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INSTRUMENT QUESTION & ANSWERS

INTRODUCTION :

It is the branch of engineering which deals with the measurement, monitoring, display etc.of the various of energy exchanges which take place during process operations. "In shortInstrumentation is the study of Instrument."

INSTRUMENT :

Instrument is a devices which is used to measure, monitor, display etc. of a processvariable.1. What are the process Variabe !The process Variable are : low. !ressure. Temperature. evel. #uality i. e. $ %&, '%&, p( etc.

". De#i$e a the process Variabe a$% state their $it o# 'easre'e$t. !(o) :)efined as volume per unit of time at specified temperature and pressure 'onditions, is generally measured by positive*displacement or rate meters.U$its :kg + hr , litter + min, gallon + min , m+ hr , -m+ hr. gases /

*ressre :orce acting per unit 0rea. ! 1 +0U$its :bar , pascal , kg + cm&, lb + in&.

+e,e :)ifferent between two heights.U$its :2eters, mm, cm, percentage.

Te'peratre :It is the degree of hotness or coldness of a body.U$its :)egree 'entigrade, )egree ahrenheit, )egree 3elvin, )egree 4ankine.

Qait- :It deals with analysis. p(, $ '%&, $ 5&, 'onductivity, Viscosity /

. What are the pri'ar- ee'e$ts se% #or #o) 'easre'e$t !The primary elements used for flow measurement are : %rifice !late. Venturi tube. !itot tube. 0nnubars.

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low -o99le. eir ; lumes.

/. What are the %i##ere$t t-pes o# ori#ice pates a$% state their ses !The different types of orifice plates are :

'oncentric. uid as well as gases and steam service. Thisorifice plate beta ratio fall between of 5.8? to 5.@? for li>uids and 5.&5 to 5.@5 for gases,and steam. Aest results occur between value of 5.6 and 5.B. beta ratio means ratio of theorifice bore to the internal pipe diameters.

6?C beveled edges are often used to minimi9efriction resistance to flowing fluid /

flowd

C

fig 8/

Ecce$tric :The eccentric orifice plate has a hole eccentric. Dse full for measuring containing solids,oil containing water and wet steam. =ccentric plates can use either flange or venacontracta taps, but the tap must be at 875C or E5C to the eccentric opening.

=ccentric orifices have the bore offset from center to2inimi9e problems in services of solids*containingmaterials.

fig & /

Se0'e$ta :The segmental orifice place has the hole in the form segment of a circle. This is used forcolloidal and slurry flow measurement. or best accuracy, the tap location should be 875Cfrom the center of tangency.

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C

C

C

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&. ual pressure on both sides of the transmitter. Jero shouldnot shift. If it is shifting carry out static alignment.. Vacuum test : 0pply e>ual vacuum to both the sides. The 9ero should not shift.6. 'alibration !rocedure: ive &5 psi air supply to the transmitter.

Vent the .!. side to atmosphere. 'onnect output of the Instrument to a standard test gauge. 0dGust 9ero.?. 0pply re>uired pressure to high pressure side of the transmitter and adGust the span.B. 0dGust 9ero again if necessary.

19. What is the sea ii% se% #or #ii$0 i'pse i$es o$ cr%e a$% ,iscos ii% !lycol.

11. 2o) %o -o carr- ot pipi$0 #or a Di##ere$t pressre #o) tra$s'itter o$ii%s;

)a- 'a$i#o% ,a,e

(o) tra$s'itter

K *

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)a- 'a$i#o% ,a,e =

*ri'ar- isoatio$ ,a,e

(o) %irectio$

Ori#ice pate

Stea' Ser,ice :%n steam service the transmitter is mounted below the orifice plate withcondensate pots. The pots should be at the same level.

Ori#ice pate

(o) %irectio$

*ri'ar- isoatio$ ,a,e

Co$%e$sate pot

)a- 'a$i#o% ,a,e

? @ (o) tra$s'itter

1". Dra) a$% epai$ a$- #o) co$tro oop !

cB,positio$er

Ori#ice pate

*ri'ar- isoatio$ ,a,e

AIR SU**+ (RC

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)a- 'a$i#o% ,a,e

(o) tra$s'itter AIRSU**+

1. A$ operator tes -o that #o) i$%icatio$ is 'ore ! 2o) )o% -o startcheci$0 !

irst flushing the transmitter. lush both the impulse lines. 0dGust the 9ero bye>uali9ing if necessary. If still the indication is more then.

'heck .!. side for choke. If that is clean then. 'heck the leaks on .!. side. If not. 'alibrate the transmitter.

1/. 2o) %o -o %o a ero chec o$ a D.*. tra$s'itter !'lose one of the valve either (.!. or .!. open the e>uali9ing valve. The %+! should read

9ero.

1. 2o) )o% -o %o uali9er valve open.

3eep the .!. side valve closed. uali9er valve. %pen both the primary isolation valves.

13. 2o) %o -o cacate $e) #actor #ro' $e) ra$0e si$0 o% #actor a$% o% ra$0e!

#8 L M!8 #8 1 %ld flowfactor/ F #&1 -ew flowfactor/#& L M!& L M!81 %ld range L M!&F -ew range

#& L M!8 1 #8 L M!&

#8 L M!& #& F

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L M!8

%ld factor L-ew range-ew factor 1

L%ld range

14. 2o) )i -o ,e$t air i$ the D.*. ce ! What i# sea pots are se% !

0ir is vented by opening the vent plugs on a li>uid service transmitter. %n services where seal pots are used isolate the primary isolation valves and

open the vent valves. ill the line from the transmitter drain plug with a pump.

17. Wh- #o) is 'easre% i$ sare root !low varies directly as the s>uare root of different pressure 1 3 s>uare root of 0!.uare root of differential pressure the pen does notdirectly indicate flow. The flow can be determined by taking the s>uare root of the pen.

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Aourdon tube. )iaphragm. 'apsule. Aellows. !ressure

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.

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Air re0ator

N "pr0e Air spp- rota'eter

The air pressure to the bubbler pipe is minutely inexcess of the li>uid pressure in the vessel, so thatair pressure indicated is a measure of the level inthe tank.

