by the scales is about 33C. Using the

conversion formula previously stated, we can convert the reading taken from the

Celsius scale to a value on the Fahrenheit scale.

Our primary concern in the discussion of the mercury-in-glass thermometer is that

you understand the principle involved in its operation. This principle is based on

the expansion of mercury when heated.

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d. Resistance Thermometer.

(1) You calibrate liquid-in-glass thermometers such as the mercury-in-glass

thermometer and thermometer calibrators with a resistance thermometer. The

resistance thermometer is preferred over liquid-in-glass thermometers as a standard

for several reasons. Among these reasons are:

- Mercury freezes at -40 C.

- The pressure of mercury vapor becomes extremely high at temperatures in

excess of 360 C.

- The measurement accuracy of the resistance thermometer is far greater

than that of a liquid-in-glass thermometer.

(2) The resistance thermometer discussed in this volume consists

essentially of a pair of platinum helical (spiral) coils, bifilar-wound on a mica

form which is sealed in a Pyrex tube containing dry air. The Pyrex tube is

approximately 0.7 cm in diameter and 46 cm long, measured from the tube end of the

mount cover. This mount cover provides the means by which a clamp may be fastened

to the thermometer to mount it while it is in use. A four-lead cable 8 feet in

length, containing two potential and two current leads, forms part of the

resistance thermometer.

(3) When you use the resistance thermometer, immerse it approximately 9

inches in the medium having its temperature measure, and allow it to remain there

long enough to reach the temperature of the medium before any readings are

attempted (usually 5 minutes in a flowing liquid or 15 minutes in slow moving air).

TABLE 4

Ranges of Common Temperature Measuring Methods

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(4) Operating Principle. The operating principle of the resistance

thermometer is based on the fact that the resistance of an electrical conductor

increases when the conductor is heated. Resistance changes due to changes in

temperature can be accurately measured with the resistance bridges you have used

for other measurements. To calibrate the thermometer calibrator, you use a bridge

similar to the Mueller temperature bridge. Since the bridge is an integral part of

the measurement circuit, we discuss the Mueller temperature bridge as it is used ina resistance thermometer circuit.

4. MUELLER TEMPERATURE BRIDGE.

Although you have had some training on the construction and use of the Mueller

temperature bridge, we would like to review its construction and the basis on which

it operates.

a. The Mueller temperature bridge is designed to measure temperatures or

temperature difference to a high degree of accuracy when used in conjunction with

precision resistance thermometers. The range of the bridge is 1 to 111.111 ohms.

b. The bridge is balanced by the use of a convenient set of six step-by-step

dial switches. Certain coils used with these dial switches must be kept at aconstant temperature while making temperature measurements. These coils are

mounted in a special thermally insulated block whose temperature is kept constant

by an electric heater controlled by a thermoregulator. The rectifier and relay for

operating temperature control are mounted in a separate container, thereby

preventing galvanometer deflections when the relay operates. With this arrangement

there is no need for temperature corrections.

c. The sensitivity of the instrument and the damping of the galvanometer can

be adjusted over a reasonably wide range. It is also possible to perform a quick

check of--and if necessary, to adjust the equality of the ratio arms of--the bridge

to read the current flowing in the resistance thermometer. A special terminal and

an adjustable resistor are used in the measurement so that the potential leads of

the thermometer can be equalized as necessary.

d. A mercury contact commutator permits reversing the connections to a four-

lead potential-terminal-type resistance thermometer so that you can cancel the

effect of lead resistance. A plug and block arrangement is used to determine

whether zero correction is necessary. Terminals are included for connections to

the external battery (for supplying the bridge), to the external galvanometer, and

to either a three- or four-lead resistance thermometer.

e. As the Mueller bridge principle is discussed, you should remember the

primary purpose for its discussion. The resistance thermometer is based on the

fact that for every degree the temperature increases or decreases a proportional

amount. These resistance changes can be detected

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and accurately converted from resistance to temperature values by the Mueller

bridges. You use the resistance thermometer to calibrate thermometers,

thermocouples, and thermometer calibrators.

f. An examination of the bridge circuit (figure 5) should help you understand

its operating principle and the operation of the controls you use to null the

bridge and obtain the temperature values. We can simplify the explanation of thebridge circuit in figure 5 by using simplified circuits of its primary components.

For instance look at figure 6.

g. The diagram in figure 6 is a simplified version of the upper portion of

the Mueller bridge in figure 5. The resistances labeled "A" and "B" in figure 6

represent the ratio arms of the Mueller bridge in figure 5. The slidewire in

figure 6 is a resistance balancing device with the same function as the slidewire

where a null is obtained in the galvanometer (G). The null indicates the equality

existing between ratio arms A and B. You check the equality of the ratio arms by

interchanging arms A and B.

h. The bridge rheostat arm shown at A in figure 7 is the same R shown in

figure 5. The shunt type decade shown at B in figure 7 is the type of decade shown

in A of figure 7 and as part of R in figure 5. The inclusion of the rheostat armshown in figure 7 (and its shunt decades) permits measurements as small as 0.001

ohm. Special shunt type decades are used so that small resistance changes which

occur in devices such as the resistance thermometer are not masked (hidden) by

contact resistance.

i. The last division of the Mueller bridge discussed is shown in figure 8.

The resistor R represents the adjustable rheostat arm labeled "F" in figure 5. The

letter X represents a four-terminal resistance such as the one in the resistance

thermometer. L1, L2, L3, and L4 are the leads which connect the resistor

(resistance thermometer) to the bridge for a measurement.

j. If the galvanometer is connected to binding post 2, as shown in figure 9,

the resistance X and the lead L4 (with its resistance) is connected in theadjustable arm R.

k. If the ratio arms A and B are equal resistances and the resistance of L1

is equal to that of L4, the bridge is balanced by adjusting the rheostat arm until

its resistance equals the resistance of the arm in which X is located. Usually,

leads L1 and L4 are interchanged and the successive readings averaged so that you

can record the reading presented when L1 is equal to L4 with the galvanometer at

its NULL position.

