INSTRUMENTATION FOR PROCESS MEASUREMENT AND CONTROL PDF

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mentals needed in process control and instrumentation. The discussion of the basic principles underlying pressure measurement has been expanded to include. Process. Measurement and Analysis. VOLUME I. Bela G. Liptak. EDITOR-IN- CHIEF Dedicated to you, my colleagues, the instrument and process control. instrumentation and control engineers at P & I Design Ltd. This experience is based on design and Accurate flow measurement is a key element in process productivity. tailamephyli.tk 3.

Top Interview Questions. Sort: Relevance Popular Date. How long will you work with the firm? Dunn McGraw-Hill Electronics and Communication Engineering - Measurements Explain what is instrumentation?

A positioner is a device put into a valve to ensure that it is at a correct position of opening as per the control signal. Instrumentation engineering interview questions and Tips and tricks for What are interview question for instrumentation engineering? Here are top interview questions for your job interviews: Question.

This is the most frequently asked question in all area. List any four objectives of process control. Suppressing the influence of external disturbances, Optimizing the performance, Increasing the productivity, Cost effective.

Process Control and Instrumentation - Chemical Engineering This is the chemical engineering questions and answers section on "Process Control and Instrumentation" with explanation for various interview, competitive examination and entrance test.

Solved examples with detailed answer description, explanation are given and … Instrumentation Engineer Interview Questions and Answers We setthe processtime constantso that it will alwaysequal1 d.

What happensspecificallydependson the type of controller,pneumatic or electronic Match the following: Controller, two-position-a. Derivative -b. Proportionalband adjustment-f.

Regulatedby control valve -g. Valve Desiredvalue Manipulatedvariable Error A proportionalcontroller will bave an otIsetditIerencebetweenset point and control point: a. At all times b. Equal to the proportionalbandsetting c. That dependsuponprocessload d. That will eventuallyvanish If it were possiblefor a proportionalcontrollerto bavea true Opercent proportionalband, the controller gainwould bave to be: a. Unity c. Infinite If the proportionalbandof the controlleris adjustedto minimum possiblevalue,the control actionis likely to be: a.

Excellent b. With maximumotIset d. Inoperative I-Il. The following symbol representsa: 8. Flow rate controller b. Fixed control point appearsin an instrumentdiagram.

Frequencyconverter d. Final control element With a proportional-onlycontroller if measurement equalsset point, the output will be: 8. Impossibleto define Oor percent,dependingon actionselected b. Unknown c. In the modemcontroller,derivative actionis appliedonly to the: 8. Error c. Setpoint b. Measurement d. Integral circuit The functionof the integral reset modeis to: 8.

Opposechangein measurement b. Automaticallyadjustthe controller's gain c. Eliminate offset d. It is alsoa conditionoflife on this planet: we live at the bottom of an atmosphericocean that extendsupward for manymiles.

This massof air has weight, and this weight pressingdownwardcausesatmosphericpressure. Water, a fundamentalnecessityof life, is suppliedto most of us underpressure. In the typical processplant, pressureinfluencesboiling point temperatures, condensingpoint temperatures,processefficiency, costs, and other important factors.

The measurementand control of pressure,or lack of it-vacuum-in the typical processplantis critical. Instruments are availableto measurea wide rangeof pressures. How theseinstruments function is the subjectof this chapter.

What Is Pressure? Pressure is often defined in terms of "head. We want to find the pressure in the boUom ofthe column. The weight ofthe column mar be calculated by first finding the volume of water. Water weighs So the weight of 23 cubic feet will be 23 times The area ofthe base is 1 square foot, or 12 inches times 12 inches, or square inches.

The pressure equals 1, In practice, we find that only the height ofthe water confits. It mar be present in a small pipe or beneath the surface of a pond. In any case, at a depth of 23 feet, the pressure will amo unt to approximately 10 pounds per square inch. If in your home the water pressure is 50 pounds per square inch and the system uses a gravity feed, the water tank, or reservoir, holds the water at a height of 50 divided by 10, or 5 times 23 equals feet above the point where the pressure measurement is made.

Head and pressure, then, mar mean the same thing. We must be able to convert from one to the other. You mar encounter reference to inches of mercury for pressure measurement.

Building your analytical and practical skills in measurement and control engineering.