The method above are suitable for open tank applications. when a li>uid is in a pressurevessel, the li>uid column pressure canFt be used unless the vessel pressure is balanced out.This is done through the use of different pressure meters.

c / )ifferential pressure meter :'onnection are made at the vessel top and bottom, and to the two columns of the ).!.meter. The top connection is made to the .!. column of the transmitter and the bottom to(.!. column of the transmitter. The difference in pressure in the vessel is balanced out,since it is fed to both the column of the meter. The difference in pressure deducted by themeter will be due only to the changing, level of the li>uid.

d / )isplacer type level measurement :The leveltrol is one of the most common instruments used measuring level in closed tanks.This instrument works of 0rchimedes principle. The displacer in immersed in the li>uiddue to which there is loss of weight depending on the specified gravity of the li>uid. Thisdisplacer hangs freely on a knife transmitted to the pneumatic or electronic counterpart atthe other end.

9. Epai$ ho) -o )i 'easre e,e )ith a %i##ere$t pressre tra$s'itter.The bottom connection of the vessel is connected to high pressure side of the transmitter.

)ifferent !ressure 1 ( H )

2%

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2* +* K *

D B * TRANSMITTER

This difference pressure is applied to (.!. side of the transmitted and calibrated.

1. 2o) is D.*. tra$s'itter appie% to a cose ta$ !In close tank the bottom of the tank is connected to the high pressure side of thetransmitter and top of the tank in connected to .!. side of the transmitter. In this way thevessel pressure is balanced.

".2o) is D.*. tra$s'itter appie% to a$ ope$ ta$ !%n an open tank level measurement the .!. side is vented to atmosphere. hateverpressure acts is on the (.!. side which is a measure of level.

2* +* ? @

. 2o) is D.* tra$s'itter appie% to a cose ta$ & ope$ ta$ )ith Dr- e0!

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=xample : %pen tank with H 1 55 inches N 1 ?5 inches J 1 85 inches 1 5.7

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(minimum level 1 ?5 / 5.7 / * ?55 / 5.E / 1 * 685 inches(maximum level 1 55 K ?5 / 5.7 / O ?55 / 5.E / 1 * 8@5 inches'alibrated range 1 * 685 to O8@5 inches head of water. minus sings indicate that the higher pressure is applied to the low pressure side of thetransmitter /

/. What is pr0e e,e s-ste' !This method is also known as bubbler method of level measurement. 0 pipe is installedvertically with its open end at the 9ero level. The other end of the pipe is connected to aregulated air supply and to a pressure gauge or to P! transmitter. To make a levelmeasurement the air supply is adGusted so that pressure is slightly higher than the pressuredue to the height of the li>uid. This is accomplished by regulating the air pressure untilbubbles can be seen slowly leaving the open end of the pipe. The gage then measures theair pressure needed to over come the pressure of the li>uid.+Q ! 1 ( H )USE : %n for corrosive li>uids where the transmitter cannot be directly connected toprocess eg... 0cids, uids.

. Epai$ the )ori$0 o# a e,etro.The leveltrol is used for measuring level of li>uids in a closed vessel.8. !4I-'I!=. : It works on 0rchimedes principle "The loss in weight of a body

immersed in a li>uid is e>ual to amount of li>uid displaced by the body". The leveltrolbasically consists of the following :

&. )I

2otion take of from Tor>ue tube. 2eans of reverse control action. 0dGustment for specific gravity.

?. !4%!=4TI%-0 D-IT. : 'onverts primary motion to a proportional output airpressure.

B. '%-T4%

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R J=4%

The variation in buoyancy resulting from a change in li>uid level, varies the net weight ofthe displacer increasing or decreasing the load on the tor>ue arm. This change is directlyproportional to the change in level and specific gravity of the li>uid. The resulting tor>uetube movement varies the angular motion of the rotor in the 4V)T 4otary Variable)ifferential. Transformer/ providing a voltage change proportional to the rotordisplacement, which is converted and amplified to a ).'. current.

4. 2o) )i -o re,erse a$ actio$ o# the e,etro.!The reversing are serves as motion take off arm from the tor>ue tube. It is provided with aslot on each side of the center so that link can be connected either for reverse or directaction.

7. What is i$ter#ace e,e ! 2o) %o -o cacate it !hen a vessel is filled with two li>uids of two different specific gravities the levelmeasurement refers to as interface level.)! 1 ( ) * d /

)=-

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8. irst close both the primary isolation valves and drain the li>uid inside the chamber.&. 0dGust the 9ero to get 5$ output.. 'onnect a transparent !V' tube to the drain point as shown in hook up.6. ill it to the center of the top flange.?. 0dGust the specific gravity or span adGustment =lectronic evel /.B. ill it up to ?5 $, check linearity.

/9. 2o) )i -o caibrate o$ i$ter#ace e,e co$tro. !%n an interface leveltrol there are two li>uid of two different specific gravities.8. The level will be 9ero when it is full of lighter li>uid. Jero $ level 1 ( H d. ( 1 )isplacer length d 1 uid.& The level will be 855 $ when it is full of heavier li>uid. 855 $ level 1 ( H ).

) 1 uid.'alibration with water :8. ill ( H d level with water adGust 9ero.&. ill ( H ) level with water adGust

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/. What are the i'itatio$s o# e,etro !The limitations of a level control that it cannot be used for lengths more than @& inches.

/3. 2o) )i -o co''issio$ D.*. tra$s'itter i$ #ie% i$ pressrie% ,esse.8. 'lose both the isolation valves, Vent the (.!. side.&. ill it with the sealing li>uid.. %pen the .!. side vent valve.6. 0dGust 9ero with suppression spring.?. 'lose the .!. side vent valve.B. %pen both the isolation valves.

/4. 2o) )i -o chec ero o# a e,e D.*. tra$s'itter )hie is i$e !8. 'lose both the isolation valves.&. %pen the vent valve on .!. leg and (.!. leg drain.. 'heck and adGust 9ero if necessary.

