Instrumentation Important Points to Remember

8/12/2019 Instrumentation Important Points to Remember

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INSTRUMENTATION IMPORTANT POINTS TO REMEMBER

CHAPTER: ANALOG VOLTMETERS AND AMMETERS REF: SAHANEY

PMMC type meters can be used for only to measure dc. Induction type meters are used to measure only ac. Pmmc is most accurate type of dc current measurements. Electro dynamo meter type instruments are useful to measure both ac and dc and scale

is common for both, hence it is useful as transfer instruments.

Thermal instruments have advantage that their calibration is same for both ac and dcthey are particularly suited for alternating type currents because they depends upon theheating effect of the alternating current. Their readings are independent of frequency

or waveform of the current, and any stray magnetic fields which may exist in the vicinity

of the meter.

The induction principle is more generally used for watt hour meters than for ammetersand voltmeters owing to their comparatively high cost and inaccuracy of induction

instruments of latter types.

The most serious error is error produced by heat generated in the instrument which canchange the resistance of coil. In ammeters it is not a big problem but in case of

voltmeters it is a serious problem.

Hence resistance of the coil must be constant A high swamping resistance of whose resistance temp coefficient is small is connected in

series with the coil, so that , although the resistance of the coil change considerably the

overall change in resistance of circuit is small.

PERMANENT MAGNET MOVING COIL (PMMC)

The pmmc instrument is most accurate type for dc measurement. TORQUE EQUATION: deflecting torque Td = NBLDI =GI Where G is a constant = NBLD The spring control provides a restoring or control torque Tc= k For final steady deflection Tc=Td $=( G/K)I As the deflection is directly propotional to the current passing through the meter we get

a uniform scale for the instrument.


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Errors in PMMC meters:- weakening of permanent magnets due to aging oftemperature effects

Weakening of springs due to ageing of temperature effects Change of resistance of the moving coil with temperature Advantages and disadvanges of PMMC: scale is uniform Power consumption is very low, between 25 micro watt to 200 micro watts Disadvantages are costlier than moving iron type and cannot be used for ac

measurements.

OHMMETERS:-

Series type: these ohmmeters will read infinite resistance at 0 current position and 0

resistance at full scale current position.

Rh = R1+ R2||Rm

Shunt type : zero mark on the left side and infinite mark on right side

Rh= R1||Rm

MOVING IRON TYPE: the most common ammeters and voltmeters for laboratory or switch

board use at power frequencies are the moving iron instruments.

Cheap in cost as compared with any other type of ac instrument of same accuracy and

ruggedness.

This is attraction type instrument mostly.

Deflection is proportional to square of current. The deflection torque is therefore unidirectional

whatever may be the polarity of the current.

Advantages and disadvantages:- universal use , cheap ,robust ,accuracy, 240 degree scale

Due to hysteresis loss dc measurements are generally avoided.

Waveform errors :- scale is rms , hence waveforms highly distorted can measure satisfactorily.

ELECTRODYNAMO METER TYPE INSTRUMENTS:-

A transfer instrument is one that may be calibrated with a dc source and then used withoutmodification to measure ac this requires a transfer type instrument to have same accuracy for

both dc and ac , which the electro-dynamo meter instruments have.

Advantages and disadvantages:-

Because of air core these are free from eddy current errors and hysteresis errors.


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Can use both ac and dc and measures rms value irrespective of waveform

Disadv: low torque/weight ratio can cause increased frictional losses and low sensitivity

They are more expensive compared to pmmc and moving iron type.

Sensitive to overloads and mechanical impacts should be handle with great care.

Have non uniform scale.

High power consumption because the current under measurement has to provide the field flux

HOT WIRE INSTRUMENTS:

Calibrated to measure rms value

.instable due to stretching of wire, lack of ambient temperature compensation, sluggish

response and high power consumption and inability to withstand overloads and mechanical

shocks

THERMO COUPLE INSTRUMENTS:

When two metals having different work functions are placed together , a voltage is generated

at the junction which is nearly proportional to the temperature. This junction is called thermo

couple.