Mercury is Therefore, ahead ofmercuryexerts apressure Because it is hazardous, mercury no longer is used commonly in manometers. The head or pressure terms cited thus far are called, collectively, "gauge pressure.

Gauge pressure makes no allowance for the fact that on earth we exist under a head of air, or an atmosphere. The height of this head of air varies with elevation, and also to some degree with weather conditions.

If rou ride an elevator from the bottom to the top floor of a tall building, rou will likely feel your ears "pop. A simple method of measuring atmospheric pressure would be to take a length of small diameter 0. Fill the tube entirely with mercury and temporarily seal the end. Invert this end into a deep dish of mercury and remo ve the seal. The result will be a column of mercury as shown in Figure with some space remaining at the top.

Atmospheric pressure on the surface of the exposed mercury will balance the height of mercury in the tube and prevent it from running out of the tube. The height of the mercury above the level in the dish is, then, a measure of atmospheric pressure. At sea level, this would amount to approximately When the etfect of the atmosphere is included in our measurement, we then must use absolute pressure gauge pressure plus atmospheric pressure.

Units ot Measurement Every major country has adopted its own favorite units of measurement. The United States has traditionally employed the English system. However, international trade has made it necessaryto standardize units of measurement throughout the world. Fortunately, during this standardization, there has be en rationalization ofthe measurementsystem. The force of common usage is so strong that the familiar English system will undoubtedly persist for many years, but the changeover is definitely underway.

The time will soon corne when process industries will deal exclusively with SI units. Proper adjustment of the proportional band can be observed by the response ofthe measurement to an upset.

Figure shows several examples ofvarying the proportional band for the beat exchanger. The proportional band that will cause onequarter wave damping will be smaller, thereby yielding tighter control over the measured variable, as the dead time in the process decreases and the capacity increases.

One consequence of the application of proportional control to the basic controlloop is offset. Offset means that the controller will maintain the measurement at avalue different from the set paint. This is most easily seen in Figure Examples of varying proportional band for beat exchanger.

But note that, because of the proportional action of the linkage, the increased open position can be achieved only at a lowered level. Stated another way, in order to restore balance between the ftow in and the ftow out, the level must stabilize at avalue below the set point. This difference, which will be maintained by the control loop, is called offset, and is characteristic of the application of proportional-only control to feedback loops.

The acceptability ofproportional-only control depends on whether this offset can be tolerated. Since the error necessary to produce any output decreases with the proportional band, the narrower the proportional band, the less the offset. For large capacity, small dead time applications accepting a very narrow proportional band, proportional-only control will probably be satisfactory, since the measurement will remain within a small percentage band around the set point. If it is essential that there be no steady state difference between measurement and set point under all load conditions, an additional function must be added to the controller.

This function is called integral action an older term is reset. Integral Action Reset The open-loop response of the integral mode is shown in Figure , which indicates a step change in the artificial measurement away from the set point at some instant in time.

As long as the measurement remains at the set point, there is no change in the output due to the integral mode in the controller. Open-loop response oCintegral mode. This function, then, causes the output to change until the proper output is achieved in order to hold the measurement at the set paint at various loads.

This response is added to the proportional response of the controller as shown in Figure The step change in the measurement first causes a proportional response, and then an integral response, which is added to the proportional. The more integral action there is in the controller, the more quickly the output changes due to the integral response. The integral adjustment determines how rapidly the output changesas a function oftime.

Among the various controllers manufactured, the amount of integral action is measured in one of two ways-either in minutes per repeat, or the number of repeats per minute.

For controllers measuring integral action in minutes per repeat, the integral time is the amount of time necessary for the integral mode to repeat the open-loop response caused by proportional mode, for a step change in error.

Thus, for these controllers, the smaller the integral number, the greater the action of the integral mode. On controllers that measure integral action in repeats per minute, the adjustment indicates how many repeats of the proportional action are generated by the integral mode in one minute.

Table p. Thus, for these controllers, the higher the integral number, the greater the integral action.

Integral time is shown in Figure l-IS. The proper amount of integral action depends on how fast the measurement can respond to the additional valve trave 1 it causes. Open-loopresponseoCproportional plus integralmodes. The result will be an integral cycle in which the valve travels from one extreme to another as the measurementoscillates around the set point.

When integral action is applied in controllers on batch processes, where the measurement is away from the set point for long periods between batches, the integral may drive the output to its maximum, resulting in "integral wind-up.