/7.Epai$ the )ori$0 o# a$ E$ra# e,e 0a0e!The =nraf precise level gauge are based on servo powered null*balance techni>ue. 0displacer serves as a continuous level sensing element.!rinciple:0 displacer with a relative density higher than that of the product to be measured, issuspended from a stainless steel wire A, that is attached to a measuring drum. 0 two phaseservo meter controlled by a capacitive balance system winds or unwinds the measuringwire until the tension in the weighing springs is in balance with the weight of the displacerpartly immersed in the li>uid. The sensing system in principle measures the twocapacitance formed by the moving center sensing rod = provided with two capacitor

plates and the side plates. In balance position the capacitances are of e>ual value. 0 levelvariation will a difference in buoyancy of the displacer. The center sensing rod will move inthe direction of one of the side capacitor plates. This causes a difference in value of thesecapacitances. Ay an electronic circuit this change is detected and integrated. )uring therotation of the servo motor the cam driven transmitter continuously change the voltagepattern to a remote indicator of which the receiver motor drives a counter indicating levelvariation.

T E M * R E T U R E

/8. What are the %i##ere$t 'etho%s o# te'peratre 'easre'e$t ! Epai$.

The different methods of temperature measurement are :8. 2echanical &. =lectrical.Mecha$ica 'etho%s:

1. Mercr- i$ 0ass ther'o'eters : This consists of a glass tube of very fine bore Goinedto a reservoir at the bottom and sealed at the top. 0 measured >uantity of mercury is theenclosed. hen the thermometer is heated the mercury expands much more than the glassand is therefore forced to rise up in the tubing 0 scale is fixed at the side.

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". 5i'etaic Ther'o'eter : Two metals whose coefficient of linear expansion isdifferent are welded and rolled together to the desire thickness. The actual movement of abimetal is its flexivity with one end fixed, a straight bimetal strip%e#ects i$proportion toits temperature, to the s>uare of its length and inversely with its thickens..*ressre Spri$0 Ther'o'eters :There are four classes of pressure springthermometers.8. i>uid filled 1 class 8&. Vapor pressure 1 class &. as filled 1 class 6. mercury filled 1 class 6+ii% #ie% & Mercr- #ie% :Aoth type, operate on the principle of thermal expansion. here the bulb is immersed in aheated substance. The li>uid expands causing the pressure spring to unwind. Theindicating, recording or controlling mechanisms are attached to pressure spring.'ompensated Thermometer uid which only partiallyfills the system. 0t low temperatures the vapor pressure increase for each unit temperaturecharge is small, at higher temperature the vapor pressure change is much greater.

Eectrica 'etho% o# te'peratre 'easre'e$t:8. Thermocouples: It is a simple device consisting of a dissimilar metal wires Goined attheir ends. when an of each wire is connected to a measuring instrument thermocouplesbecomes an accurate and sensitive temperature measuring device.

cold or reference Gunction

(ot or measuring Gunction

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Thermocouples Types and 4ange:

Type T+' !ositive wire ; color -egative wire ; color 4ange U / Iron ; hite 'onstantan ; 4ed *55 to8655 3 / 'hromel ; Nellow 0lumel ; 4ed *55 to&55 = / 'hromel ; !urple 'onstantan ; 4ed *55 to8B55 T / 'opper ; Alue 'onstantan ; 4ed *55 to B?5 4 / !latinum and 85$4hodium ; Alack !latinum ; 4ed & to &@55 < / !latinum and 8$4hodium ; Alack !latinum ; 4ed & to &@55

4esistance O Temperature )etectors 4T)/:4T)Fs are generally used for precise temperature measurement. It consists of a five wire

wrapped around an insulator and enclosed in a metal. The most sheath of a resistancethermometer resembles that of bimetallic thermometer bulb.!4I-'I!= : "4esistance increases as temperature increase"Rt. F Ro =1 ? J t >4t. 1 4esistance of Temperature to measured.4o. 1 4esistance of 9ero temperature. 1 'o. off of thermal expansion/.t 1 Temperature to be measured.These metals have a positive temperature co*efficient of expansion. Therefore resistanceincreases as the temperature increases.

Types of material used: 8/ !latinum &/ -ickelThese metals have a positive temperature co*efficient of expansion. Therefore resistanceincreases as the temp. increases.

'alculation of 4esistance or !t855.4o. 1 855x for platinum 1 5.557? +c.To calculate 4esistance at 855Fc.4855 1 855 S 8K 7.? x 85 6 x 855 / 1 855 K 855 x 5.7?/4855 1 87.?Resista$ce at 1996c F 17.

9. What is *t 199 'ea$. !!t855 means 855 %(2< at 5F' for a platinum resistance bulb.

1. What is t)o )ire a$% three )ire R.T.D. s-ste' !Two wire 4 .T .). system :Two wire 4T) system use for short distance like a compressor field local panel.Three wire

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Three wire system use for long distance coke a field to control 4un.The third wire is used for compensation of lead wire resistance.

T)o@)ire R.T.D Three@)ire R.T.D

? @ ? @

R1 R" R1R"

R R

R.T.D R.T.D

". Dra) a pote$tio'eter te'p. 'easri$0 circits a$% epai$ its!

Ther'ocope T)o %i##ere$t si0$a AM*

U$ $o) si0$a

Ser,o baa$ci$0 'otor

Meas. Co$sta$t ,ota0e H$o)$ circit si0$a si0$a

O*ERATION :The input to the instrument is a measurement of some in the processes using a sensingelement such as thermocouple / or a device to produce direct voltage, which is thevoltage signal/. This voltage is subtracted from a voltage developed by a known constant

voltage in a potentiometer measuring circuit. The subtraction occurs by connecting twovoltages in series with the opposing polarity, difference between these two voltagesproduces signal, the voltage going to the amplifier. The error will positive or negativedepending on which of the two voltages greater. hen amplified, the error signal willdrive servo balancing motor in appropriate direction to adGust circuit actually drive theslide wire / until the difference between the feedback voltage and the input voltage isbalance out. 0n error signal e>ual to 9ero results null point / the balancing servo motoris be longer driven /

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al

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. What is the co$sta$t ,ota0e $it ! 4&1&E.6 42 4c

'r8 48 K &k 'r 6

'84 'r *

@B6W 4esistance lead of '4T1 6.

