ME 320 lab 1

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ME 320: Heat Transfer Laboratory

Temperature Measurement Investigation

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TemperatureMeasurementInvestigation

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I. ObjectiveThe objectives of this experiment are to become familiar with general temperature measurement

instrumentation used in the laboratory, and to conduct various measurements with commonly used thermometers in

order to identify the differences between them. In addition, a thermocouple reference junction will be built and

compared to an electronic one, and the basic concepts of psychometrics.

II. BackgroundTemperature measurement is involved in many fields of study including thermodynamics, fluid mechanics,

heat transfer, aerodynamics, chemistry, electronics, and physics. To measure temperature, it is necessary to measure

some physical property that changes with temperature. Any instrument used to measure temperature is called a

thermometer. Examples of different types of thermometers that you will be investigating in this laboratory exercise

are described below.

A. Liquid ThermometerA commonly used instrument which uses liquid-in-glass as the measuring device. As the temperature on the

outside of the glass-bulb section of the thermometer increases, the liquid inside the glass-bulb expands. When the

temperature decreases, the liquid contracts. Thus, the instrument uses the differential expansion or contraction of the

liquid and the glass. There is a scale on the glass that relates the expansion of the liquid in the glass to the

temperature.

B. Thermocouple

Consists of an electrical circuit, which uses two unlike metals joined together to form a complete circuit. The

junction of dissimilar metals generates a temperature-dependent electromotive force.

Thermocouples are very important and commonly used temperature measurement devices and are based on the

Seebeck effect. By definition, the Seebeck effect is the production of an electromotive force (emf) and consequently

an electric current in a loop of material consisting of at least two dissimilar conductors when two junctions are

maintained at different temperatures. As shown in Figure 1, if two wires composed of unlike metals are connected at

both ends and one of the ends is heated, a continuous current flows through the circuit. If the circuit is broken, as

shown in Figure 2, the net open circuit voltage (the Seebeck voltage) is a function of the temperature of the junction

and the composition of the two metals. A chart that correlates temperature with the corresponding open circuit

voltage can be used to determine the temperature at the thermocouple junction.

Metal A

Metal B

Metal A

Heat

Figure 1 Seebeck Effect

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All dissimilar metals exhibit this effect. There are many standard combinations of metals that have been

developed, for which voltage correlations with temperature are well established. For small changes in temperature,

the Seebeck voltage is linearly proportional to temperature, =e TAB

where , the Seebeck coefficient, is the

constant of proportionality.

Metal B

Metal A

Heat

+

-

eAB

Figure 2 Seebeck Voltage

C. RTD (Resistance Temperature Detector)

Resistance thermometers, also known as RTDs, are wire-wound and thin-film devices that work on the

physical principle of the temperature coefficient of electrical resistance of metals. They are nearly linear over a wide

range of temperature and can be made small enough to have response times of a fraction of a second. They require

an electrical current to produce a voltage drop across the sensor that can be then measured by a calibrated display

device.

D. Thermistor

Thermistors are thermally sensitive resistors whose prime function is to exhibit a large, predictable, and precise

change in electrical resistance when subjected to a corresponding change in temperature.

E. Psychrometer

This device measures both dry and wet bulb temperatures of the air. One thermometer is wrapped in cloth,

which when wet and placed in an air stream, will indicate the wet bulb temperature of the air. Using the wet bulb

and dry bulb temperatures, the amount of moisture in the air can be determined.

F. Device Selection

Selection of a temperature measurement device is a common challenge in engineering. Table 1 contains a brief

summary of the advantages and disadvantages of the more commonly used instruments. Considerations will also

include the environment in which the temperature measurement is being made, e.g. liquid, gas, oxidizing

environment; also electrical noise, such as in the presence of high energy ignition systems in engines; also the long-

term maintenance and reliability required. Sometimes the choice is simple and obvious; other times, it is very

difficult, and numerous iterations may be required until a satisfactory solution is found.

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Table 1 Comparison between different types of thermometers

Glassthermometer Thermocouple RTD Thermistor

Advantages

Simpletouse

Inexpensive

Selfpowered

Simple

Rugged

Inexpensive

Widevariety

Widetemperature

range

Moststable

Mostaccurate

Morelinearthan

thermocouple

Highoutput

Fastresponse

Two-wireOhms

measurement

Disadvantages

Breakable

Lowresolution

Slowresponse

Noelectricaloutput

signal

Non-linear

Low-voltage

Reference

required

Leaststable

Leastsensitive

Expensive

Currentsource

required

Smallchangein

resistance

Lowabsolute

resistance

Self-heating

Non-linear

Limited

temperaturerange

Fragile

Currentsource

required

Self-heating

Applications

Commonlyusedto

measurethe

temperatureofaliquid

orgasinalow

temperaturerange

Bestoptionin

applicationswithhigh

temperatureranges

(upto1800K)suchas:

steelandiron

industry,heating

appliances,lab

environment,

medical,etc.