This problem can be prevented by including a "batch function" in the controller, a function specifically designed to prevent "wind-up. Whereasthe proportional mode respondsto the size of the error and the integral mode respondsto the size and time duration of the error, the derivative moderespondsto how quickly the error is changing.

The first is a response to a stepchangeof the measurementaway from the set point. For a step, the measurementis changinginfinitely fast, and the derivative mode in the controller causesa considerablechangeor spike in the output, which dies immediatelybecausethe measurement hasstopped changingarterthe step.

The secondresponseshowsthe responseofthe derivative mode to a measurementthat is changingat a constantrate. The derivative output is proportionalto the rate of changeofthis error. The greaterthe rate of change,the greaterthe output dueto the deriva Two derivative responses.

The derivative holds this output as long as the measurement is changing. Thus, the greater the derivative number, the greater the derivative response. To avoid a large output spike caused by step changes in the set point, most modem controllers apply derivative action only to ehanges in the measurement. Derivative action in controllers helps to control processes with especially large time constants.

Open-loop response oCproportional plus derivative modes. This will cause large and rapid variations in the controller output, which will keep the valve constantly moving up and down, wearing the valve and causing the measurement to cycle.

As previously described, the response of an element is commonly expressed in terms of a time constant, defined as the time thai will elapse until an exponential curve reaches Although the transmitter does nat bave an exact exponential response, the time constant for the pneumatic temperature transmitter and its associated thermal system is approximately 2 seconds.

The reaction curve shown in Figure incorporates the response of the beat exchanger and measuring system, and it represents the signal thai will actually reach the controller.

This curve indicates thai, given a sudden change inside the beat exchanger, more than 30 seconds will elapse before the controller receives a signal thai is a true representation of thai change. From the reaction curve, or characteristic, we can determine the type of controller required for satisfactory control under ibis difficult, but common, delayed response characteristic.

Selecting the Controller The beat exchanger acts as a small-capacity process; thai is, a small change in steam can cause a large change in temperature. Accurate Fig. The processreactioncurve is obtainedby imposinga stepchangeat input. Variations in water rate cause load changes that produce offset, as described previously.

Thus, the integral mode should also be used. Whether or Dot to include the derivative mode requires additional investigation of the process characteristic.

The time interval between the start of the upset and the intersection of the tangentialline is marked TA;the time interval from this point to the point of inftection is TB. IfTB exceeds TA,some derivative action will prove advantageous.

If TBis less than TA,derivative action mar lead to instability because of the lags involved. The reaction curve of Figure clearly indicates that some derivative will improve control action.

Thus, a three-mode controller with proportional, integral, and derivative modes satisfies the needs of the beat exchanger processo Figure shows the combined proportional, integral, and deriva- Fig.

When the measurement begins to deviate from the set point, the tirst response from the controller is a derivative response proportional to the rate of change of measurement thai opposes the movement of the measurement away from the set point. This derivative response is combined with the proportional response. In addition, as the integral mode in the controller sees the error increase, it drives the valve further still. This action continues until the measurement stops changing, at which point the derivative response ceases.

Since there is still an error, the measurement continues to change due to integral action, until the measurement begins to move back toward the set point. As soon as the measurement begins to move back toward the set point, there is a derivative response proportional to the rate of change in the measurement opposing the return of the measurement toward the set point. The integral response continues, because there is still error, although its contribution decreases with the error.

AIso, the output due to proportional is changing. Thus, the measurement comes back toward the set point. As soon as the measurement reaches the set point and stops changing, derivative response again ceases and the proportional output returns to 50 percent. With the measurement back at the set point, there is no longer any changing response due to integral action. However, the output is at a new value. This new value is the reguli ofthe integral action during the time thai the measurement was away from the set point, and compengates for the load change thai caused the original upset.

Conclusion This chapter has describedthe responsesof a three-modecontroller when it is used in the feedbackcontrol of industrial measurements.

The readershouldbave a clear understandingof the following points: In order to achieve automatic control, the control loop musi be closed. In order to maintainastable feedbackcontrolloop, the most important adjustmentto the controller is the selectionof the proper action, either reverseor direct, on the controller. Properselection of tbis action will causethe controller output to changein sucha way thai the movementofthe valve will opposeany changein the measurementseenby the controller.