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Figure 5. Mueller temperature bridge.

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Figure 6. Mueller bridge ratio arms.

Figure 7. Mueller bridge rheostat arm and shunt decades.

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Figure 8. Mueller bridge four-terminal connections.

i. The last division of the Mueller bridge discussed is shown in figure 8.

The resistor R represents the adjustable rheostat arm labeled "R" in figure 5. The

letter X represents a four-terminal resistance such as the one in the resistance

thermometer. L1, L2, L3 and L4 are the leads which connect the resistor

(resistance thermometer) to the bridge for a measurement.

j. If the galvanometer is connected to binding post 2, as shown in figure 9,

the resistance X and the lead L4 (with its resistance) are connected into the

right-arm of the bridge. The lead L1 (with its resistance) is connected in the

adjustable arm R.

k. If the ratio arms A and B are equal resistances and the resistance of L1

is equal to that of L4, the bridge is balanced by adjusting the rheostat arm until

its resistance equals the resistance of the arm in which X is located. Usually,

leads L1 and L4 are interchanged and the successive readings averaged so that you

can record the reading presented when L1 is equal to L4 with the galvanometer at

its NULL position.

Figure 8. Mueller bridge four-terminal connections.

1. Measurement principle. Our primary purpose in discussing the Mueller

bridge is to complete the concept of the resistance thermometer measurement

principle.

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TABLE 5

Resistance-Temperature Conversion Values for Platinum Resistance Thermometer

(3) At 0

C. the resistance ratio indicated is 1.000000. Since the

resistance of the thermometer at 0 measures 25.54900, the calibration accuracy of

the thermometer is correct. At 1C. the resistance ratio indicated is 1.003982.

When you multiply the ratio indicated by 25.54900, the product is very near the

value 25.65075. If you didn't know the temperature value (1C.), you could find it

by using the product of the ratio and resistance (ohm) columns. If the ratio

indicated on your measuring device is 1.1977710 for 50C. (value shown in table 5,

A, for 50C), you

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determine the resistance of the thermometer by multiplying the ratio by 25.54900

(thermometer resistance at 0). This product is nearly 30.60030. When you locate

the resistance value 30.60030 on the section of the resistance-temperature

conversion chart shown in table 5, A, you see that the temperature measured by the

resistance thermometer is 50C.

5. THERMOMETER CALIBRATION

a. One of the requirements is that you learn more about the calibration of

thermometers. You must increase your ability to apply thermal measurement

principles in the calibration of thermometers. When we use the term "thermal

measurement principles," we are thinking of the physical laws and concepts of heat

and temperature which we have discussed. These theories and concepts form the

basis for the thermal principles applied when you calibrate thermometers. The

physical laws we have in mind are as follows:

Heat is the total kinetic energy of moving molecules.

Heat is transferred from one body to another by means of conduction,

convection, radiation-absorption, or some combination of these processes.

The intensity of heat in a body is called its temperature.

The intensity of heat in a body is indicated on scales which relate the

intensity of the heat in that body to the intensity of heat necessary to raise

water to its boiling point or lower water to its freezing point.

Electrical energy can be transformed into heat (kinetic) energy.

The absence of heat is cold.

b. The thermometer scales in figure 3 show that the freezing points and the

boiling points of water are reference points for thermometers regardless of the

scale used. From table 4 it should be obvious that you may be required tocalibrate thermometers other than the mercury-in-glass type. Regardless of the

type, the measurement principle and the laws of physics which support this

principle remain the same. For example, when you calibrate a thermometer, what

does the measurement process include? Regardless of the type of thermometer

calibrated, you merely insert the thermometer being calibrated into a measurement

chamber and compare the thermometer reading with the reading indicated on the

measurement chamber readout device. The answer to the question you are thinking is

yes; the measurement chamber readout device has to be calibrated with the

resistance thermometer before it is used.

c. Your laboratory uses a temperature calibrator to check the accuracy of

other temperature devices. The rough sketch in figure 10 represents one of the

calibrators which you may use.

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Figure 10. Thermometer calibrator.

d. No attempt is made in figure 10 to duplicate any of the calibrators you

may have seen. We want you to become familiar with the basic components of a

typical thermometer calibrator and the purpose of these components. The scale

shown in figure 10 illustrates some limitations of the typical calibrator.

Calibration checkpoints on the scale of the thermometer calibrated must be within

the limits of the calibrator scale.

e. Comparing the limits of the calibrator scale of figure 10 with the ranges

of the mercury-in-glass and alcohol-in-glass thermometers listed in table 4, you

see that all points on the scale of the mercury-in-glass thermometer are within therange of the calibrator. Even though the lower limit of alcohol-in-glass

thermometer scale is lower than that of the calibrator, the alcohol-in-glass

thermometer can be calibrated at the minimum limit of its calibrator and maximum

point of the thermometer.

NOTE: When the boiling point of alcohol is attained, the thermometer may

explode.

f. The calibrator's operating principle is simple. The instrument contains

the components necessary for a refrigeration unit. Since cold is the absence of

heat, a refrigerant is used in the refrigeration unit to remove heat as necessary.

The amount of heat removed and the degree of cold reached depends on the position

of the calibrator indicator. The position of the indicator determines the amount

of refrigeration just as the thermostat in your home refrigerator determines theamount of refrigeration and the degree of coldness.

g. To calibrate thermometers at or near the boiling point of water, you

adjust the necessary controls on the calibrator so that the refrigeration unit is

replaced by heating units. Just as you selected the degree of coldness with the

calibrator indicator, you select the temperature (intensity of heat)

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at which you wish the thermometers calibrated.

h. Although the "wells" (openings into which the thermometers are placed for

calibration) are not shown in figure 10, they are located in such a position in the

calibrator that they can be completely enclosed by a synthetic non-flammable oil

which is used in the heat transfer process. It doesn't matter whether you are

calibrating thermometer scales at the freezing point of water; the calibrator scalemust first be calibrated with the standard thermometer which is your reference.