Can be used to measure both ac and dc measurements and this type of instruments are very

usefull for measurements at RF frequencies.

ADVANTAGES: indicates rms value irrespective of waveform, not effected by stray magnetic

fields, free from frequency errors can be used at microwave frequencies also.

The overload capacity of these instruments is small compared to other type of instruments.

RECTIFIER TYPE INSTRUMENTS:

Convert ac to pulsating dc and uses PMMC ,and this is attractive method because pmmc has

good sensitivity compared to other instruments.

The rectifier type of instruments are calibrated for rms values of sinusoidal voltages or

currents . in order that a rectifier type instrument should read true rms value of thesinusoidal input , the scale markings are made with using a multiplying factor of 1.11 in case

of instruments using full wave rectifier 2.22 in case of instruments using halfwave rectifiers.

Waveform errors are present in case of other than sinusoidal measurements.(see pg no 295

for an example).


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Sensitivity for ac signals is lower than that of dc signals.

METER TYPE SUITABILITY MAJOR USES

PMMC DC Most widely used for dc in low and medium

impedance ckts.

MOVING IRON

AC OR DC

Inexpensive type used for rough indication of

currents and voltages. Widely used in

indicating type applications such as panels.

ELECTRO DYNAMO METER Widely used for precise ac current and

voltage measurements at power frequencies.

Transfer instrument

ELECTROSTATIC High voltages and little currents can be

supplied by ciruit under measurement

THERMO COUPLE Rf signals.

RECTIFIER Medium sensitivity and medium impedance

ckts.

Note: electrostatic type instruments do not take any current , they measure rms value of

voltage.


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CATHODE RAY OSCILLOSCOPE

The normal form of a cro uses a horizontal input which is an internally generated rampvoltage called TIMEBASE

Electro static deflection = D= LldEd/2dEa . L= distance between screen and center of the deflecting plates. Ed = potential between deflection plates. D= distance between deflecting plates. ld length of deflecting plates, Ea voltage of pre

accelerating anode.

DEFLECTIN SENSITIVITY S= D/Ed and deflection factor = 1/S.

Graticule is a grid of lines that serves as a scale for measurements purpose AQUADAG: the bombarding electrons , striking the screen release secondary emission

electrons are collected by an aqueous solution of graphite called aquadag which is

connected to the second anode, collection of secondary electrons is necessary to keep

the crt screen in a state of electrical equilibrium.

Vertical amplifier is the principal factor in determining the sensitivity and bandwidth ofan oscilloscope.

The gain of the vertical amplifier determines the smallest signal that the oscilloscopecan satisfactorily reproduce on the CRT screen, the sensitivity of an oscilloscope is

directly proportional to gain of the vertical amplifier that is, as gain of the vertical

amplifier increases sensitivity increases, which allows us to observe smallest amplitude

signals.

HORIZONTAL AMPS:- the horizontal amplifiers basically serves two purposes. 1. When the oscilloscope being used in the ordinary mode of operation to display a

signal applied to the vertical input, the horizontal amplifier will amplify the sweep

generator output.

2. when the oscilloscope is being used in the X-Y mode , the signal applied to thehorizontal input terminal will be amplified by the horizontal amplifier.

MEASUREMENTS OF PHASE AND FREQUENCY(LISSAJOUS PATTERNS)


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Fy/Fx = number of horizontal tangencies/ number of vertical tangencies.

ans ()

NOTE:


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DUAL TRACE OSCILLOSCOPE: there are two separate vertical channels

There are two common operating modes alternate mode and chop mode.

DUAL BEAM OSCILOOSCOPE: the dual trace oscilloscope cannot capture two fast transient

events, as it cannot switch quickly enough between traces

The dual beam cro has two separate electron beams and two separate vertical channels

SAMPLING OSCILLOSCOPES: these are to examine very fast signals

The horizontal amplifier should be designed for high amplitude and slow rise timesignals.

