chapter 1general measurement system

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SEGI UNIVERSITY COLLEGE MEASUREMENT AND INSTRUMENTATION

CHAPTER 1

GENERAL MEASUREMENT SYSTEM

Measurement has been of great relevance to humankind since the earliest days of human civilization, when it

was first used as a means of quantifying the exchange of goods in barter trade systems. Today, measurement

systems, and the instruments and transducers used in them, are of immense importance in a wide variety of

domestic and industrial activities. The growth in the number and sophistication of instruments used in the

industry has been particularly significant over the last two decades as automation schemes have been

developed. A similar rapid expansion in their use has also been evident in military and medical applications

over the same period.

The last decade has seen a large and rapid growth in new industrial technology, encouraged by developments

in electronics in general and computers in particular. This decade of rapid growth, often referred to as the

electronics revolution, represents a step improvement in production techniques of a magnitude similar to that

brought about by the !ndustrial "evolution in the last century. The massive growth in the application of

computers to process control and monitoring tasks has spawned a parallel growth in the requirement for

instruments to measure, record and control process variables. As modern production techniques dictate

working to tighter and tighter limits of accuracy, and as economic forces limiting production costs become

more severe, so the requirement for instruments to be both accurate and cheap becomes ever harder to

satisfy. This latter problem is at the focal point of the research and development efforts of all instrument

manufacturers. !n the past few years, the most cost#effective means of improving instrument accuracy has

been found in many cases to be the inclusion of digital computing power within the instruments themselves.

These intelligent instruments therefore feature prominently in current instrument manufacturers catalogues.

$uch intelligent instruments represent the latest stage in the present era of measurement technology. This era

goes back to the start of the !ndustrial "evolution in the nineteenth century when measuring instruments first

began to be developed to satisfy the needs of industrialized production techniques. %owever, the complete

history of measurement techniques goes back much further, in fact by thousands of years to the very start of

human civilization.

Measurement is essentially the act, or the result, of a quantitative comparison between a given quantity and aquantity of the same kind chosen as a standard or a unit. The result of measurement is expressed by anumber representing the ratio of the unknown quantity to the adopted unit of measurement.

Measurement provides us with a means of describing a natural phenomena in quantitative terms. As afundamental principle of science, &ord 'elvin stated(

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CHAPTER 1 , GENERAL MEASUREMENT SYSTEM )

1-1 INTRODUCTION

1-. MEASUREMENT


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The device used for comparing the unknown quantity with the unit of measurement or a standard quantity is

called a measuring instrument.

The art of measurement is a wide discipline in both engineering and science, encompassing the areas ofdetection, acquisition, control and analysis of data.

*resent#day applications of measuring instruments can be classified into three ma+or areas.

The first of these is their use in regulating trade, and includes instruments which measure physicalquantities such as length, volume and mass in terms of standard units.

The second area for the application of measuring instruments is in monitoring functions.

These provide information which enables human beings to take some prescribed action accordingly.-hilst there are thus many uses of instrumentation in our normal domestic lives, the ma+ority of

monitoring functions exist to provide the information necessary to allow a human being to control someindustrial operation or process.

The third area for the application of measurement systems is the use as part of automatic control systemsforms.The characteristics of measuring instruments in such feedback control systems are of fundamentalimportance to the quality of control achieved. The accuracy and resolution with which an output variableof a process is controlled can never be better than the accuracy and resolution of the measuringinstruments used.

There are two methods of measurements

i/ Direct comparison methodsandii/ Indirect comparison methods

!n direct comparison methods0 the unknown quantity is determined by direct comparison with a standard ofthe given quantity. The result is expressed in terms of a chosen unit for the standard. and a numericalmultiplier.

1or example(Aa length can be measured in terms of metre and a numerical constant. Thus a 2m length means a length of

2 times of metre. A human being can make direct length comparison with precision of about 3.42 mm.

5irect comparison methods of measurement are though simple, but it is not always possible, feasible andpracticable to use them. The involvement of a person in these methods makes them inaccurate and lesssensitive. %ence direct comparison methods are not preferred and are rarely used.

!n engineering applications use of measurement systems indirect methods of measurement, is used.

CHAPTER 1 , GENERAL MEASUREMENT SYSTEM 4

1-3 MEASUREMENT PRINCIPLE

1-4 METHODS O5 MEASUREMENTS


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). Mechan&ca/ In#!umen# , $uch instruments are very reliable for static and stable conditions but theysuffer from the ma+or drawback of inability of responding rapidly to the measurements of dynamic andtransient conditions.