Precision

measurementswhere

thetemperaturesdo

notexceed600C.

Fortemperaturecontrol

ofprecisionprocesses

Automatictemperature

controlofoven,

injectionmolding,and

testchambers.

G. Thermocouple Reference Junctions

The Seebeck voltage cannot be measured directly. A voltmeter must be first connected to the thermocouple,

however the voltmeter leads themselves create two new thermoelectric circuits. Consider a voltmeter connected

across a copper-constantan (Cu-C) type T thermocouple, as shown in Figure 3.

Figure 3 Measuring Junction Voltage with a DMM

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We would like the voltmeter to read only V1, but connecting the voltmeter to measure the output ofJ1 has

resulted in the creation of two more metallic junctions:J2 and J3. SinceJ3 is a copper-copper junction, it creates no

thermal EMF (V3=0) but J2 is a copper-constantan junction, which will add an EMF (V2) in opposition to V1. The

resultant voltmeter reading Vwill be proportional to the temperature difference betweenJ1 andJ2. This says that the

temperature atJ1

cannot be found unless the temperature ofJ2

is known.

H. Ice Bath

One way to determine the temperature ofJ2 is to physically put the junction into an ice bath, forcing its

temperature to be 0C and establishingJ2as the reference junction. Since both voltmeter terminal junctions are now

copper-copper, they create no thermal EMF and the reading Von the voltmeter is proportional to the temperature

difference betweenJ1 andJ2.

Now the voltmeter reading is:

! = !! !! = !(!!! !!!) (1)

where tJ1 and tJ2 are absolute temperatures. If we define TJ as junction temperature specified in degrees

Celsius, then:

!! ! + 273.15 = !! (2)

V then becomes:

! = !! !! = ! (!!! + 273.15 (!!! + 273.15)] (3)But since TJ2=0C

! = ! !!! !!! = !(!!! 0) (4)

!=!!!! (5)

This protracted derivation is used to emphasize that the ice bath junction output, V2, is notzero volts: it is a

function of absolute temperature. By adding the voltage of the ice point reference junction, we have now referenced

the reading Vto 0C. This method is very accurate, because the ice point temperature can be precisely controlled.

The ice point is used by the National Institute of Standards and Technology (NIST) as the fundamental reference

point for their thermocouple tables, so we can now look at the NIST tables and directly convert from voltage to

temperature. Figure 4 shows a schematic of the circuit representing an ice bath reference junction.

Figure 4 External Reference Junction

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The Cu-C thermocouple shown in Figure 4 is a unique example because the copper wire is the same metal as

the voltmeter terminals. How do you think that the circuit will be affected if an iron constantan (Type J)

thermocouple were used instead?

I. Isothermal Block

Consider using an iron-constantan (Type J) thermocouple instead. The iron wire, shown in Figure 5, increases

the number of dissimilar metal junctions in the circuit, as both voltmeter terminals become Cu-Fe thermocouple

junctions.

Figure 5 Iron-Constantan Thermocouple

Figure 6 Junction Voltage Cancellation

If both voltmeter terminals are at the same temperature, the voltages cancel, as shown in Figure 6. However, if

the terminals are at different temperatures, there will be an error introduced. For a more precise measure the copper

voltmeter leads should be extended so the copper-iron junctions are made on an isothermal block, as shown in

Figure 7. An isothermal block is an electrically insulating but thermally conducting material, which serves to hold J3

and J4 at the same temperature. The absolute block temperature is unimportant because the two Cu-Fe junctions act

in opposition. We will have:

! = !(!!! !!"#) (6)

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Figure 7 Using Isothermal Block to Remove Junctions from DMM Terminals

Replacing the ice bath with another isothermal block results in the circuit shown in Figure 8:

Figure 8 Elimination of the Ice Bath

The new block is at some reference temperature, TREF, and becauseJ3 and J4 are still at the same temperature,

we can again show that:

! = !(!!!!!"#) (7)

This is still a rather inconvenient circuit because we have to connect two thermocouples. Lets eliminate the

extra Fe wire in the negative (low) lead by combining the Cu-Fe junction (J4) and the Fe-C junction (JREF). We can

do this by first joining the two isothermal blocks, as shown in Figure 9.