The more narrow the proportional band, the more the controller reacts to changes in the measurement. If too narrow a proportional band is used, the measurement cycles excessively. If too wide a proportional band is used, the measurement will wander and the offset will be too large. The function of the integral mode is to eliminate offset. If too much integral action is used, the res uIt will be an oscillation of the measurement as the controller drives the valve from one extreme to the other.

If too little integral action is used, the measurement will retum to the set point too slowly. The derivative mode opposes any change in the measurement.

Too little derivative action has no significant effect. Too much derivative action causes excessive response of the controller and cycling in the measurement. Questions l-I. All control systemsthat fit into the usual patternare: Open-Ioop c. Closed-loop b. Nonself-regulating d. If operatingproperly,automaticcontrol will always: Reducemanpower b. Reducecosts c.

Make the processoperatemore uniformly d. Decreasemaintenance Automatic controllersoperateon the differencebetweenset point and measurement,which is called: Offset c. Error b.

Bias d. Feedback Controls with a fixed offset b. Controlsarounda point c. Automaticallyadjustsits integraltime d. Requiresprecisetuning Gainand proportionalbandsare: Reciprocallyrelated b.

Two differentcontrol modes c. Adjusted independentlyof one another d. Whenwe adjustintegraltime in a controller: We determineanRC time constantin the controller's internalfeedback path b. We adjustthe time it will take for integralto equalderivative c. We setthe processtime constantso that it will alwaysequal1 d. What happensspecificallydependson the type of controller,pneumatic or electronic Match the following: Controller, two-position-a.

Derivative -b. Proportionalband adjustment-f. Regulatedby control valve -g. Valve Desiredvalue Manipulatedvariable Error A proportionalcontroller will bave an otIsetditIerencebetweenset point and control point: At all times b.

Equal to the proportionalbandsetting c. That dependsuponprocessload d. That will eventuallyvanish If it were possiblefor a proportionalcontrollerto bavea true Opercent proportionalband, the controller gainwould bave to be: Unity c. Infinite If the proportionalbandof the controlleris adjustedto minimum possiblevalue,the control actionis likely to be: Excellent b. With maximumotIset d. Inoperative I-Il. The following symbol representsa: Flow rate controller b.

Fixed control point appearsin an instrumentdiagram. Frequencyconverter d. Final control element With a proportional-onlycontroller if measurement equalsset point, the output will be: Impossibleto define If in a proportional-plus-integral controller measurement is away from the set point for a long period,the controller's output will be: Oor percent,dependingon actionselected b. Unknown c. In the modemcontroller,derivative actionis appliedonly to the: Error c.

Setpoint b. Measurement d. Integral circuit The functionof the integral reset modeis to: Opposechangein measurement b. Automaticallyadjustthe controller's gain c.

Eliminate offset d. It is alsoa conditionoflife on this planet: This massof air has weight, and this weight pressingdownwardcausesatmosphericpressure. Water, a fundamentalnecessityof life, is suppliedto most of us underpressure. In the typical processplant, pressureinfluencesboiling point temperatures, condensingpoint temperatures,processefficiency, costs, and other important factors. The measurementand control of pressure,or lack of it-vacuum-in the typical processplantis critical. Instruments are availableto measurea wide rangeof pressures.

How theseinstruments function is the subjectof this chapter. What Is Pressure? Pressure is often defined in terms of "head. We want to find the pressure in the boUom ofthe column. The weight ofthe column mar be calculated by first finding the volume of water.

Water weighs So the weight of 23 cubic feet will be 23 times The area ofthe base is 1 square foot, or 12 inches times 12 inches, or square inches. The pressure equals 1, In practice, we find that only the height ofthe water confits. It mar be present in a small pipe or beneath the surface of a pond. In any case, at a depth of 23 feet, the pressure will amo unt to approximately 10 pounds per square inch.

If in your home the water pressure is 50 pounds per square inch and the system uses a gravity feed, the water tank, or reservoir, holds the water at a height of 50 divided by 10, or 5 times 23 equals feet above the point where the pressure measurement is made. Head and pressure, then, mar mean the same thing. We must be able to convert from one to the other.

