Chapter 1 - DCE5038

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Chapter 1Introduction to Instrumentation

NOOR ZIELA ABD RAHMANB.Eng (Hons) Electronics

Chapters1) Introduction to Instrumentation2) Indicating Instruments 3) Measurement of Power and Energy 4) Measurement of R, L and C 5) Magnetic Measurement 6) Measurement Of Non-Electrical Quantities

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DCE5018 Electrical Measurement and Instrumentation Techniques

Assessment Scheme

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Coursework Test 1 10% (Week 6)

Test 2 10% (Week 11)

Quiz / Assignment

10% (Week 9)

Laboratories 30%

Final Exam Written Test 50%


Course Outline1.1 Functional Elements of a measurement system

1.2 Performance Terms

1.3 Types of Errors

1.4 Classification of Instrument

1.5 Selection of Instruments

1.6 Impedance Loading and Matching

1.7 Error Analysis4


1.1 Functional Elements of a Measurement SystemObjectives

1. To state the purpose of measurement system.

2. To list the functional elements of a measurement system and explain their function.

3. To give example of the functional elements of a measurement system.

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Measurement System

A measurement system converts the unknown quantity under measurement to a numerical unit using instrument.Number + measured unit6.8 kg/(ms)2

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•Measurement - Comparison between an unknown quantity and a predefined standard

Terms

•Measurand- the unknown quantity to be measured

•Instrument - physical device uses to determine measurand numerically.

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Element of measurement system

All measurement systems consist of three mainelements:

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Sensing Element

Frequently called transducer.It is in some way ‘in contact’ with what is being

measured and produces some signal which is related to the quantity being measured.

Signal Converter

The output from the sensing element then passes through a second element before reaching the display.

This second element can take a number of forms.In general it can be considered the signal converter

in that the signal from the sensing element is converted into a form which is suitable for the display or control element.

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Display

The third part of the measuring system could be a display or control system.

The display element is where the output from the measuring system is displayed.

The display element takes the information from the signal converter and presents it in a form which enables an observer to recognize it.

The control system is where the output from the measuring system is used to control a process.

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1.2 Performance Terms

Objective

1. To differentiate the terms used to describe the performance of a measurement system.

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Error : Difference between the result of the measurement and the true value of the quantity being measured.

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Accuracy of the instruments can be specified in either ofthe following terms.

i) Percentage of true value =

measured value – true value x 100 true value

ii) Percentage of full-scale deflection =

measured value – true value x 100 maximum scale value

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•Before the manufacturer could come out any new products, that manufacturer must always guarantees a certain accuracy of their product numerically to ensure the quality of the instrument.

•The manufacturer specify the deviations from the nominal value of a particular quality.

•The limits of these deviations from the specified value are defined as limiting errors or guarantees errors.


1.3 Types of Error

Objectives:1. To list two types of error.2. To distinguish the characteristics of the

two types of error.3. To identify and explain the causes of the

types of error.4. To give examples of the causes of the

types of error. 5. To contrast the methods to eliminate or

reduce the two types of error

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Type of Errors

Random Errors Systematic Errors

a) Instrument Error b) Environmental Error

Avoidable errors that creep into every measurement

i) Due to inherent shortcomings of instruments.

ii) Due to misuse of instruments.

iii) Due to loading effect of instruments.

Effects of temperature,pressure, humidity, dust,vibrations, or externalmagnetic or electrostaticfields.

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i) The random motion of air molecules.

ii) An unstable voltage source from main supplied by TNB.

iii) Vibration caused by a passing motor vehicle.

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1.4 Classification of Instruments

1.4.1 Active/Passive Instrument

1.4.2Deflection/Null Instrument

1.4.3Monitoring/Control Instrument

1.4.4Analog/Digital Instrument

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1.4.1Active/Passive Instrument

Active type:Instrument that requires some source of auxiliary powerie. compressed air, electricity, hydraulic supply etc.

Eg: Petrol Tank Level Indicator Simple Bourdon gauge (pressure) Clinic Thermometer (temperature)

Passive type:Energy requirements of the instruments are met entirelyfrom the input signal.

Eg: Differential Transformer (displacement, force, pressure, etc)

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Chapter 1 - Intoduction to Instrumentation 26


1.4.2Deflection/Null Instrument

Deflection type:Value of the quantity being measured is displayed in termsof the amount of movement of a pointer.