You should realize that your greatest concern over your thermometer calibrator is

with its stability and repeatability, not its accuracy.

i. Let us check to see if any of the laws of physics listed or principles

based on those laws are involved in the use of the thermometer calibrator. The

first law concerns heat and kinetic energy. Heat is generated by electrical

currents. These currents consist of electrons which do move and do possess kinetic

energy. Therefore, you can see that the first law listed has an application. The

second law listed concerns the transfer of heat. When the calibrator is used for

thermometer calibration, a synthetic non-flammable oil was mentioned. One of the

purposes for using this oil is its heat conducting capabilities. So you can see

that the second law has an application in the use of the thermometer calibrator.

6. RELATED HEAT SENSING AND MEASURING INSTRUMENTS

You should know the thermal measurement principles and methods applied in the

operation and calibration of heat measuring devices such as thermocouples and

pyrometers.

a. Thermocouples. Remember that when two unlike metals, such as copper and

iron, are connected as shown in figure 11, the junction of the metals can be used

as a part of a temperature measuring device. If heat is applied to the left-hand

joint (A) shown in figure 11 while the right-hand joint (B) remains at room

temperature (cold), a voltage is generated which causes the galvanometer to

deflect. The amount of deflection is proportional to the difference in the

temperatures at A and B. This device is known as a thermocouple.

Figure 11. A simple thermocouple.

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(1) One typical thermocouple application is the cylinder temperature

measurement of an air-cooled engine. The connections for the indicating instrument

are shown in figure 12.

Figure 12. Thermocouple application

(2) The thermocouple unit represented in figure 12 is made of iron and

constantan, which is an alloy of copper and nickel. The leads to the engine

cylinder are insulated with asbestos and covered with a cotton braid, which is

impregnated with a fire and moisture resistant lacquer. One of the two junctions

of the unlike-metal leads is formed into a copper ring which serves as a spark plug

gasket. The other, which is the cold junction, is inside the galvanometer. The

instrument is calibrated to read temperature in degrees Celsius. Commonly used

thermocouples are made of iron vs constantan, chromel vs alumel (chromel is an

alloy of nickel and chromium while alumel is an alloy of nickel and aluminum), and

copper vs constantan.

(3) Some thermocouples (figure 13) are constructed by connecting a wire

made of platinum alloy to a wire made of pure platinum. Such a device may be

called a noble metal thermocouple. A very satisfactory thermocouple of this type

consists of one wire made of 90 percent platinum and 10 percent rhodium and another

wire made of pure platinum. Thermocouples often have ranges extending to 3000F.

These thermocouples are often used as a standard for calibrating less expensive

thermocouples, or for special installations.

(4) The platinum-platinum rhodium thermocouple, shown in figure 13, is

typical of the thermocouples used for calibration purposes. Figure 13, A, shows

the outer configuration of the instrument, whereas figure 13, B, is a cutaway used

to show the internal composition and construction of the thermocouple. The

principle parts of the thermocouple shown in figure 13, B, are the head (5), a

primary protecting tube (11), and the thermocouple element (7). The thermocouplewires are 25 1/2 inches long and extend up through an insulator (9), lava-insulator

spacer (8), and fish-spine

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Figure 13. Platinum-platinum rhodium thermocouple.

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insulator (6) and are clamped under the screws of the two terminals (3) in the

head. These terminals connect the lead wires to the thermocouple. The thermometer

(2) is also mounted on the head. The thermometer, covered by a protecting sheath,

extends into the head to measure the temperature at this point (reference junction).

(5) We are interested in the principle used as a basis for the construction

of the thermocouple. From our discussion of the simple thermocouple of figure 11you can reason that there is a similarity on principle and construction of the

thermocouples shown in figures 11 and 13. Let's examine the corresponding

measurement circuits of the two thermocouples and see if we can establish a useful

comparison between the two.

(6) When you examine the measurement circuits A and B in figure 14, you see

that both circuits have hot junctions which are heated by an oven or other device

whose temperature is to be measured. Both measurement circuits use a galvanometer

as a readout device. However, in the measurement circuit B, a precision

potentiometer and a galvanometer are used to measure and indicate the voltage

developed across the thermojunction. The voltage developed is proportional to the

difference in temperatures between points a and b in both measurement circuits.

The relationships of junction voltages developed by various junction temperatures

are shown graphically in figure 15.

Figure 14. Thermocouple measurement circuits.

(7) Although the temperature scale indicated on the graph extends to

2000C., the platinum thermocouple is used for precise measurements between 0 and

1500. The temperature values and corresponding voltage values included in Figure

15 support the operating principles of the measurement circuits in Figure 14. We

are referring specifically to the B measurement circuit. In this circuit the

reference junction is maintained at 0

by means of an ice bath (two ice bottles

could be used). Any

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Figure 15. Junction temperatures versus junction emf.

difference in temperature between junctions a and b is indicated at the temperature

of junction a with respect to 0 (junction b reference temperature). Figure 15 is a

graphical presentation of how the thermocouple junction voltage increases as

junction temperature increases. The complete measurement circuit of Figure 14 is

shown in Figure 16.

Figure 16. Thermocouple measurement circuit using ice bath reference junction.

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(8) In addition to the readout device represented in Figure 14, Figure 16

includes connections for the standard cell and the battery (BA) which are a part of

the measurement circuit. A potentiometer of the type shown in Figure 16 should be

standardized before each measurement. The portion of a millivolt-temperature

conversion table (Table 6) shows how potentiometer scale values in millivolts are

converted to values in millivolts.