You mar encounter reference to inches of mercury for pressure measurement. Mercury is Therefore, ahead ofmercuryexerts apressure Because it is hazardous, mercury no longer is used commonly in manometers. The head or pressure terms cited thus far are called, collectively, "gauge pressure. Gauge pressure makes no allowance for the fact that on earth we exist under a head of air, or an atmosphere. The height of this head of air varies with elevation, and also to some degree with weather conditions.

If rou ride an elevator from the bottom to the top floor of a tall building, rou will likely feel your ears "pop. A simple method of measuring atmospheric pressure would be to take a length of small diameter 0. Fill the tube entirely with mercury and temporarily seal the end. Invert this end into a deep dish of mercury and remo ve the seal.

The result will be a column of mercury as shown in Figure with some space remaining at the top. Atmospheric pressure on the surface of the exposed mercury will balance the height of mercury in the tube and prevent it from running out of the tube. The height of the mercury above the level in the dish is, then, a measure of atmospheric pressure. At sea level, this would amount to approximately When the etfect of the atmosphere is included in our measurement, we then must use absolute pressure gauge pressure plus atmospheric pressure.

Units ot Measurement Every major country has adopted its own favorite units of measurement. The United States has traditionally employed the English system. However, international trade has made it necessaryto standardize units of measurement throughout the world. Fortunately, during this standardization, there has be en rationalization ofthe measurementsystem.

The force of common usage is so strong that the familiar English system will undoubtedly persist for many years, but the changeover is definitely underway. The time will soon corne when process industries will deal exclusively with SI units. Pressure Measurement Perhaps the area that has caused the most concern in the change to SI units is pressure measurement. The new unit of pressure, the pascal, is unfamiliar even to those who have worked in the older CGS centimetre, gram, second metric system.

Once it is accepted and understood, it willlead to a great simplification of pressure measurement Cromthe extremes of full vacuum to ultrahigh pressure. It will reduce the multiplicity ofunits now common in industry to one standard that is compatible with other measurements and calculations.

To understand the pascal and its relationship to other units of pressure measurement, we must return to a basic understanding of pressure. As noted previously, pressure is force per unit area. In the English system, the distinction between mags and force became blurred with common usage of terms such as weight and mags. We live in an environment in which every object is subject to gravity.

Every object is accelerated toward the center of the earth, unless it is restrained. The force acting on each object is proportional to its mags. However, in faci, force and mass, as quantities, are as different as apples and pears, as the astronauts bave observed. A numbec of schemes bave be en devised to overcome ibis problem. For example, a quantity called the pound-force was invented and made equal to the force on a mass of one pound under a specified acceleration due to gravity.

The very similarity between these two units led to more confusiott. The pascal, by its definition, removes all these problems. The Pascal The SI unit of pressure is defined as the pressure or stress that arises when a force of one newton N is applied uniformly over an area of one square metre m2.

This pressure has been designated one pascal Pa. This is a small unit, but the kilopascal KPa , 1, pascals, and the megapascal MPa , one million pascals, permit easy expression of common pressures. The definition is simple, because gravity has been eliminated. The pascal is exactly the same at every point, even on the moon, despite changes in gravitational acceleration.

In SI units, the unit offorce is derived from the basic unit for mass, the kilogram kg , and the unit of acceleration metres per second per second, mls2. At thai time, the SI unit was called the "newton per square metre. The use ofthe millibar in meteorology lent weight to the acceptance of the bar.

The kilopascal kPa , 1, pascals, equals 0.

Introduction

The megapascal mPa equals psi and is convenient for expressing high pressures. The pascal may be regarded as a "measuring gauge," the size ofwhich has been defined and is constant. This gauge can be used to measure pressure quantities relative to absolute vacuum.

Used in this way, the results will be in pascal absolute. The gauge may also be used to measure pressures relati ve to the prevailing atmospheric pressure, and the results will be pascal gauge. Ifthe gauge is used to measure the difference between pressures, it becomes pascal differen- tial. The use of gauge pressure is extremely important in industry, since it is a measure ofthe stress within a vesseland the tendency offtuids to leak auto It is really a special case of differential pressure measurement, inside versus outside pressure.

Where there is any doubt about whether a pressure is gauge, differential, or absolute, it should be specified in full.

However, it is common practice to shaw gauge pressure without specifying, and to specify by saying "absolute" or "differential" only for absolute or differential pressures.

The use of "g" as in psig is disappearing, and the use of "a" as in psia is frowned upon. Neither g flor a is recognized in SI unit symbols. However, M is recognized for differential pressure in all units.