The constant voltage circuit consists of a rectifier, '4, a filter capacitor '8, followed bytwo stages of 9ener regulation. 0bridge configuration is provided to 8amp line voltageregulation 9ener '4, 48 and 4& combine provide relatively constant current to 9ener'46, Thus variations. 4esisters 4& and 4 form a bridge that any remoment line voltageeffects.

/. Epai$ the )ori$0 o# a baa$ci$0 'otor.

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0 burnout resistor is provided which develops a voltage drop between the measuringcircuit and the amplifier. The polarity of the signal determines the direction of the servodrive upon an open circuit in the input.Dpscale burnout : 4 value 85 2)ownscale burnout : 4 value &.& 2

3. Epai$ the boc %ia0ra' o# a$ a'pi#ier i$ a te'p. recor%er.

4. Wh- is a co$,erter se% i$ a te'p. recor%er !The converter is designed to convert ). '. input voltage into an 0. '. input voltageproportional in amplitude to the input.

7. Wh- are Ther'o)es se% !In numerous application it is neither desirable nor practical to expose a temperature sensordirectly to a process material. ells are therefore used to protect against damagecorrosion, erosion, abrasion and high pressure processes. 0 thermowell is also useful inprotecting a sensor from physical damage during handling and normal operation.

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.0dd the ambient temp. millvolts to the corr. input millvolts measured. ind out from thechart the corresponding temperature.

39. What t-pe o# se$si$0 ee'e$t )o% -o se to 'easre ,er- o) te'peratre !The sensing element used for measuring very low temperature is 4. T. ). 4esistance Temperature )etector /

31. What are si$ te'peratre ther'ocopes !

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The polarity of the signal at 00 determines the phase of the alternating voltage at AAwhich in turn determines the direction of rotation of the balancing motor.

3. 2o) is ato'atic Re#ere$ce K$ctio$ co'pe$satio$ carrie% ot i$ te'p.recor%ers !

RheostatK *

scale

Variabe Resistor

K *

Re#ere$ce 2ot

K$ctio$ K$ctio$L*

or automatic reference Gunction compensation a variable nickel resister is used. 0s thetemperature changes, so does its resistance. This reference Gunction compensatory islocated, so that it will be at the temperature of the reference Gunction. The referenceGunction is at the position where the dissimilar wire of the thermocouple is reGoined, whichinvariably is at the terminal strip of the instrument.

CONTRO+ SSTEMS

33. Epai$ the appicatio$ o# proportio$a i$te0ra a$% %eri,ati,e actio$!*roportio$a co$tro o$- :!roportional control only attempts to return a measurement to the set point after a loadupset has occurred. (ow ever it is impossible for a proportional controller to return themeasurement exactly to the set point.Use: It is normally used for level controls. It reduces the effect of a load change but it cannot eliminate it.*roportio$a ps reset co$tro:

4eset action is introduced to eliminate offset. It will integrate any difference betweenmeasurement and setpoint and cause the controllerFs output to change until the differencebetween the measurement and set point is 9ero. 4eset will act as long as the error exists.Use: !roportional K 4eset controllers are by far the common types used in industrialprocess control and where predominate dead times occur.*roportio$a ps reset ps %eri,ati,e:)erivative or rate action helps the controller overcome system inertia and result in faster,more precise control. )erivative action occurs whenever the measurement signal changes.

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Dnder study conditions the rate action does not act. )erivative allows the controller toinGect more corrective action.Use : %n temperature controls.

34. What is %i##ere$ce 0ap co$tro !)ifferential gap control is similar to on off control except that a band or gap exists aroundthe control point.Use: In industry differential gap control is often found in non critical level controlapplications where it is desirable only to prevent a tank from flooding or drying.Whe$ ameasured variable exceeds the upper gap the control valve will open fully or be closedfully.

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The term ultimate gain was attached to this method because its use re>uire thedetermination of the ultimate gain sensitivity/ and ultimate period. The ultimate sensitivity3u is the maximum allowable value of gain for a controller with only !roportional mode/for which the system is stable. The ultimate period is the period of the response with thegain set at its ultimate value.*rocess reactio$ cr,e :

To deter mine the process reaction curve, the following steps are recommended. :8. et the system come to steady state at the normal load level.&. !lace the controller on manual.. 2anually set the output of the controller at the value at which it was operating in the

automatic mode.6. 0llow the system to reach the steady state.?. ith controller on manual, impose a step changes in the output of controller, which is

an signal to value.B. 4ecord the response of controlled variable.@. 4eturn the controller output to its previous value and return the controller to auto

operation.4". Epai$ the )ori$0 o# a$ eectro$ic *.I.D. co$troer.Input from the measurement transmitter is compared with the set point voltage to producea deviation signal. The deviation signal is combined with a characteri9ed feed back signalto provide the input for the function generator amplifier. This amplifiers output is deliveredto the feed back network, and to the final output which is a 85*?5m.a. do signal foractuation of final operators.

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R C Vi$ ?

AV @1 @ VotV9F Vi$%t RCA$ao0 %i##ere$tiator:

R

C % ?V9F @ RC Vi$ Vi$ AV %t @ Vot

4/. What is a$ti reset )i$% p !

If the limit acts in the feed back section of the control amplifiers integral circuit, thecontroller output will immediately begin to drive in the opposite direction as soon as theprocess signal crosses the set point. This approach is referred to as antireset wind up.

4. What are De@satrators !hen, in some processes, e.g. batch process, long transient responses are expected duringwhich a sustained deviation is present the controller integral action continuously drives theoutput to a minimum or maximum value. This phenomenon is called "integral saturation ofthe control unit". hen this condition.

43.What is the e##ect o# )eep hoe o$ cacatio$ o# ori#ice bore!

=%'>" )here: %' F Measre% %ia'eter o# ori#ice.