TABLE 6

Millivolt-Temperature Conversion Chart

(9) As an example of how Table 6 is used in a thermocouple temperature

measurement, suppose that you have filled the ice bath thermos bottles, shown in

Figure 16 with ice and water. Also suppose that after zeroing the galvanometer and

standardizing the potentiometer, you adjust the controls of the potentiometer for a

reading of 4.859. What is the value of the temperature measured? In Table 6,

4.859 (millivolts) is located opposite 560. The value 4.859 is also located in the

fourth column. This means that the temperature measured is 564. If you examine

the temperature values in the left column of the graph, you can see that there are

ten divisions between values. The in-between temperature values are obtained by

taking the left-hand column value (560) and adding the value indicated by the

column in which the potentiometer value is located (0.004).

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(10) A pyrometer is usually associated with the measurement of extremely

high temperatures. When properly designed, a pyrometer can be used for relatively

low temperature measurements. Some of the principle types of pyrometers in use are

as follows:

Optical pyrometers.

Seger cone pyrometers.Thermoelectric pyrometers.

Direct-radiation pyrometers.

Resistance pyrometers.

Our discussion is limited to the principles involved in the operation of the

thermoelectric and optical pyrometers.

(a) Thermoelectric pyrometers. You should remember how a thermocouple

is designed to measure the difference in temperature between a cold and a heated

junction. A millivoltmeter equipped with a scale which is graduated to read in

degrees of temperature is often used with the thermocouple. This thermocouple-

millivoltmeter combination is sometimes called a millivoltmeter pyrometer and is a

typical thermoelectric pyrometer.

(b) Optical pyrometers. Temperature measurement with an optical

pyrometer consists of comparing the monochromatic illumination from the source

being measured with the illumination from the filament of a standard lamp. When

the intensity of illumination from the standard source is equal to the intensity of

illumination from the hot body (source measured), you can assume that both bodies

are at proportionate temperatures. The proportionate temperature of the hot body

is determined by the graduations on the pyrometer scale.

(2) The type of pyrometer you use will vary with the need. Optical

pyrometers may have serial numbers such as 8621, 8622, 8623, and 8626. These

pyrometers may have a suffix C, indicating that the instrument is designed for

centigrade temperature measurements. In all other respects the capabilities of the

instrument are the same as those using the Fahrenheit scale. Temperature rangesfor three of these instruments are as shown in Table 7.

(3) The two major parts of this instrument are the telescope and a control

box. The telescope is designed to fit the hand. The telescope has an eyeshield

which is used when sighting a body. A flexible cable connects the control box to

the telescope. The control box contains a galvanometer, a standard cell, and a

breather. The telescope consists of a lamp, a switch, and a means of focusing the

lamp filament and the image of the hot body. The pyrometer measurement principle

is included in the steps of the measurement procedure which follow. Examine the

steps to see if you can identify the measurement principle.

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TABLE 7

Leads Northup Optical Pyrometers

(a) Sight on the object whose temperature is to be measured and adjust

the focus knob for the sharpest image.

(b) Close the contacts by pressing the switch, which is located on the

lower right side of the telescope sight piece.

(c) Rotate the knurled knob until the filament of the lamp blends with

(has the same brilliance as) the image of the hot object (until an optical balance

is obtained).

(d) In making the optical balance, use the section of the lamp filament

which is opposite the index of the lamp.

(e) Move the telescope from the line of sight, keeping the switch

closed, and press in on the knob which is located on the lower left-hand corner of

the front panel. While holding the knob in, rotate it until the galvanometer

pointer balances at zero on its scale.

(f) Read the value of temperature (on the proper scale).

(4) We hope that you included the processes of reaching an optical balance

and adjusting the galvanometer pointer for a zero balance in your determination of

the measurement principle. When the processes mentioned are performed, the

intensity of light from the source whose temperature is to be measured equals the

intensity of light from the standard source; the

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temperatures of the two sources are proportionate. Because of the proportionate

relationships, the pyrometer is a direct reading instrument.

(c) Now that you have completed the material on the effects and

measurement of heat and temperature, let's proceed to the study of humidity to see

how it is related to temperature and heat and how it is measured.

7. HUMIDITY

a. Humidity is nothing more than water vapor, and there is always some water

vapor in the air. The term "humidity" is used to describe the amount of water in

the air. However, the amount of water vapor may differ from place to place. It

will vary in a given location depending on the temperature, wind, rainfall, and

other weather factors. Before we discuss the instruments used in measuring

humidity, let's look a little closer at the nature of humidity.

b. The Nature of Humidity. There are two types of humidity measurements

usually associated with the measurements of the moisture content in the air. They

are relative humidity and absolute humidity. Both express the amount of moisture

content in the air, but in different forms. Let's define these and other common

terms that are associated with humidity.

(1) Capacity of Air - Capacity of air is the amount of moisture which the

air can hold when it is saturated. Capacity usually is measured in grains per

cubic foot (gr/ft3). The capacity increases with an increase in temperature.

(2) Absolute Humidity - Absolute humidity is the amount of water vapor in a

cubic foot of air at any given time.

(3) Relative Humidity - Relative humidity of the air is defined as the

ratio of the amount of moisture which the air actually contains to its capacity.

EXAMPLE: When a quart bottle contains one pint of a liquid, it is 50 percent full.

If a cubic foot of air that could hold four grains of water vapor holds only two

grains of water vapor, it is 50 percent full or half saturated. Such air has a

relative humidity of 50 percent.

(a) Another term commonly used in relative humidity is vapor pressure,

or partial vapor pressure. Vapor or partial vapor pressure is the number of pounds

per square inch of the atmospheric pressure which consists of water vapor. Partial

vapor pressure cannot be greater than the saturation value of the air at any given

temperature. For all practical purposes, it is the percentage of water vapor

relative to the saturation value of a given volume of air at a given temperature.

in other words,

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the term "partial pressure" can be interchanged with relative humidity. The

meanings are the same.