This is because such instruments have moving parts that are rigid, heavy and bulky and consequentlyhave a large mass. Mass results in inertia problems and, therefore, such instruments cannot faithfully

follow the rapid changes which are involved in dynamic measurements. Thus, it would be almostimpossible to measure a 23 %z voltage by a mechanical instrument. Another drawback of mechanicalinstruments is that most of them are a potential source of noise and hence cause noise pollution.

4. E/ec#!&ca/ In#!umen#,$uch instruments are more rapid in indicating the output of detectors ascompared to mechanical instruments but unfortunately electrical instruments are also dependent uponmechanical meter movement as indicating device. $ince the mechanical movements have some inertia,they have limited time and hence frequency response.

3- E/ec#!on&c In#!umen#

The mechanical and electrical instruments and systems cannot cope up with the very fast responserequirements of the scientific and industrial measurements carried out nowadays. The necessity to step#up

response time and also the detection of dynamic changes in certain parameters, which needs themonitoring time of the order of mill#seconds and quite often micro#seconds, have led to the developmentof electronic instruments and their associated circuitry. $uch instruments make use of semiconductordevices. The response time of such instruments is extremely small as the movement involved in electronicdevices is only that of electrons and electrons have very small inertia.

A)an#a'e

6lectronic instruments are light, compact and have a high degree of reliability. Their power consumption is very small. The most important use of electronic instruments is in measurement of non#electrical

quantities, where the non#electrical quantity is converted into electrical form with the use of transducers.

6lectronic instruments are widely employed in detection of electro#magnetically producedsignals such as radio, video and microwave.

6lectronic instruments also have the advantage of obtaining indication at a remote locationthat helps in monitoring inaccessible or hazardous locations.

The instruments or measurement systems according to their functions are divided into three categories

In)&ca#&n' &n#!umen#

An instrument that supplies the information in the form of deflection of a pointer is known as anindicating instrument.

For example, the deflection of pointer of an ammeter indicates the current flowing through the branch ofan electric circuit in which it is connected. Pressure gauges, speedometers, thermometers, ammeters,voltmeters, wattmeters etc. fall under this category.

Reco!)&n' &n#!umen#"ecording instruments are those which keep a continuous record of the variations of the magnitude of anunknown quantity to be observed over a definite period of time.!n such instruments the moving system carries inked pen which touches lightly a sheet of paper wrappedover a drum moving with uniform slow motion in a direction perpendicular to that of the deflection of the


1-6 HISTORY O5 DEVELOPMENT O5 INSTRUMENTS

1-7 5UNCTIONS O5 INSTRUMENTS AND MEASUREMENT SYSTEMS


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pointer. Thus, a curve is traced which shows the variation in the magnitude of the quantity underobservation over a definite period of time.

For example, a potentiometric type recorder, employed for monitoring temperature, records the

instantaneous values of temperature on a strip chart recorder, nother example is a speed log, usuallyprovided with commercial vehicles, that draws a graph of speed against time or distance. Temperatureand pressure recorders etc. fall under this category.

Con#!o//&n' &n#!umen#

8ontrolling function is one of the most important functions, especially in the field of industrial processcontrol. !n controlling instruments, the information is used to control the original measured quantity.

For example, a carburettor or fuel in!ection system, measures the fuel requirements of the engine fordifferent loads, speeds, and accelerator" pedal positions, and supplies the necessary air and fuel to theengine. Thermostats, float type level control, machine tool carriage"position control etc. fail under thiscategory.

The purpose of measurement system is to present an observer with a numerical value corresponding to thevariable being measured.

The measurement system consists of several elements or blocks as shown in 1igure ).9a/. !t is possible toidentify four types of elements.

Figure 1.7(a)

P!&ma!y Sen&n' E/emen#

This is in contact with the process and gives an output which depends in some way on the variable to be

measured.

The primary sensing element is that which makes contact with the physical quantity under measurement,called the measurand,receives energy from the measured medium and produces an output depending insome way on the measurand.

6xamples are thermocouple output e.m.f depends upon input temperature/, strain gauge resistance dependson mechanical strain/, orifice plate pressure drop depends on flow rate/. !t is important to note that aninstrument always extracts some energy from the measured medium. Thus the measured quantity is alwaysdisturbed by the act of measurement, which makes a perfect measurement theoretically impossible. :oodinstruments are designed to minimise this effect, but it is always present to some degree.

*rimary sensing elements may have a non#electrical input and output such as an orifice plate, a spring, amanometer or may have an electrical input and output such as a rectifier. !n case the primary sensingelement has a non#electrical input and output, then it is converted into an electrical signal by means of atransducer. The transducer is device which, when actuated by one form of energy, is capable of converting itinto another form of energy.