% F %' 1 ? 9. %h F Drai$ hoe %ia'eter.%h % F Correcte% %ia'eter ori#ice sie.

44. Epai$ the )ori$0 o# Rota'eter!

OUT +ET

The flow rate varies directly

Tapered glass tube as the float rises and falls in

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Variable area meters are special form of head meters. here in the area of flow restrictoris varied. ually applicable to amoving, electrically conductive li>uid. The primary device of commercial magnetic metersconsists of a straight cylindrical electrically insulated tube with a pair of electrodes nearlyflush with the tube wall and located at opposite ends of a tube diameter. 0 uniform a.c.magnetic field is provided at right angles to electrode diameter and to the axis of the tube.

The a.c. voltage developed at the electrodes is proportional to the volume flow rate offluid, and to a magnetic field strength. This device is limited to electrically conductingli>uids. The magnetic meter is particularly suited to measurement of slurries and dirtyfluids, since there are no location for solids to collect except the walls of the tube itself.

48. Epai$ the )ori$0 o# a trbi$e 'eter.Turbine meters consist of a straight flow tube within which a turbine or fan is free torotate, about its axis which is fixed along the center line of the tube. uency proportional to flow rate. Turbine meter are primarily applied to measurementof clean and non*corrosive hydrocarbons.79. Epai$ the )ori$0 o# a *itot tbe.The pitot tube measures the velocity at point in the conduct. If >uantity rate measurementis desired, it must be calculated from the ratio of average velocity to the velocity at thepoint of measurement.*ri$cipe : If a tube is placed with its open and facing into a stream of fluid, then the fluidimpinging on the open end will be brought to rest, and the kinetic energy converted topressure energy. This the pressure built up in the tube will be greaterthan that in the freestream by the impact pressure or pressure produced by loss of kinetic energy. The increasein pressure will depend upon the s>uare of the velocity of the stream. The difference ismeasured between the pressure in the tube and static pressure of the stream. The staticpressure is measured by a tapping in the wall of the main or by a tapping incorporated inthe pitot static tube itself. The difference between the pressure in the tube and staticpressure will be a measure of the impact pressure and therefore of the velocity of thestream oil.

71. Where is the i$te0ra ori#ice se% !

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Integral orifice is used to measure small flow rates. It is mounted directly on the secondarydevice. The integral orifice diameter varies between 5.5&5 inch and 5.&?5 inch diameter.The integral orifice finds considerable use in laboratory and pitot plants.'alculation of flow rate :

Q$ B (c F Hs C)i (a ('

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these are inertial viscous, gravitational, compressibility, pressure and elastic forces. 'ertaindimensionless ratio are developed based on fluid properties. Velocities and dimension,which are essentially force ratio.The more important of these are 4eynolds number

V F ,eocit- , D D F i$si%e %ia'eter o# pipe R F F #i% %e$sit- P P F ,iscosit-

or most applications in practical flow measurement the 4eynolds number is taken to besufficient criterion of dynamic similarly. The magnitude of 4eynolds number not onlyindicates whether the flow is laminar or turbulent but also furnishes the probable shape ofvelocity profile. )ue to the strong role it plays as an indicator of varying flowcharacteristics, many of the deviation from the theoretical e>uations are called 4eynaldonumber effects.

73. 2o) )o% -o choose %i##ere$tia ra$0e !

The most common diff. range for li>uid measurement is 5*855" (&%. This range is highenough to minimi9e the errors caused by une>ual heads in the seal chambers, differences intemps. of load lines etc. The 855" range permits an increase in capacity up to 655" and adecrease down up to &5" by merely changing range tubes or range adGustments.

74. What is positi,e Dispace'e$t 'eters !

pri$cipe: The principle of measurement is that as the li>uid flows through the meter itmoves a measuring element which seals off the measuring chamber into a series ofmeasuring compartments each holding a definite volume. 0s the measuring element'o,es; these compartments are successively filled and emptied. Thus for each complete ofthe measuring element a fixed >uantity of li>uid is permitted to pass from the inlet to the

outlet of the meter. The seal between measuring element and the measuring chamber isprovided by a film of measured li>uid. The number of cycle of the measuring element isindicated by means of a pointer moving over the dial, a digital totali9er or some other formof register, driven from the measuring element through an adGustable gearing.The 'ost co''o$ #or's o# positi,e %ispace'e$t 'eters are :8. 4eciprocating !iston type.&. 4otating or %scillating !iston type.. -utating )isc type.6. luted

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CONTRO+ VA+VES

78. What is a co$tro ,a,es !0 control valve is the final control element, which directly changes the valve of themanipulated variable by changing the rate of flow of control agent.0 control valve consists of an operator and valve body. The operator provides the powerto vary the position of the valve plug inside the body. The plug is connected to theoperator by a stem, which slides through a stuffing box. The air signal from the controller

is applied above the diaphragm. The increasing air signal from the controller is appliedabove the diaphragm. 0n increasing air signal will push the operator stem downwardsagainst the force exerted by the spring on the diaphragm plate. The valve is adGusted insuch a way that the plug starts moving when psi is applied to the diaphragm and touchesthe seat when 8? psi is applied to the diaphragm. Thus an increase in air pressure will closethe valve. (ence the home "0ir to 'lose". 0nother type is "0ir to open", such that psion the diaphragm the value is closed and 8? psi air signal it in fully open.

89. What are the %i##ere$t t-pes o# co$tro ,a,es !The commonly used control valves can be divided as follows.8. )epending on 0ction.

&. )epending on the Aody.1. Depe$%i$0 o$ actio$:)epending on action there are two types of control valves, 8/ 0ir to close, &/ 0ir toopen.". Depe$%i$0 o$ bo%-:8. lobe valves single or double seated.&. 0ngle valves.. Autterfly valves.6. Three way valves.

81. What is the se o# si$0e seate% ,a,e !The single seated valve is used on smaller si9es, and in valve of larger si9es, where anabsolute shut off is re>uired. The use of single seated valve is limited by pressure dropacross the valve in the closed or almost closed position.