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Figure 9 Joining the Isothermal Blocks

We havent changed the output voltage V. It is still

! = !(!!!!!"#) (8)

Now we call upon the law of intermediate metals to eliminate the extra junction. This empirical law states that

a third metal (in this case, iron) inserted between the two dissimilar metals of a thermocouple junction will have no

effect upon the output voltage as long as the two junctions formed by the additional metal are at the same

temperature. Figure 10 illustrates the lat of intermediate materials.

Figure 10 Law of Intermediate Metals

This is a useful conclusion, as it completely eliminates the need for the iron (Fe) wire in the low lead.

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Figure 11. Equivalent Circuit

Again,! = !(!!! !!"#) where is the Seebeck coefficient for a Fe-C thermocouple. Junctions J3 and J4 inFigure 11 take the place of the ice bath. These two junctions become the Reference Junction.

J. Software Compensation

Now we can proceed to the next logical step: directly measure the temperature of the isothermal block and use

that information to compute the unknown temperature, TJ1as indicated in Figure 12.

Figure 12 External Reference JunctionNo Ice Bath

A thermistor whose resistance RT is a function of temperature provides us a means to measure the absolute

temperature of the reference junction. Junctions J3 and J4 and the thermistor are all assumed to be at the same

temperature, due to the design of the isothermal block. Using a digital multimeter (DMM) , we simply:

Measure RT to find TREF and convert TREF to its equivalent reference junction voltage, VREF using the

NIST tables.

Measure V and subtract VREF to find V1, and convert V1 to temperature TJagain using the NIST tables.

This procedure is known as software compensation because it relies upon the software of a computer or

controller to compensate for the effect of the reference junction. The isothermal terminal block temperature sensor

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can be any device which has a characteristic proportional to absolute temperature: an RTD, thermistor, or an IC

sensor.

K. Electronic Ice Point

Rather than measuring the temperature of the reference junction and computing its equivalent voltage as we

did with software compensation, we could insert a battery to cancel the offset voltage of the reference junction. The

combination of this hardware compensation voltage and the reference junction voltage is equal to that of a 0C

junction.

The compensation voltage, e, is a function of the temperature sensor resistor, Rt. The voltage V is now

referenced to 0C and may be read directly and converted to temperature using the NIST tables.

Another name for this circuit is electronic ice point reference. These circuits are commercially available for use

with any voltmeter and with a wide variety of thermocouples. The major drawback is that a unique ice point

reference is usually needed for each individual thermocouple wire.

Figure 13 Hardware Compensation Circuit

It seems logical to ask that if we already have a device that will measure absolute temperature, like an RTD or

thermistor, why do we even bother with a thermocouple that requires reference junction compensation? The single

most important answer to this question is that the other devices are only useful over a narrow temperature range.

Thermocouples, on the other hand, can be used over a wide range of temperatures, and optimized for various

environments. They are much more rugged than thermistors, as evidenced by the fact that they are often welded to a

metal part or clamped under a screw. They can be manufactured quickly and easily, even in the field. They are the

most versatile temperature transducer available and since the measurement system performs the entire task of

reference compensation and software voltage to temperature conversion, using a thermocouple becomes as easy as

connecting a pair of wires.

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III. EquipmentApictureoftheexperimentalapparatuswithimportantfeatureslabeledisshowninFigure14.

Figure 14. Photo of the main control panel and its components

IV. Experimental ProcedureA. Air Temperature Measurement

1. Turn on the main circuit breaker. Remove the glass thermometer from its holder, and determine the

ambient room temperature. Record this value in the provided table. Place the thermometer back into

the plastic tube for safekeeping.

2. Connect a type T thermocouple (blue color code) into the thermocouple input socket labeled 1. Switch

on the Thermocouple Digital Indicator, and ensure that the rotary switch is set to #1. Use the

thermocouple to measure the ambient air temperature, displayed on the digital meter above the selector

switch, and record the data.

3. Plug the thermistor probe handle into the thermistor digital indicator socket, and insert the thermistor

with the perforated tip (used only for measuring air) into the handle. Do not use the air thermistor

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for measuring water temperature. Turn on the digital display labeled Thermistor, and measure and

record the ambient room temperature. Note that the temperature limit for the air thermistor is 100C.