The "standard" gravitational acceleration is 9. This is an arbitrary figure selected as a near average of the actual acceleration due to gravity found all over the earth. The following are typical values at different places: This is oflittle practicat importance in industrial applications.

However, with some transmitters being sold with a rated accuracy of: Gravity-Dependent Units Units such as psi, kglcm2, inches of water, and inches of mercury Hg are all gravity dependent. The English unit pounds per square inch psi is the pressure generated when the force of gravity acts on a mass of one pound distributed aveT one square inch.

Consider a dead weight tester and a standard mass of one pound which is transported around the earth's surface: The same applies to units such as inches of water and inches of mercury. The force at the bottom of each column is proportional to the height, density, and gravitational acceleration. Dead weight testers are primary pressure standards.

They generate pressure by applying weight to a piston that is supported by a fluid, generally oil or air. By selecting the weights and the cross-sectional aTea of the piston, the pressure generated in any gravity field can be calculated.

Therefore, dead weight testers are gravity dependent. For accurate laboratory work, the gravity under which the tester was calibrated and that at the place of use must be taken into account. Similarly, the pressure obtained by a certain height of fluid in a manometer depends on density and gravity.

Instrumentation Books

Factors given in the conversion tables in the Appendix, it should be noted, deal with units of force, noi weight. Dead weight testers will be discussed in more detaillater in ibis chapter. Gravity-lndependent Units While gravity plays no part in the definition of the pascal, it has the same value wherever it is measured.

Units such as pounds-force per square inch and kilogram-force per square centimetre are also independent of gravity because a specific value of gravitational acceleration was selected in defining these units. Under equal gravity conditions, the pound-mass and pound-force are numerically equal which is the cause of considerable confusion. Under nonstandard gravity conditions the usual case , correction factors are required to compensate for the departure from standard. It should be noted that the standard value of actual gravity acceleration is llot recognized as such in the SI unit system, where only the SI unit of acceleration of one metre per second per second is used.

In the future, only the measured actual gravity at the location of measurement G will be used when gravity plays a part in the system under investigation. The pascal is a truly gravity-independent unit and will be used to avoid the presently confusing question of whether a stated quantity is gravity dependent.

Pressure Standards Now let us consider the calibration standards that are employed with pressure-measuringinstruments and the basic instruments that are used to measure pressure. It mar help to look at the ways in which the standards for pressure calibration are established. You will recalI that head is the same as pressure. A measure of head, then, can be a dependable measure of pressure. Perhaps the oldest, simplest, and, in many respects, one of the most accurate and reliable ways of measuring pressure is the liquid manometer.

Figure shows a differential manometer. When only a visual indication is needed and static pressures are in a range that does Dot constitute a safety hazard, a transparent tube is satisfactory. When conditions for the visual manometer are unsuitable, a variety of ftoat-type liquid manometers are often employed.

Simple U-tube manometer. The simplest differential gauge is the liquid-filled manometer: It is often used to calibrate other instruments. The most elementary type is the U-gauge, which consists of a glass tube bent in the form of a U, or two straight glass tubes with a pressure connection at the bottom.

In the more advanced designs, vertical displacement of one side of the manometer is suppressed by using a chamber of large surface area on that side. Figure shows such a manometer. If the area ratio is in the vicinity of 1, to 1, the displacement in the large chamber becomes quite small and the reading on the glass tube will become extremely close to true inches or true millimetres.

The large side would have to be of infinite area for the reading in the glass tube to be exact. This problem is sometimes overcome with a special calibration of the scale.

However, if the glass tube becomes broken and must be replaced, the scale must be recalibrated.

Instrumentation for Process Measurement and Control, Third Editon

A more common and quite reliable design features a zeroing gauge glass as shown in Figure The scale mar be adjusted to zero for each differential pressure change, and the reading mar be taken from a scale graduated in actual units of measurement arter rezeroing.

Well or reservoirmanometer. Well manometerwith zeroingadjustment. Incline manometer tubes, such as those shown in Figure , will give magnified readings, but must be made and mounted carefully to avoid errors due to the irregularities ofthe tube. It is also essential that the manometers be precisely positioned to avoid errors due to level.

Still other types of manometers for functions other than simple indication, including those used with high pressure and hazardous fluids, employ a float on one leg of the manometer. When reading a manometer, there are several potential sources of error. One is the effect of gravity, and another is the effect of temperature on the material contained within the manometer.