8". What is the se o# %obe seate% ,a,e !In double seated valves the upward and downward forces on the plug due to reduction offluid pressure are nearly e>uali9ed. It is generally used on bigger si9e valves and highpressure systems. 0ctuator forces re>uired are less i.e. 0 small si9e actuator.

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8. What is C, o# a ,a,e !'v is the capacity of a valve and is defined as :"-o of gallons per minute of water which passes through a fully open valve at a pressuredrop of 8 psi. 'V 1 > Z! + /

here: 'v 1 Valve co*efficient > 1 Volumetric flow rate gallons minute / Z! 1 !ressure drop across the valve in psi.

1

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The valve coefficient 'v is proportional to the area F0F between the plug and valve seatmeasured perpendicularly to the direction of flow.

8/. What are the %i##ere$t t-pes o# actators !The different types of actuators are :8. )iaphragm %perated. &. !iston %perated.8. What t-pes o# bo$$ets )o% -o se o# hi0h te'p. a$% ,er- o) te'p. !

2i0h te'peratre: Aonnets are provided with radiation fins to prevent glad packing fromgetting damaged.O$ ,er- o) te'peratre: =xtended bonnets are used to prevent gland packing fromgetting free9ed.

83. 2o) )i -o )or o$ a co$tro ,a,e )hie it is i$e !hile the control valve is in line or in service, it has to be by passed and secondly the lineto be depressuri9ed and drained.

84. What is the se o# a ,a,e positio$er !

The valve positioner is used for following reasons :8. #uick 0ction control valve.&. Valve hysteresis.. Valves used on viscous li>uids.6.

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Autterfly valves are used only in systems where a small pressure drop across the valve isallowed. The butterfly is fully open when the disc rotates by E5. 0%ra)bac o# this valveis that even a very small angular displacement produces a big change in flow.

199. What is the se o# three )a- ,a,es !Three way control valves are only used on special systems, where a dividing or mixture offlows according to a controlled ratio is re>uired.

191. What are the %i##ere$t t-pes o# p0s !The different types of plugs are generally used are : 8 / V. port plug & / 'ontoured plugV@port p0:!orted plug are generally used on double seated valves. This is because ported plugs, havea more constant off balance area.Co$tore% p0:'ontoured plugs are generally used on single seated valve with small trim si9es.

19". What is a ca0e ,a,e !0 cage valve uses a piston with piston ring seal attached to the single seated valve "plug".(ere the hydrostatic forces acting on the top or the piston or below the valve plug tend tocancel out. The seat ring is clamped in by a cage. 'age valves are generally used for noise

reduction.

19. What are the a%,a$ta0es o# Ca'#e ,a,es !'amflex valves are intermediates between globe valve and butterfly valve. The plugrotates B5F for full opening.A%,a$ta0es:8. 0ctuator forces re>uired are very less.&.=xtended bonnet and hence can be used on any service i.e. on high temp. and very low

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Temperature.. Variations in flow.6. ight weight.

19/. What is the se o# i$ co$$ecte% to the ,a,e positio$er !The link serves as the feed back to the value. 0nt valve movement is sensed by this link.

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b/ ual percentage valve is used.+ii% +e,e Co$tro :-ormally linear valve is used.Aasically in selecting a valve characteristic two important point have to be taken intoaccount.a/ There should be a linear relationship between the position of the plug and the flowthrough the valve in a wide range of change in the pressure drop across the valve.b/ The pressure drop across a valve should be as low as possible.

Co$tro ,a,e sii$011". What is the e##ect o# pipe re%cers o$ ,a,e capacit-!hen control valves are mounted between pipe reducers, there is a decrease in the actualvalve capacity. The reducers create an additional pressure drop in the system by acting ascontractions of enlargements in series with the valve.2etric formula: for inlet and outlet reduces.

" % " C,

"

R F 1 1. 1 @

D" 9.9/%"

or outlet reducer only or inlet reducer with entrance angle less than 65 inches.

" %" C,"

R" F 1 1. 1 @

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D" 9.9/%"% F ,a,e sie ''. D F i$e sie ''. C, F reire% ,a,e co@e##icie$t.To compensate for reducer losses at sub*critical flow, divide 'v calculated by 4.

11. A$ operator tes -o that a co$tro ,a,e i$ a stc ! 2o) )i -o startcheci$0 !8. irst of all get the control valve is passed from operation.&. 'heck the lingual to the diaphragm of the control valve.. )isconnect it possible the actuator stem from the control valve stem.6.

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(ee% )ater

(e 0as

*V

'ascade means two controllers is series. %ne of them is the 2aster or !rimary and thesecond is the secondary of slave controller. The output of the secondary controlleroperates the final control element, that is the valve.+oop epa$atio$:The output of the temp. transmitter goes as measurement signal to the TI' which is themaster controller. uality due to cascading is a function of relative speeds and time lags. 0 slowprimary 2aster/ variable and a secondary uickly todisturbances represent a desirable combination for this type of control. If the slave canrespond >uickly to fast disturbances then these will not be allowed to enter the processand thereby will not upset the control of primary master/ variable. It can be said that useof cascade control on heat transfer e>uipment contributes to fast recovery from loadchanges or other disturbances.

114. Epai$ ratio co$tro s-ste'.

U$ co$troe% #o) 0 / (T Ratio co$troer

(1? ("Seco$%ar- co$troer

A / (TCo$troe% #o) (V

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0 ratio control system is characteri9ed by the fact that variations in the secondary variabledo not reflect back on the primary variable. In the above diagram 5 a ratio control systemthe secondary flow is hold in some proportion to a primary uncontrollable flow.If we assume that the output of primary transmitter is 0, and the output of the secondarytransmitter is A, 0nd that multiplication factor of the ratio relay is 3, then for e>uilibriumconditions which means set valve is e>ual to measured valve, we find the followingrelation :30 * A 1 5or A+0 1 3, where F3F is the ratio setting of the relay.