(5) Now that we have defined the common terms associated with humidity,

let's see how temperature affects humidity. A cold room in a house usually feels

damp. However, if the same room is heated the dampness seems to disappear and the

room becomes dry. The amount of moisture in the room has not been reduced byheating the room. However, the capacity of the air in the room for moisture

increases when the temperature is raised.

EXAMPLE: Suppose the air in a room at 32F. contains 2 grains of water vapor per

cubic foot. As shown in Table 8, the capacity of air at 32F. is 2.118 gr/ft3 or 94

percent. Such air will feel damp. When the temperature is raised to 68F., the

amount of moisture is unchanged, but the capacity of the air is now 7.56 gr/ft 3

divided by 7.56 gr/ft3, or 26 percent. This air feels dry. We see that increasing

the temperature decreases the relative humidity. On the other hand, if the

temperature is lowered, the relative humidity will increase.

TABLE 8

Water vapor capacity of air

(c) Humidity Measurement Instruments. Instruments that are used to

determine relative humidity are called either psychrometers or hygrometers.

Generally, the act and dry bulb instrument is called a psychrometer and direct

indicators of relative humidity are called hygrometers. Hygrographs determine and

record relative humidity.

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(1) The psychrometer.

(a) The psychrometric method of determining humidity is of importance in

the field of meteorology, air conditioning, laboratories, etc. Basically, the

psychrometer consists of two thermometers, one having a dry bulb, the other a wet

bulb. The dry-bulb thermometer indicates the actual temperature of the air. The

wet-bulb thermometer has a cloth wick surrounding its bulb. This wick is moistenedwith water. When the air is dry, the rapid evaporation of the water from the wick

cools the bulb and lowers the temperature indicated by the wet-bulb thermometer.

The lower the relative humidity, the more rapidly the water evaporates to produce a

greater difference between the wet- and the dry-bulb thermometer readings.

(b) Figure 17 illustrates a typical wet- and dry-bulb psychrometer. The

function of the saturated wick is to retain a thin film of water on the wet bulb so

that evaporation may continue until the wet-bulb temperature reaches a minimum.

Cotton or soft mesh linen is normally used because of excellent water absorbing

properties. Sizing the wick, encrustations due to mineral content of the water and

the thermometer will interfere with a continuous film of water on the thermometer

bulb. Foreign substances in the water or on the wick change the saturation vapor

pressure of the water and affect the results. Therefore, wicks should be clean and

should be replaced frequently, and distilled water should be used for moisteningthe wick. The wick should extend beyond the bulb and on the stem of the

thermometer for an inch or so in order to reduce heat conduction along the stem to

the bulb.

Figure 17. Wet and dry bulb psychrometer

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(c) The temperature of the water used to moisten the wick should be at,

or slightly higher than, the wet-bulb temperature. This is especially important

when the ambient temperature is high and the relative humidity is low. If the

temperature of the water used to wet the bulb is too high, it may take a long time

for the bulb to cool to wet-bulb temperature. Before this point is reached, the

water may have evaporated sufficiently so that the thermometer never reaches the

wet-bulb temperature. If the moistening water temperature is appreciable lowerthan the wet-bulb temperature, the thermometer temperature will climb throughout

the period of ventilation, remaining constant at the wet-bulb temperature only as

long as there is sufficient water to keep the bulb surrounded with a film of water.

If the temperature of the water used for moistening is at, or slightly above, the

wet-bulb temperature, the wet bulb will quickly attain the wet-bulb temperature and

remain at this value long enough to be easily and accurately read.

(d) Ventilation is obtained by swinging, slinging, or whirling the

thermometers at such rates as to produce the minimum velocity of 900 ft/min.

Stationary thermometers may be ventilated with a motor driven fan, so long as

minimum velocity is attained. Unventilated psychrometers are unreliable and hence

rarely used.

(e) The dry-and wet-bulbs of the psychrometer must be separated. Thisis to prevent the air that passes the wet-bulb (and is thereby cooled by

evaporation) from contacting the dry-bulb and causing an erroneous dry-bulb

reading. In the case of a sling, or whirled, psychrometer this may be avoided by

placing the thermometers so that the air will flow across the dry bulb before

reaching the wet bulb. Therefore, a sling psychrometer should be swung in only one

direction, depending upon its construction and the placement of the thermometers.

(f) The heat absorbed by the wet bulb, due to radiation, tends to raise

the wet bulb temperature so that a true depression is not attained. This can be

minimized by radiation shielding. One method, as shown in Figure 18, is to

surround the wet bulb with an external primary metal shield and insert an auxiliary

shield with a moist wick. When the thermometer is ventilated, the auxiliary shield

attains a temperature close to that of the wet bulb. This practically eliminatesthe source of radiation and conduction due to the difference in dry- and wet-bulb

temperatures.

(g) After the psychrometer is ventilated and the difference between the

dry-bulb and wet-bulb thermometers is determined, a chart similar to Table 9 is

used to compute relative humidity. Notice that with a given temperature

difference, the percentage of relative humidity depends upon

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Figure 18. Cross section of a shielded psychrometer.

the temperature of the dry-bulb thermometer. Keep in mind that relative humidity

varies with the air temperature.

TABLE 9

Relative Humidity

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(2) Mechanical Hygrometer. A mechanical hygrometer indicates relative

humidity directly; it is about 3 percent accurate if both temperature and humidity

are held constant at room temperature. The sensing element is usually a bundle of

human hair that has been put through some process to remove the oil from the hair.

The midpoint of several strands of hair is connected, through a lever arrangement,

to a pointer. As the humidity increases, the length of the hairs increases and

causes the pointer to move across the dial. Figure 19 shows this type ofhygrometer.