4. Plug the RTD handle into the RTD Digital Indicator socket, and insert the probe into the handle.

Switch on the RTD display meter and measure and record the ambient room temperature using the

RTD digital display.

5. Turn the air heater on using the toggle switch, and set the Air Heater Temperature Control to 3.

Toggle the Hot/Cold switch to the Hot position. The air blower will discharge hot air across the brass

thermowell located at the blower outlet.

6. Individually place the temperature measurement devices (glass thermometer, type T thermocouple, air

thermistor, and RTD) in the thermowell. After ensuring that steady state has been reached, record the

temperature using all four devices. Switch the Cold/Hot selector switch to Cold when all data have

been collected and allow the blower to run for a couple of minutes to cool the heater. Switch OFF the

air heater switch. Caution: The thermowell may remain hot after the blower is turned off.

B. Water Temperature Measurements

1. Fill the water heater tank approximately full of water, and turn the heater control to 10. Be careful

not to touch this as it heats, as you could get burned. When the water begins to boil, reduce the setting

to 7just enough to keep the water boiling. Note: Do not turn the water heater on until you have

finished the hot air temperature measurements, since the water vapor could affect the

experimental results.

2. Change the thermistor probe from the air thermistor to the water thermistor, which has a sealed or

closed probe tip. Take temperature measurements of the vessel of ambient temperature water with all

four devices, and record.

3. The water in the heater tank should be boiling by now. Individually insert the temperature

measurement devices into the boiling water, taking care not to allow the probes to touch the bottom of

the tank. Record the temperature using all four devices. Turn down the water heater control when

measurements are complete.

4. Prepare an ice water bath by placing crushed ice with a small amount of water into the Thermos

container. Allow this to stand for a few minutes before proceeding with measurements.

5. Individually insert the four devices into the ice water bath. Measure and record each observed

temperature.

C. Thermocouple Reference Junction

1. Fill the water heater tank approximately full of water, and turn the heater control to 10. Be careful

not to touch this as it heats, as you could get burned. When the water begins to boil, reduce the

setting to 7just enough to keep the water boiling.

2. Prepare an ice water bath by placing crushed ice with a small amount of water into the Thermos

container. Allow this to stand for a few minutes before proceeding with measurements.

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3. In order to use the Millivolt Digital Meter, connect the white extension lead into the socket using the

red line (+) and black line (-). Measure and record the voltage indicated for ice water. Compare this

value with handbook values. A picture of the experimental setup and a circuit schematic are shown in

Figures 15 and Figure 16.

Figure15Photoofsetupforexperiment3-step3

Figure16EquivalentcircuitcorrespondingtoFigure15

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4. In order to use the ice bath reference assembly, disconnect the white extension lead with

thermocouple probe then connect the white lead to the male socket (white) on the hand-held reference

junction assembly.. Insert the type T thermocouple probe into the thermocouple female socket (blue)

on the hand-held reference junction assembly. Measure and record voltage readings in millivolts for

ambient, ice bath, and boiling water. Compare your measurements with the type T thermocouple

table in Appendix A, which relates millivolts to temperature. A picture of the experimental setup and

the corresponding circuit schematic are shown in Figures 17 and Figure 18.

Figure 17 Setup for Experiment 3-step 4


5. Connect one end of the red extension wire to the output socket of the Thermal Reference Junction

labeled (+). Connect the other end to the red wire to the (+) input of the Millivolt Digital Meter.

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Connect the black extension wire to the (-) terminals. Switch on the MIllivolt Digital Meter and

switch on the panel-mounted Thermal Ref. Junction. Using the type T thermocouple probe, measure

and record voltage readings for the room temperature, ice water, and boiling water. Compare these

readings to those published in Appendix A and those recorded in step (d). A picture of the

experimental setup and a circuit schematic are shown in Figures 19 and Figure 20.

Figure19SetupforExperiment3-step5


D. Psychrometrics

1. Make sure the center glass tube is filled with water and that the cloth around the wet-bulb

thermometer is wet.

2. Wait until the temperature measurement on the wet-bulb thermometer comes to a steady state.

Record the dry-bulb and wet-bulb temperatures in the data table

3. Use the data table on the front panel of the thermometers to find relative humidity.

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V. Technical ReportFor this experiment, a full written report is not required. Please answer the following questions and present

your results using tables as indicated. Your submission must be prepared using a computerhandwritten

documents are not acceptable.