117. Epai$ #e to air ratio co$tro o# #r$aces. Air

(RC Master stea'

(RC (C

(T (RCRS* pri'ar-

seco$%ar-

(V RR (T

Ratio co$troer(e 0as

118. What is (r$ace Dra#t co$tro !Aalanced draft boilers are generally used negative furnace pressure. hen both forceddraft and induced draft are used together, at some point in the systemthepressure will bethe same as that of atmosphere. Therefore the furnace pressure must be negative toprevent hot gas leakage. =xcessive vacuum in the furnace however produces heat losses

through air infiltration. The most desirable condition is that the one have is a very slightabout 5.8" (&5 / negative pressure of the top of furnace.

1"9. What is #ee% bac co$tro ! What is #ee% #or)ar% co$tro ! Discss itsappicatio$ !

(ee% bac co$tro: Co$troer=(IC> Measri$0 ee'e$t Set poi$t Tra$s'itter

Co$tro ,a,e Ori#ice pateeed back control involves the detection of the controlled variable and counteracting ofcharges its it[s value relative to set point, by adGustment of a manipulated variable. Thismode of control necessities that the disturbance variable must affect the controlled variableitself before correction can take place. (ence the term FfeedbackF can imply a correctionFbackF in terms of time, a correction that should have taken place earlier when thedisturbance occurred.

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*ROCESS

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(ee% #or)ar% co$tro : Ori#ice

Otpt (T

Co$troer A%%iti,e re0ator

eed forward control system is a system in which corrective action is based onmeasurement of disturbances inputs into the process. This mode of control responds to adisturbance such that is instantly compensates for that error which the disturbance wouldhave otherwise caused in the controlled variable letter in time.eed forward control relies on a prediction. 0s can be seen from the figure of feed forwardcontrol a necessary amount of input goes to the process. This measurement goes to the

controller which gives output to the control valve. The control valve regulates the flow.

(ee% bac co$tro :

In feed forward control no difference between the desired result and actual result needexist before corrective action is taken in feed back control a difference must exist. (ence,open loop or feed forward control is capable of perfect control, but feed back is not. )ueto economic impartibility of precision , predicting the amount of correction necessary toachieve satisfactory results with feed forward control, feed back control is most often

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*ROCESS

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used. In order to properly choose the type of feed back controller for a particular processapplication, two factors time and gain must be considered.

1"1. Epai$ three ee'e$t #ee% )ater co$tro s-ste'!(ee% )ater co$tro Dr' e,e co$tro Stea' co$tro

*" *1 */

Sare root Sare root etractorEtractor *

Co'pti$0 eatio$:*F R = *"@ *1 @ H > ? */? H9

*F Otpt.*1; *"& */ F I$ptH1F A%Kstabe sppressio$.H9F A%K.5ias.

1"". Epai$ A$ti@sr0e co$tro!

Co'pressor

*DT

*i$et *otet

Ratio

(T ( (RC= (o) co$,erter>

5-@*ass ,a,eThis method of surge control uses the ratio of compressor pressure rise to inlet flow rateto set the flow in by*bass loop. hen the suction pressure drops and discharge shoots up,the compressor starts surging. The pdt senses this and gives the signal to the 4' whichwill open the by*pass valve.

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ue for designing electrical e>uipment for safe use inlocations made ha9ardous by the presence of flammable gas or vapors in air.

")efn. :" Intrinsically safe circuit is one in which any spark or thermal effect produceeither normally or under specified fault conditions is incapable of causing ignition of aspecified gas or vapor in air mixture at the most easily ignited concentration.

2AGARDOUS AREAS :The specification of products or systems sold as intrinsically safe must state in whatha9ardous areas they are infect intrinsically safe. Dniversal cooling of ha9ardous areas hasnot, unfortunately, been adopted in all countries. (owever two sets of codes in common

use are.

Qe. : What %oes a tra$s'itter otpt start #ro' @1 psi or =9." @ 1 H0BC'"> or / @"9 'a. etc. !A$s. : The transmitter output stance from what is known as "live 9ero". This system hasspecific advantages :8. The systems automatically alarms when the signal system becomes inoperative.&. The output areas is linear 4atio of 8 : ? /.

DEAD GERO SI

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MOTION 5A+ANCE *RINCI*+E :"0 controller which generates an output signal by motion of its parts".

The increase in input signal will cause the baffle to move towards the no99le. The no99leback pressure will increase. This increase in back pressure acting on the balancing bellows,will expands the bellows, there by moving the no99le upward. The no99le will move untillmotion almost/ e>uals the input baffle/ motion.A%,a$ta0es o# #orce 5aa$ce :8. 2oving parts are fever.&. Aaffle movement is negligible.. rictional losses are less.

AUTOMATIC CONTRO++ER :It is a device which measured the value of variable >uantity or condition and operates to

correct or lie it deviation of this measured value from a selected reference.AUTOMATIC CONTRO+ SSTEM :It is any operable arrangement of one or more automatic controllers in closed loops withone or more processes.

SE+( O*ERATED CONTRO++ER :It is one in which all the energy needed to operate the final control element is derived fromthe controlled medium through the primary element.

RE+A O*ERATED CONTRO++ER :It is one in which the energy transmitted through the primary element is eithersupplemented or amplified for operating the final control element by employing energyfrom another sources.

*ROCESS :0 process comprises the collective function performed in and by the e>uipment in which avariable is to be controlled.

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SE+( REuantity and condition which is measured and controlled.

CONTRO++ED MIDIUM :It is that process energy or material in which a variable is controlled. The controlledvariable is a condition or characteristic of the controlled medium. or e.g. wheretemperature of water in a tank is automatically controlled, the controlled variable istemperature and controlled medium is water.

MANI*U+ATED VARIA5+E :It is that >uantity or condition which is varied by the automatic controller so as to affectthe value of the controlled variable.

CONTRO+ A

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CONTRO+ *OINT :It is the value of the controlled variable which under any fixed set of conditions theautomatic controller operates to maintain.

D E ( I N A T I O N.

ACCURAC :

0 number or >uantity which defines the limit of error under reference conditions.

ATTENUATION :

0 decrease in signal magnitude between two points, or between two fre>uencies.