Figure 19. Hair type hygrometer

(a) The hair hygrometer indicates relative humidity over a wide range of

temperature, but its reliability decreases rapidly as the ambient temperature

decreases below freezing. Under changing humidity conditions there is a

considerable lag between the dial reading and the actual humidity. With

temperature and humidity stable at 77F., a change in relative humidity will require

approximately 5 minutes for the hair hygrometer to indicate 90 percent of the

change. This time lag greatly affects accurate measurement under changing humidity

conditions.

(b) Many organic materials are hygroscopic. Such materials as wood

fibers and cotton string have been used. None of these seems to be any better thanhuman hair.

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(c) Hygrographs are recorders of relative humidity. A pen arm, which is

substituted for the pointer, traces an ink record on a clock-driven drum and graph-

paper mechanism.

(3) Dew point hygrometer. The dew point is that point where the humidity

in the air just starts to condense and form water droplets on exposed surfaces.

This condition exists when the relative humidity reaches 100 percent. Basically,the procedure for detecting dew point is to cool a mirror until dew or frost just

condenses room surrounding air. The temperature at the surface of the mirror at

the instant dew or frost appears is defined as dew point. By using tables such as

tables 8 and 9, relative and absolute humidity may be calculated when the dew point

is known. One difficulty with this method is in measuring the exact temperature of

the mirror when dew or frost first occurs. Another difficulty is that any two

observers would probably not detect dew or frost at the same instant. It is

therefore, common practice for the dew point to be taken as the average temperature

at which dew or frost is first detected, on cooling of the mirror, and the

temperature at which the dew or frost vanishes when the mirror is warmed. This

procedure does not assure a correct answer, but it is close.

TABLE 8

Water Vapor Capacity of Air

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TABLE 9

Relative Humidity

(a) Photoelectric detection of the dew point has been used and is based

upon achieving an equilibrium condition of the mirror surface during which the

amount of dew or frost remains constant. This method agrees with visual methods,

within .1 C., down to -35 C.

(b) When the dew point is at or below the freezing point, the formation

of frost is not always positive due to the lack of a crystal nucleus for the frost

crystals to form on. In this case, supersaturation of the

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air may occur.

(c) A very simple dew point indicator is illustrated in Figure 20. Air

from the squeeze bulb is forced into the tank. This causes rapid evaporation of

the ether contained in the tank, which in turn cools the ether and the glass tube.

When the air in the tank is cooled to the dew point, dew forms on the glass tube.

When dew formation starts, the temperature of the ether may be read on thethermometer. This is the dew point temperature.

Figure 20. Dew Point Hygrometer

(4) Other types of hygrometers. The gravimetric method of determining

humidity is employed only when calibrating instruments or in determining the exact

water vapor content of air. In the gravimetric method, a measured volume of air ata known pressure and temperature is passed over a moisture absorbing chemical, such

as phosphorus pentoxide, and its increase in weight is measured.

(a) Other methods involving a color change of cobaltous salts have been

used. A very simple indicator may be made based on the change in color, from blue

to pink, of a cloth of paper impregnated with cobaltous

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chloride, as the humidity increases. A color comparison scale, when employed with

this indicator, gives a rough estimate of the relative humidity. Various other

similar chemicals have been used, more or less successfully.

(b) A great many experiments have been performed with various types of

materials for use in electrical hygrometers. Electrical hygrometry is based upon a

change in electric resistance of a hygroscopic material with changes in humidity.The material may be metal wires with various chemical compounds placed between the

wires, such as polyvinyl acetate, polyvinyl alcohol and lithium chloride,

phosphoric and sulfuric acid, and others. Plastics, underfired clays, cotton

impregnated with various solutions, and cotton wool and human hair have been

investigated. In some of these materials, the resistance appears to vary directly

with changes in humidity. In others, resistance appears to vary logarithmically

with changes in humidity.

(c) For specialized purposes, electrical hygrometers have proven to be

superior to other measuring devices, but for weather forecasting and general

humidity control the other types of hygrometers are better suited. However, it

would seem that, eventually, electrical hygrometers will replace most of the other

types of humidity measuring devices.

(5) This completes our discussion of humidity measuring instruments.

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ACCP SUBCOURSE NUMBER SM0486

MECHANICAL AND ELECTRO-MECHANICAL MEASUREMENT PRINCIPLES

EXERCISES FOR LESSON 1

1. Which statement concerning heat is not true?

a. Heat causes metal to expand

b. Heat is generated by compression

c. Heat is a form of energy

d. Heat is the partial kinetic energy of moving molecules

2. Excessive friction in a car engine

a. may prevent movement of some parts.

b. increases oil viscosity.

c. decreases wear.

d. improve gas mileage.

3. As a gas is compressed

a. space between the molecules decrease.

b. temperature decreases.

c. partial kinetic energy is expended.

d. condensation is removed from the compressed gas to reduce corrosion

4. Electricity

a. is heat.

b. is obtained from kinetic energy.

c. when applied to a resistance reduces heat by electromotive force.

d. can be used to produce heat.

5. As molecules are pressed closer together

a. the forces of repulsion and attraction equalize.

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b. repulsion force increases.

c. attraction force increases.

d. repulsion force decreases.

6. Refer to Figure 2 and Table 2. When the viper is moved from position 1 toposition 2, kinetic energy is increases

a. 2 times. c. .05 watts.

b. 4 times. d. .2 watts.

Figure 2. Energy, heat, and power in an electrical circuit.

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TABLE 2

Applied energy--Kinetic energy--Heat

7. With an increase in temperature molecular motion in material

a. decreases. c. remains constant.

b. increases. d. is transferred to the tip of an

all-metal poker.