A. Experiments 1 & 2

1. Prepare a data table using Excel or other similar application which contains all of the temperature

measurements. Show the average temperature measured for each case.

2. Do all of the temperature measurements agree? Report the greatest observed difference from the

average for each case in a table. Report these both in terms of percentage and C. Which device

exhibited the greatest deviation from the average? Offer an explanation as to why

3. According to your results, is it better to report the observed difference from the average in terms of

percentage, C or Kelvin?

4. Which thermometer seemed to respond faster/slower in each situation? Offer an explanation regarding

the characteristics of each device.

B. Experiment 3

5. Prepare a table as above with the recorded data and add the corresponding temperature as determined

from the NIST table in Appendix A.

6. To which temperature does the recorded value in mV from Experiment 3.1 of the Data Sheet

corresponds? Is this value different from 0C? Why?

7. Do the ice water compensation and electronic cold reference junction temperature measurements

agree? Which technique do you think is more accurate, and why?

C. Experiment 4

8. Using the psychrometric chart provided in Appendix B of your lab manual or another source such as

your thermodynamics text book or the internet, determine the following parameters for the wet-bulb

and dry-bulb temperatures you recorded:

a. Relative Humidity

b. Humidity Ratio

c. Specific Volume

d. Enthalpy

9. If the wet-bulb temperature is kept constant but the dry bulb temperature increased by 5 degrees, what

would you expect the change to the above parameters to be? Would they increase, decrease, or

remain the same?

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Thermoelectric Voltage (absolute mV)

C 0 1 2 3 4 5 6 7 8 9 10

C

-40 -1.475 -1.510 -1.544 -1.579 -1.614 -1.648 -1.682 -1.717 -1.751 -1.785 -1.819 -40

-30 -1.121 -1.157 -1.192 -1.228 -1.263 -1.299 -1.334 -1.370 -1.405 -1.440 -1.475 -30

-20 -0.757 -0.794 -0.830 -0.867 -0.903 -0.940 -0.976 -1.013 -1.049 -1.085 -1.121 -20

-10 -0.383 -0.421 -0.458 -0.496 -0.534 -0.571 -0.608 -0.646 -0.683 -0.720 -0.757 -10

0 0.000 -0.039 -0.077 -0.116 -0.154 -0.193 -0.231 -0.269 -0.307 -.0345 -0.383 0

0 0.000 0.039 0.078 0.117 0.156 0.195 0.234 0.273 0.312 0.351 0.391 0

10 0.391 0.430 0.470 0.510 0.549 0.589 0.629 0.669 0.709 0.749 0.789 10

20 0.789 0.830 0.870 0.911 0.951 0.992 1.032 1.073 1.114 1.155 1.196 20

30 1.196 1.237 1.279 1.320 1.361 1.403 1.444 1.486 l.528 1.569 1.611 30

40 1.611 1.653 1.695 l.738 1.780 1.822 1.865 1.907 1.950 1.992 2.035 40

50 2.035 2.078 2.121 2.164 2.207 2.250 2.294 2.337 2.380 2.424 2.467 50

60 2.467 2.511 2.555 2.599 2.643 2.6872.731 2.775 2.819 2.864 2.908

60

70 2.908 2.953 2.997 3.042 3.087 3.131 3.176 3.221 3.266 3.312 3.357 70

80 3.357 3.402 3.447 3.493 3.538 3.584 3.630 3.676 3.721 3.767 3.813 80

90 3.813 3.859 3.906 3.952 3.998 4.044 4.091 4.137 4.184 4.231 4.277 90

100 4.277 4.324 4.371 4.418 4.485 4.512 4.559 4.607 4.654 4.701 4.749 100

110 4.749 4.796 4.844 4.891 4.939 4.987 5.035 5.083 5.131 5.179 5.227 110

120 5.227 5.275 5.324 5.372 5.420 5.469 5.517 5.566 5.615 5.663 5.712 120

130 5.712 5.761 5.810 5.859 5.908 5.957 6.007 6.056 6.105 6.155 6.204 130

140 6.204 6.254 6.303 6.353 6.403 6.452 6.502 6.552 6.602 6.652 6.702 140

150

6.702 6.753 6.803 6.853 6.903 6.954 7.004 7.055 7.106 7.156 7.207 150

C 0 1 2 3 4 5 6 7 8 9 10

C

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Free Air Jet Investigation

Appendix B. Psychrometric Chart

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