Eg: Spring Balance (weight)

Null type:Instrument that generates an equivalent opposing effectsto nullify the physical effect caused by the quantity to bemeasured.

Eg: Equal-arm Beam Balance (weight)

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10

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30

01

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Standard mass

Measured mass

Deflection Type Null Type

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1.4.3 Monitoring/Control Instrument

Monitoring type:Instrument which only give an audio/visual indication ofthe magnitude of the physical quantity being measured.

Control type:Output of the instruments being input to the controller.

1.4.4 Analog/Digital Instrument

Analog type:Present the variables of interest in the form of continuouswith respect to time.

Digital type:Physical variables are represented by digital quantities.

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1.5 Selection of Instruments

Various consideration involved in the selection of the

instrument including:

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Instrument’s Qualities:

(a) Accuracy and precision characteristics including other specifications like sensitivity etc.

(b) Nature and type of data available, ie. whether analog, digital, continuous or sampled.

(c) Nature and type of read out, ie. whether indicating or recording type.

(d) Susceptibility to environment disturbances.

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Convenience Aspects

(a) Suitability for the given application, ie. whether for laboratory use, field use or both.

(b) Adaptability to different sizes of inputs, ie. scale expansion, range changes, etc.

(c) Ease in calibration, when needed.

(d) Simplicity and ease of instrument behavior diagnosis.

(e) Material durability.

(f) Ready self-indication or check determination in case of instrument malfunction.

(g) Safety in use.

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Costs, Initial and Cumulative Total(a) Initial cost of instrument procurement, installation including the various attachments and accessories.

(b) Maintenance, repair, recalibration, etc.

(c) Running cost.

(d) Expected life span.

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1.6 Impedance Loading & Matching

Any measuring instrument with an input signal

source would extract some energy, thereby changing

the value of the measured variable. This implies that

the input signal suffers a change by virtue of the fact

that it is being measured. This effect is termed loading.

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1.6.1 Loading Effect Due To Shunt Connected Instruments

In Figure 1, E0 is the open circuit voltage ie. the voltage that appears across the terminals A and B when the load which is a voltmeter in this case is not connected to the terminals.

Ideally, when the load is connected across terminals A and B the output voltage should remain the same.

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In Figure 2, when a voltmeter with an input impedance ZL is connected across A and B, a current IL flows and the actual voltage measured, EL, is given as

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Example

Example 1:A multimeter having a sensitivity of 2,000 /V is used for the measurement of voltage across a circuit having an output resistance of 10 k. The open circuit voltage of the circuit is 6 V. Find the reading of the multimeter when it is set to its 10 V scale. Find the percentage of error of true value of the measurement.

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Example 2: A 50 V range voltmeter is connected across the terminals A and B of the circuit shown in figure below. Find the reading of the voltmeter under open circuit and loaded conditions. Find the percentage of error of true value of the measurement due to the loading effect. The voltmeter has a resistance of 1,000 k.

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1.6.2 Loading Effect Due To Series Connected InstrumentsIn Figure 3, the value ofcurrent flowing between terminals A and B under ideal conditions is I0. It is

the current that flows when terminals A and B are shorted and given as

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In Figure 4, when an ammeter is placedbetween output

terminals, it adds to the

impedance ofcircuit. Suppose ZL =

Input impedance of ammeter, therefore, themeasured value of

current,IL, is given as

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Example 1:It is desired to measure the value of current in the 500 resistor as shown in figure below by connecting a 100 ammeter. Find (a) the actual value of current,(b) the measured value of current, and(c) the percentage of error of true value of the measurement due to the loading effect.

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1.7 Error Analysis1. The magnitude of a quantity having a nominal value As and a

limiting error of ± A must have a magnitude Aa between the limits As - A and As + A or actual value of quantity, Aa = As ± A.

2. The relative limiting error is defined as the ratio of the error to the nominal magnitude of a quantity.

3. To find the limiting error of a function, find the partial derivative of the function with respect to all the variables.

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Example 1: Two resistors have the following ratings:R1 = 37 ± 5% and R2 = 75 ± 5%If the resistors are connected in series, find the limiting error and the percentage of relative limiting error.

Example 2:Repeat Example 1 if the resistors are connected in parallel.

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Chapter 1 - DCE5038