CHAPTER 1 , GENERAL MEASUREMENT SYSTEM ;

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1-8 ELEMENTS O5 GENERALISED MEASUREMENT SYSTEM


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S&'na/ Con)&on&n' E/emen#

This ta#es the output of the sensing element and converts it into a form more suitable for further processing,usually a dc voltage, dc current or frequency signal.

Many a times certain operations are to be performed on the signal before its further transmission so thatinterfering sources are removed and the signal may not get distorted. The process may be linear such asamplification, attenuation, integration, differentiation, addition and subtraction or non#linear such asmodulation, detection, sampling, filtering, chopping and clipping etc. The process is called the signalconditioning. $o signal conditioner follows the primary sensing element or transducer, as the case may be.The examples of signal conditioning element are deflection bridge which converts an impedance change into avoltage change0 amplifierwhich amplifies millivolts to volts0 oscillator which converts an impedance changeinto a variable frequency voltage.

S&'na/ P!oce&n' E/emen#

This ta#es the output of the conditioning element and converts it into a form more suitable for presentation.


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Figure 1.7(b): Rudimentary pressure gauge

Figure 1.7(c)

Measurement of non#electrical quantities by electrical methods was given greater attention since themiddle of twentieth century and this marked the beginning alone of the most fruitful and vigorous areas ofactivity concerning the scope of their applications. $imultaneously, the advances made in the field ofelectronics substantially contributed to the birth of a new discipline known as instrumentation.

!nstrumentation deals with the science and technology of measurement of a large number of variablesembracing the disciplines of physical sciences such as physics and chemistry and engineering disciplines likemechanical, electrical, electronics, communication and computer engineering. !nstrumentation refers to theart and science of collection of several instruments and auxiliary equipment and their utilization for conductingsuccessfully a test or an experiment on a system, process or plant.

An !nstrumentation system is a physical system, which is a collection of physical ob+ects connected in sucha way as to give the desired output response. 6xamples of a physical system may be cited from a laboratorysuch as an electronic amplifier composed of many components from an industrial plant such as a steamturbine or from utility services such as a communications satellite orbiting the earth. Thus an instrumentationsystem may be defined as an assembly of various instruments and other components interconnected tomeasure, analyse and control the electrical, thermal, hydraulic and other non#electrical physical quantities.


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the stress is on distortion analysis. >nce a physical model of a physical system is obtained, the next step is tohave a appropriate physical laws. 5epending upon the choice of variables and the coordinate system, a givenphysical model may lead to different mathematical models. 1or example, a network may mathematical modeli.e. mathematical representation of the physical model by making use of be modelled as a set of nodal

equations employing 'irchhoff ?s fhst law or current law/ or a set of mesh equations employing 'irchhoff@ssecond law or voltage law/. Then the mathematical model is solved for various types of inputs to have thedynamic response of the system.

!nstrumentation systems can be classified into two main categories namely, analog systems and digitalsystems.

Analog systems deal with measurement information in analog form. An analog signal may be defined as

a continuation function, such as a plot of voltage against time or displacement against force.

5igital systems handle measurement information in digital form. A digital quantity may consist of a

number of discrete and discontinuous pulses whose time relationship contains information regardingmagnitude or the nature of quantity.

The instrumentation system plays an important role in automation. An automatic control system orautomation/ requires a comparator or an error detector/, which measures the difference between the actualand desired performance and actuates the control elements. :eneral block diagram of an automatic controlsystem is shown in 1igure ).a/ . An error detector or a comparator/ compares a signal obtained throughfeedback elements, which is a function of the output response, with the reference input. Any differencebetween these two signals constitutes an error or actuating signal, which actuates the control elements. Thecontrol elements in turn alter the conditions in the plant controlled member/ in such a manner that theoriginal difference or error is reduced.

Figure 1.9(a)

There are numerous examples of this type of application. A common one is the typical simple tank level control systemshown in 1igure ).b/. -ith this control system liquid level controlled output/ in the tank can be maintained withinaccurate tolerance of the desired level of liquid even though the out put flow rate through the valve $%is varied. The float

feedback path element/ senses the liquid level and positions the slider arm B on a potentiometer. The slider arm A ofanother potentiometer is positioned corresponding to the desired liquid level hthe reference input/. -hen the liquid levelrises or falls, the potentiometers error detector/ give an error voltage error or actuating signal/ proportional to the changein liquid level. The error voltage actuates the motor through a power amplifier control elements/ which in turn conditionsthe


1-: INSTRUMENTATION AUTOMATION


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Figure 1.9(b)

CHAPTER 1 , GENERAL MEASUREMENT SYSTEM C

chapter 1general measurement system

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