8. Most of the air is evacuated from the column of a mercury-in-glassthermometer to

a. prevent contamination of the mercury.

b. permit use of a non-linear scale.

c. permit free expansion of the mercury to the top of the tube.

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d. maintain the mercury at a constant viscosity.

9. You obtain a resistance reading from the Mueller bridge of 26.111 ohms, the

temperature is approximately

a. 5.5 F. c. 5.5 C.

b. .5 C. d. 55 C.

10. What statement is true when calibrating an alcohol-in-glass thermometer?

a. Reduce the temperature of the calibrator so the alcohol freezes before

taking the first reading.

b. Insure that the thermometer is within the range of the calibrator.

c. Alcohol-in-glass thermometers are not as accurate as mercury-in-glass

thermometers.

d. Exercise caution when approaching the boiling point of alcohol.

11. A synthetic nonflammable oil is used with the thermometer calibrator to

a. prevent corrosion of the mercury contacts.

b. act as a media for heat transfer.

c. lubricate the thermometer calibrator.

d. protect the heating unit.

12. You are making a thermocouple temperature measurement and obtain a reading of

5.790 millivolts from the potentiometer. What is the value of temperature?

a. 655 K. c. 654 K.

b. 654 C. d. 655 C.

13. Measurement with an optical pyrometer consists of

a. comparing the illumination of the standard source to that of the hot

body.

b. determining the difference between the hot and cold junctions.

c. reading the millivolt output of the cold junction.

d. thermoelectric and optical computations.

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l4. The amount of moisture that air can hold when it is saturated

a. remains constant.

b. decreases with temperature.

c. is inversely proportional to air pressure.

d. increases with temperature.

15. When the relative humidity reaches 100%

a. increasing the temperature will form water droplets on exposed surfaces.

b. water droplets will form on exposed surfaces.

c. frost forms on exposed surfaces.

d. decreasing the temperature will increase the capacity of air.

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LESSON 2. PRESSURE MEASUREMENT AND DEVICES

AIPD Subcourse Number SM0486...........MECHANICAL AND ELECTRO-MECHANICAL


Lesson Objectives......................Given lesson objectives and supportive text,

you should be able to answer all exercisequestions pertaining to pressure principles,

pressure gages, barometers, and manometers

with no errors.

Credit Hours...........................Two

TEXT

1. INTRODUCTION

Your job assignment is the mechanical-electromechanical section includes the

calibration of all types of pressure measuring and indicating devices. You must

know the principles related to the operation and calibration of the pressure

measuring equipment sent to your section. The information in this lesson explainsthe pressure principles associated with pressure measurements and the operating

principles of some pressure measuring devices.

2. PRESSURE PRINCIPLES

a. When the term "pressure principles" is used in this section, we are

referring to the theories and laws of physics which are applied to the following:

Definitions and terminology associated with the physical forces which affect

and are affected by pressure.

The nature of fluid pressure and its transmission in and through fluids.

Atmospheric pressure and vacuum.

b. Definitions and Associated Terminology. Most of the definitions for terms

used in this chapter are identified as they are needed. One of the paragraphs

which follows extends the definitions for mass and weight as they apply to pressure

measurements. The remaining paragraphs explain additional terms associated with

pressure measurements.

c. Mass and weight are two of the fundamental quantities which must be

clearly defined when used with pressure measurements. The mass of a body is a

measure of the matter which the body possesses. Although the mass of a body is

expressed in the same units as weight (grams, pounds, etc.), they are not the same.

The weight of a body is the pull or force

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of gravity acting on that body. The difference between mass and weight can be seen

when we compare a 5-pound weight with a 10-pound weight. If both weights are

dropped from heights in the same locality, their accelerations are the same (32

ft/sec/sec). However, the inertia (force resisting any change in the velocity of a

body) of the 10-pound weight is twice that of the 5-pound weight because of the

inertia of a body is directly proportional to its weight. Since the gravitational

pull on a body varies inversely with the square of the distance from the earth, theweight of a body is not constant.

d. The force which tends to pull a body toward the center of the earth is

known as gravity. The intensity of force varies inversely as the square of the

distance that a body is moved away from the center of the earth. This means that

the "pull" of gravity on a body which is situated at sea level is greater than it

would be on the same body at a point 5000 feet above sea level.

e. There are several terms which are more directly related to the pressure

measuring instruments used in your laboratory. These terms and the corresponding

definitions are included in the list which follows:

Buoyancy--The "upward" force that pushes against a body which is submerged

wholly or partially in a liquid. The force with which the body is buoyed up isequal to the weight to the liquid displaced by the body.

Resolution--The sensitivity of an instrument. The smallest alteration in the

quantity to be measured which produces any change whatever (response) in the

indication of the instrument.

Sensitivity--The degree of responsiveness. The rate of displacement of the

indicating element with respect to changes in the measured quantity.

Repeatability--Performance relative to the instrument itself. A measure of the

consistency of performance. The quality of repeatability is usually expressed in

terms of percentage variation of reading.

Hydrostatic Head--Sometimes referred to as hydraulic head, oil head, or head.

The height of a column of body of fluid above a given point considered as causing,

counteracting, or measuring pressure. In determining the quality of pressure

caused by a certain head, multiply the height of the column of fluid by the fluid

density.

Tolerance--A specified allowance for error or variation from standard operation

or measurement.

f. The terms included in the preceding list are used in technical documents

which provide information concerning equipment changes and calibration procedures.

When you are sure that you understand all of the terms listed, you should have very

little trouble understanding the material which follows in this chapter. Let's see

how these terms (some of which represent theories and laws of physics) are relatedto other physi-

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cal conditions and laws which you must know and consider when you make pressure

measurements.