Post acceleration is needed in a CRO if the frequency of the signal is greater than10MHZ.

The source of emission of electrons in a CRT is a barium and strontium oxide coatedcathode.


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In sweep mode operation of cro fsignal > = fsweep in order to get stable output and reasonable

accuracy.

MEASUREMENT OF RESISTANCE:-

Low resistance : order of 1 ohm or below that MEDIUM RESISTANCE:- 1- 0.1 M Ohm

High resistance :- above 0.1 M ohm.

Measurement of medium resistance:- 1. Ammetervoltmeter method, 2. Substitution method

3. wheatstone bridge 4. Ohmmeter method.

Wheatstone bridge:- bridge sensitivity is defined as the deflection of the galvanometer per unit

fractional change in unknown resistance.

SB= SVE/(P/Q + 2+ S/R)

SB = bridge sensitivity and Sv = galvanometer voltage sensitivity.

Q-Meter:

q-meter is designed to measure some coil properties and capacitor properties.

Operation based on series-resonance. At resonance Xc= XL then V=IR

Q= XL /R = XC /R = Vc /V since at resonance VC =Q V

If a fixes voltage applied to the circuit, a voltmeter across the capacitor can be calibrated to

read Q directly.

SERIES CONNECTION:- Low- impedance components such as low value resistors , small coils

and large capacitors, are measured in series with the measuring circuit.

PARALLEL CONNECTION:- high impedance components such as high value resistors ,, certain

inductors and small capacitors are measured by connecting them in parallel with the

measuring circuit.

SOURCES OF ERROR:- distributed capacitance or self capacitance of measuring circuit.

And insertion resistance of Q-meter.


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ERRORS IN MEASUREMENT AND THEIR STATISTICAL ANALYSIS

Relative limiting error =

.

Combination of quantities with limiting errors:

SUM OF TWO QUANTITIES:- X= X1+X2 then the relative limiting error is

= (

+

)

If a limiting error is mentioned in % then convert it into limiting error form and proceed.


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Previous ies questions:-

Measurement of an unknown voltage with a dc potentiometer loses its advantage ofopen-circuit measurement when Voltage is larger than the range of the

potentiometer.

Harmonic distortion analyzer measures the rms value of all the harmonic componentexcept the fundamental frequency component.

Load cells utilize elastic member as the primary transducer and strain gauges assecondary transducer.

IES 2000 EC

PMMC ------- linear scale over the entire range.Moving IRON -------- Square law type scale because Torque I

2.

Thermo couple ---------suitable for high frequency measurements.

Electrostatic type -----------voltmeter.

Wagners earth device removes all the earth capacitances from the bridge network. When temperatures being measured are high and physical contact with the process to

be measured is impossible , use is made of thermal radiation methods or optical

pyrometers are used, these pyrometers are used under conditions where corrosive

vapours or liquids would destroy thermo couple and thermometers and thermistors if

made to come in contact with the measured medium, this type of pyro meters are used

for rapidly moving objects. Temperature range is 12000C to 1500

0C.

In the pH cell, one of the electrodes, called reference electrode is at constant potentialregardless of pH value.

Pirani gauge:- the operation of pirani gauge depends on variation of the thermalconductivity of a gas with pressure. Pirani gauge is useful for pressures ranging from

10-1

mm to 10-3

mm

The CRO uses electrostatic type of focusing as compared to a TV picture tube which iselectromagnetic focusing.

IES 2001 EC

Flash type A/D converter is fastest, dual slope converter is slow but most accurate. Maxwell bridge can be used to measure Q< 10, and hays bridge Q>10, and

disadvantage of Anderson bridge is complex than its proto type Maxwell bridge. But

using Anderson bridge we can measure both self and mutual inductance.


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Anemometeris used to measure gas flow or gas velocity and stroboscopeis used tomeasure rotational speed.

Piezoelectric crystal sensitivity is field developed per unit stress. Potentiometric accelerometer:-major drawback is low resolution and limited to low

frequency measurements.