Correction tables are available which provide the necessary correction for the conditions under which the manometer is to be read. Perhaps even more important is the meniscus correction Figure A meniscus surface should always be read at its center-the bottom, in the case of water, and the top, in the case of mercury.

To be practical, gravity and temperature corrections are seldom made in everyday work, but the meniscus correction, or proper reading, must always be taken into account. Inclined manometer. Readinga manometer. The dead weight tester is shown in Figure The principIe of a dead weight is similar to thai of a balance. Gravity acts on a calibrated weight, which in tom exerts a force on a known area.

A known pressure then exists throughout the fluid contained in the system. This fluid is generally a suitable oil. Good accuracy is possible, bot requires thai several factors be well established: Perhaps the most important part of the procedure is to keep the piston floating.

This is accomplished generally by spinning the weight platform. An accuratetestgaugemar beusedwith hydraulic pumpin a similarsetup. Still another type of dead weight tester is the pneumatic dead weight tester.

This is a self-regulating primary pressure standard. An accurate calibrating pressure is produced by establishing equilibrium between the air pressure on the underside of the ball against weights of known mass on the top.

A diagram of an Ametek pneumatic tester is shown in Figure In this construction, a precision ceramic ball is ftoated within a tapered stainless steel nozzle. A ftow regulator introduces pressure under the ball, lifting it toward the annulus between the ball and the nozzle.

Equilibrium is achieved as soon as the ball beginsto lift. The ball ftoats when the vented ftow equals the fixed ftow from the supply regulator. This pressure, which is also the output pressure, is proportional to the weight load.

During operation, the ball is centered with a dynamic film of air, eliminating physical contact between the ball and the nozzle. The regulator sensesthe change in ftow and adjusts the pressure beneath the ball to bring the system into equilibrium, changing the output pressure accordingly.

Thus, regulation of output pressure is automatic with change of weight mass on the spherical piston or ball. Plant Instruments That Measure Pressure Directly Thus far in tbis chapter we bave been concemed with the detinition of pressure, and some of the standards used bave been described. In the plant, manometers and dead weight testers are used as standards for comparison and calibration.

The working instruments in the plant usually include simple mechanical pressure gauges, precision pressure recorders and indicators, and pneumatic and electronic pressure transmitters.

A pressure transmitter makes a pressure measurementand generates either a pneumatic or electrical signal output thai is proportional to the pressure being sensed. We will discuss transmitters in detaillater in ibis chapter.

Now we will deal with the basic mechanical instruments used for pressure measurement, how they operate and how they are calibrated.

When the amount of pressure to be measured is very small, the following instruments might be used. Bell Instrument This instrument measures the pressure difIerence in the compartment on each gide ora bell-shaped chamber.

Ifthe pressure to be measured is gauge pressure, the lower compartment is vented to atmosphere. Ifthe lower compartment is evacuated, the pressure measured will be in absolute units.

If the difIerential pressure is to be measured, the higher pressure is applied to the top of the chamber and the lower pressure to the bottom. The bell chamber is shown in Figure Pressure ranges as low as Oto 1 inch Oto Pa of water can be measured with this instrument. Calibration adjustments are zero and span. The difficulty in reading a manometer accurately to fractions of an inch are obvious, ret the manometer is the usual standard to which the bell difIerential instrument is calibrated.

The bell instrument finds applications where very low pressures must be measured and recorded with reasonable accuracy. Slack or Limp-Diaphragm The slack or limp-diaphragminstrument is used when very small pressuresare to be sensed. The most commonapplicationofthis gauge Fig. Bell instrument.It is common practice to measure only the span of actual density changes.

The time will soon corne when process industries will deal exclusively with SI units. If it were possiblefor a proportionalcontrollerto bavea true Opercent proportionalband, the controller gainwould bave to be: a.

You will recalI that head is the same as pressure. The excursion of the tube tip moves linearly with internal pressure and is converted to pointer position with the mechanism shown. This will provide a steady reading and prolong the life of the gauge. Carefullyestimatethe averageof the meniscus Proportional Action Proportional response is the basis for the three-mode controller. In Figure , for safety reasons the valve must shut if there is a failure in the plant air supply.

LORRINE from Fitchburg
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