3. The Nature of Fluid Pressure and Its Transmission. In order to understand the

nature of pressure in a fluid, you must recall and understand Pascal's law

(principle) which concerns fluid pressure. We are referring to the law which

states: "Pressure applied to an inclosed fluid is transmitted equally in alldirections without loss and acts with equal force on equal surfaces." From this

law we can reason that the pressure existing in the fluid in an inclosed system

exerts a force at right angles against the walls surrounding the fluid, is shown in

Figure 1.

Figure 1. Fluid pressure within a system

a. The shape of a fire hose before the application of pressure from the fire

hydrant is illustrated in Figure 1, A. Figure 1, B, shows how equal forces are

applied in all directions on equal surfaces of the walls surrounding the water.

Figure 2 supports that portion of Pascal's law (principle) which concerns the

ability of a fluid to transmit pressure without a loss. The 5 lbs/in2 output of

the grease gun in Figure 2 depends on the seal made between the moving gasket and

the inner walls of the gun. It is obvious that the area of the piston surface is

four or five times that of the pimp opening. However, since the pressure exerted

by the piston is 5 pounds for each in2 of surface, the pressure exerted at the 1

in2 pump opening is 5 pounds.

Figure 2. A fluid transmits pressure without a loss

b. Hydraulic (Hydrostatic) Press Operating Principle. Let's examine Pascal's

principle as it is applied to the hydraulic (hydrostatic) press in Figure 3.

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Figure 3. Hydraulic (hydrostatic) press principle

c. The pressure transmitted to the bottom of the movable piston and the

distance the top of the piston moves is determined as follows. Since pressure in a

fluid is transmitted equally in all directions and acts with equal force on equal

surfaces, the 3-pound pressure applied by the hose piston whose inside dimension is

1 in2 is transmitted to each square inch of surface on the bottom of the movable

piston. Since there are 9 square inches of surface, the total upward force

(pressure) is 3 X 9, or 27 pounds. (See example A below.)

d. In our second example (example B) based upon Figure 3, if the area of the

large piston is 50 times the area of the small piston, then a force of 5 pounds

applied to the small piston applies a total force of 250 pounds/in2 upward against

the large piston. In simple terms, the total upward force is the product of the

applied force and the ratio of the output (upward) piston area to the input piston

area. Simple ratios can be used to obtain the same answer. Referring again to

Figure 3, let the force applied to the piston whose cross-sectional area is 1 in 2

be represented by F1 and let F2 represent the total force on the bottom of the

piston. Let the cross-sectional area of the hose piston be represented by A1 and

the area of the large movable piston be represented by A2. The total force is the

proportion:

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e. The total force applied to the bottom of the movable piston in Figure 3 is

many times the force applied to the input, but the piston movement is very small.

This is true because the distances moved by the two pistons are inversely

proportional to the piston areas.

4. Atmospheric Pressure and Vacuum. You depend on both of these when a pressure

device such as the A-1 barometer is used to calibrate altimeters. As you study the

material on atmospheric pressure and vacuum and their effects on different devices,

you should attempt to establish relationships between the following:

Force and atmospheric pressure.

Fluid pressure and atmospheric pressure.

a. When we speak of air (atmosphere) pressure, we refer to the intensity of

the force, per unit area, which air exerts on an object. In most instances you

express pressure values as a given number if inches, centimeters, or millimeters of

mercury. These units were derived by pouring a quantity of mercury into a glass

tube. The open end of the tube was tilted so that the mercury could be poured into

an open dish or pan as the closed end of the tube was turned to an upright

position. The mercury from the tube was emptied into the dish until its level in

the tube dropped to 30 inches (76 cm or 760 mm), as shown in Figure 4.

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Figure 4. One atmosphere--standard pressure

6. The arrows in Figure L indicate that atmospheric pressure exerted on a body at

sea level is in a downward direction (toward the center of gravity of the earth).

This downward force (atmospheric pressure) is a measure of the weight of the air at

a given point (relative to sea level) at a given temperature. The average

atmospheric pressure exerted on a body at sea level is approximately 14.7 lbs/in 2

and is called one atmosphere. This principle, illustrated in Figure 4, is based on

the fact that, in fluids, pressure is exerted in all directions by equal amounts.

Atmospheric pressure also exerts force in all directions. The air over ahorizontal surface exerts a force equal to the weight of all the air over the

surface. If the surface is not horizontal, the air still presses perpendicularly

against it with a force equal to the weight of all the air that would press on it

if it were in a horizontal position.

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c. Because the atmosphere is heavier at sea level that at points above sea

level, the standard pressure of one atmosphere (l4.7 lbs/in2 at 0 Celsius) exists

for sea level only. As the measuring instrument is moved from sea level to a

location above sea level, the level of mercury in a tube drops from 30 inches (76

cm or 760 mm) because the weight of the air above the tube becomes less as the

height increases. The decrease in the weight of air is indicated by a decrease in

atmospheric pressure.

d. The change in atmospheric pressure can be calculated if the total change

in height and the density of the air at the new location are known. You use the

following equation:

e. If the density of air is not known, divide the increase in height

(converted to feet) by 90 and multiply the quotient by 0.1. The answer representsthe number of inches of drop in the column of mercury. This means that the mercury

in the inverted glass tube of Figure 4 drops 0.1 inch for every 90-foot increase in

altitude, as shown in Figure 5.

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Figure 5. Atmospheric pressure changes

f. The values in Figure 5 show that the increase in altitude is greater than90 feet. Therefore, the mercury level decrease in the tube is greater than 0.1

inch. To determine the drop (in inches), the total increase in altitude (500) is

divided by 90 and the result is multiplied by 0.1 to determine the decrease in the

mercury level. Now that you know the basic principles associated with force, fluid

pressure, and atmospheric pressure, we continue your study of pressure principles

by teaching the principles associated with pressure gages.

5. Pressure Gages

a. Normally when we think of measuring pressure, the gage is the first

instrument th

US Army Course (1986) Mechanical and Electromechanical Measurement Principles MM0486

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