LVDT accelerometer:- much smaller mass and thus a higher natural frequency .therefore can be used for high frequency applications. Lvdt offers low resistance to the

motion than the potentiometric and is capable of much smaller resolutions, no problem

of moving contacts.

Piezo-electric accelerometer:- useful for high input frequencies , response is poor atfrequencies less than 10Hz.

IES 2002:-

Ventri tube is used to measure flow.

Rochelle salt is a piezo-electric material. Period measurements are preferred over frequency measurements at lower

frequencies due to gating error of 1 count ambiguity.

Ramp type ADC-------------->> integrating type Dual slope ADC---------------------->> voltage dependent conversion time. Successive approximation ------------>> null balance type. Microwave oven is based on the principle of dielectric heating.

IES 2003

Range of thermistor -600 C to 150 C Hot-wire anemometer is used to measure gas velocities. Photoelectric threshold of a metal is E=

where is work function of

metal.

In a metal conductivity increases with temperature at low temperatures and constantat some temperatures and at high temperatures conductivity decreases with

increasing temperature.

THERMISTORS:- Thermistor means thermal resistor, most of the thermistors have negative temperature

coefficients.

Theve temperature coefficient of resistance can be as large as several % per degreecelsius. This allows the thermistor circuits to detect very small change in temperature

which could not be observed with an RTD or Thermo couple.

Operating range -600 cC to 150 C and the resistance of thermistor ranges from 0.5 ohmto 0.75 Mega Ohms.

Thermistor is highly sensitive device but highly non-linear.


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Thermo couples:- it works on the principle of seebeck effect, seebeck discovered that when

heat applies to junction of two dissimilar metals form an electric circuit and current flows as a

result of generated EMF.

BASEMETAL couples RARE METAL COUPLES.

Positive

wire

copper iron Chromel

90% Cr

10% Ni

chromel Platinum

90%

rhodium

10%

Tungsten

95%

Rhenium

5%

Rhodium

Iridium

Negative

wire

Constantan

40% Ni,

60% CU

constantan Alumel

94% Ni,

2% Al, +Si

and Mn

constantan platinum Tungsten

72%

Rhenium

26%

Iridium.

Temp.

Range0C

-2500

to +

4000

C

-200 to +

850

-200 to

+1100

-200 to

+850

0 to

+1400

0 to

+ 2600

0 to

+ 2100

Note:- it should be home in mind that thermocouples are active transducers unlike RTD and

thermistors which are passive.

Advantages:- cheaper than the resistance thermometers, suitable for rapid changes in

temperature.

Disadvantage:- low accuracy and hence they cannot be used for precision work.

To prevent contamination of thermocouple it should be kept in protecting tube or closed end

metal.

When temperatures near ambient are to be measured with a thermocouple and it is

inconvenient to use a fixed reference junction and therefore compensating circuits must beemployed in the measuring system

RESISTANCE THERMOMETER :-

The resistance of a conductor changes when its temperature is changed. This property is

utilized for measurement of temperature.


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Platinum is best suited for RTD applications as it can withstand for high temperatures and

being noble metal it shows limited susceptibility to contamination

THE REQUIREMENTS OF CONDUCTOR MATERIAL TO BE USED IN RTD:-

Change in resistance of material per unit change in temperature is as large as possible The material should have high value of resistivity so that minimum volume of material is

used for the construction of RTD.

The resistance of material should have a continuous and stable relationship withtemperature .

Gold and silver are rarely used for construction of RTD on account of low resistivities Tugsten has relatively high resistivity but is reserved for high temperature applications

and it is extremely brittle and difficult to work.

Copper is occasionally used for RTD its low resistivity forces conductor to be longer thanplatinum but copper RTDs are economical alternatives to platinum one

Most pf the RTDs are made either platinum or nickel or nickel alloys.

The best use of RTD is in temperature range between 6000 C to 11000 C.IES 2004:-

Lloyd-fisher square is used to measure iron losses.ENERGY METERS:-

Motor meters:- motor meters are used for both ac and dc circuits, for dc circuits metermay be ampere-hour meter or watt hour meter

In motor meters the moving system is revolving continuously unlike the indicatingmeters .

In a motor meter the speed of the moving system is controlled by braking system . thebraking system consists of a permanent magnet so placed that it induces eddy currents

produce a braking torque which is proportional to speed of moving system.

Creeping:-in some meters a slow but continuous rotation is obtained even when thereis no current flowing through the current coil and only pressure coil is energized . this is

called creeping.

ENERGY METERS FOR AC CIRCUITS:- Induction type of energy meters are universally used for measurement of energy in

domestic and industrial ac circuits. Induction type of meters posses lower friction and

higher torque /weight ratio . also induction type meters are inexpensive and accurate

and retain their accuracy over a wide range of loads and temperature conditions.

LVDT:- Advantages:- high range, can be measured from 1.25mm to 250mm, and LVDT is

Friction less device.

The LVDT gives a high output and many a times there is no need for amplification . thetransducer produces a high sensitivity which is typically about 40 V/cm.


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LVDTS show low hysteresis and hence repeatability is good in all conditions. Most of the LVDTs consume less than 1Watt. Disadvantages:- Relatively large displacements are needed to get appreciable differential output. There are sensitive to stray magnetic fields but shielding is possible. This is done by

providing magnetic shields with longitudinal slots.

Many a times the transducer performance is affected by vibrations. Piezo-resistive effect:change in the value of resistivity of a conductor when it is

strained.

IES 2005:-

Rotameter is used to measure flow rate.

IES 2006:-

Among the following meters which does not exhibit square law response?

Moving coil, moving iron, electro dynamo meter, hot wire instrument.

Ans:- PMMC

The operation of a pirani gauge depends on variation of the thermal conductivity of a gas with

pressure.

Resistive hygrometer:- some hygroscopic salts exhibit a change in resistivity with humidity .the most common is lithium chloride.

Capacitive hygrometer:- these exhibit a change in dielectric constant with humidity changes.

Microwave refractometer:-for secondary standards , a system is consisting of two cavities ,

each of which is coupled to a klystron may be employed , one cavity contains dry air , the other

contains the mixture to be measured , the change in dielectric constant changes the frequency

of one oscillator.

Crystal hygrometer: it uses quartz crystal

Which one of the following is most sensitive device?

Thermo couple, RTD, thermistor , pyrometerANS:- THERMISTER

Ies 2007:-

Potentiometric transducer is a zero order displacement transducer.


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ACE MOCK TESTS:-

Problem:- a fullwave bridge rectifier ac voltmeter is used to measure a dc voltage of10V , the amount of error in reading will be?

Ans:- vrms indicated = 1.11 * 10 V= 11.11 volts

Error is +11 %.

Problem :- a half wave type rectifier voltmeter is measuring 10V DC what is the

measured value?

Ans:- Vrms= 2.22 Vdc. 22.22V.

TRANSDUCERS:-

When actuated transforms energy from one form to other form. Passive transducers:- passive transducers derive power required for transduction from

an auxiliary power source. They also derive part of the power required for conversion

from the physical quantity under measurement. Typical examples of passive transducers

are resistive, inductive, capacitive.

Active transducers:- active transducers those which do not require an auxiliary powersource to produce their output . velocity, temperature , light intensity and force can be

transduced with the help of active transducers include tacho-generators, thermo

couples, photo voltaic cells and piezoelectric crystals.

Capacitive transducers:- these type transducers are commonly used to measuredisplacements. These transducers use following effects

1. Change in capacitance due to change in overlapping area of plats and2. Change in capacitance due to change in distance between the two plates.

Advantages:- extremely sensitive, good frequency response till 50kHzand hence

they are very useful for dynamic studies. High i/p impedance hence no loading,

Disadvantages:- non linear, output impedance is high and can cause loading effect

on next stage devices.


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Instrumentation Important Points to Remember

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