DEAD TIME :

The interval of time between initiation of an impact change or stimulus and the start of theresulting response.

DRI(T :

0s undesired change in output over a period of time, which change is unrelated to input,operating conditions, or load.

ERROR :

The difference between the indication and the true value of the measured signal.

S*AN ERROR :

It is the difference between the actual span and the specified span and is expressed as thepercent of specified span.

GERO ERROR :

It is the error of device operating under the specified conditions of use when the input is atthe lower range value.

STATIC

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The maximum difference between the upscale and downscale indications of the measuredsignal during a full range traverse for the same input.

INTER(ERENCE :

Interference is any spurious voltage or current arising from external sources and appearingin the circuits of a device.

COMMON MODE INTER(ERENCE :

It is the form of interference which appears between the measuring circuit terminals andground.

NORMA+ MODE INTER(ERENCE :

It is the form of interference which appears between measuring circuit terminals.

+INEARIT :

The closeness to which a curve approximate a straight line.

RANuantity is measured received or transmitted,expressed by stating the lower and upper range values.

RE*EATA5I+IT :

The closeness of agreement among a number of consecutive measurements of the outputfor the same value of the measured signal under the same operating conditions.

RE*RODUCI5I+IT :

The closeness of agreement among repeated measurements of the output for the samevalue of the input made under the same operating conditions.

RES*ONSE :

It is the general behavior of the output of a device as a function of input both with respectto time.

SI

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The time re>uired for the output to complete B.& $ of the total rise or decay.

S*AN :

The algebraic difference between upper and lower range values.

GERO S2I(T :0ny parallel shift of the input output curve.

* R E S S U R E.

*RESSURE CONVERSIONS :

1psi F "4.4/ 2"O1

H0Bc'"

F

1/."" psi

1 5ar F 1/.9/ psi

1 Hpa F 9.1/ psi1

H0Bc'"F

19.999'' o#

2"9

1 5ar F 1.9184H0Bc'"

1H0Bc'"

F

9.87 5ar

1 Torr F 1 '' o# 20.

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Qe. : Epai$ the )ori$0 o# a$ E$ra# e,e 0a0e !A$s. : The =nraf level precise level gauges are based on servo powered null balancetechni>ue. 0 displacer serves as continuos level sensing element.*ri$cipe :0 displacer 0 with a relative density higher than that of a product to be measured, issuspended from a stainless steal wire A tat is attached to a measuring drum. 0 two phaseservo motor controlled by a capacitive balance system winds unwinds the measuring wireuntil the tension on the weight springs is in balance with the wt. of the displace partimmersed in the li>uid. The sensing system in principle measures the two capacitanceformed by the moving central sensing rod = provided with two capacitor plates and the siplates. In balance position the capacitance are of e>uip value. 0 level variation will cause adifference in buoyancy of the displacer. The center sensing rod will move in to direction ofone of the side capacitor plates. This causes difference in value of this capacitance. Ay anelectrolyte rotation of the servo motors the can driven transmitter continuously change thevoltage pattern to remote indicate of which the receiver motor drives a counter indicatinglow variation.

*RIMAR (EED5ACH :It is the signal which is related to the reference input to obtain the actuating signal.

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It is that in which there is a continuos linear relation between the rate of change ofcontrolled variable and position of final control element. 4ate action produces valuemotion proportional to the rate of change of actual measurement.

*RO*ORTIONA+ 5AND :It is the range of values of the controlled variable which correspond to the full operatingrange of the final control element.

RESET RATE :It is the number of times+minute that the effect of proportional position action upon thefinal control element is repeated by proportional speed floating action.

There are two ways of expressing reset action :8. 4eset time and &. 4eset 4ate

1. Reset Rate : It is commonly expressed as a number of "repeats" per minute. It isdetermined by dividing.

a/ Travel of final control element Value stroke / in one minute as a result of the effect ofproportional speed floating action.b/ The travel as a result of the effect of proportional position action with the samedeviation in both cases.

". Reset Ti'e : It is the time interval by which the rate is commonly expressed in minutes.It is determined by subtracting.a/ The time re>uired for a selected motion of the final control element resulting fromcombined effect of the proportional position plus rate action.b/ The time re>uired for the same motion as a result of the effect of proportional positionaction alone with the same rate of change of controlled variable in both cases or expressed

in another way. It is the time lead in terms of air pressure on the control value produced byrate action compared with proportional position action for the same rate of change ofactual measurement in both cases.

E + E C T R O N I C S

Qe. : What is a %io%e!A$s. : 0 diode consists of two electrodes 8/ 0node &/ 'athode. The current flow is onlyin one direction.0 diode is the most basic solid state semi conductor/ device. The above figure shows a!.-. Gunction. The !. material has holes and the -. material has electrons.

(ORWARD 5IAS :

REVERSE 5IAS :

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8 / here the applied voltage overcomes the barrier potential the p side is more positivethan the n side/ the current produce is large because maGority carriers cross the Gunction inlarge numbers. This condition is called forward bias. & / hen the applied voltage aids the barrier potential n side K ve than p side/ thecurrent in small. This state is known as 4everse Aias.

Qe. : What is a ha# )a,e; # )a,e a$% bri%0e recti#ier !

A$s. :2A+( WAVE RECTI(IER :

(U++ WAVE RECTI(IER :

5RID

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Auffer : 0 device or a circuit used to isolate two ptFne circuits or stages. The emitterfollows is a example of buffer.0lpha 1 I' + I=Aeta 1 I' + IA

Qe. : 2o) )i -o test a tra$sistor )ith a 'ti'eter. !

0ns. :8. =mitter Kve of meter and Aase *ve output 1 ow resistance&. =mitter *ve of meter and base Kve output 1 (igh resistance.. 'ollector Kve and Aase *ve output 1 ow.6. 'ollector *ve and base Kve output 1 (igh.=mitter : 'ollector 1 (igh 4esistance.*N* :%pposite 4esults.

Qe. : What is a th-ristor ! What are its ses !0ns. : 0 thyristor is a special kind of semi conductor device that uses internal feedback toproduce latching action.Use : Dsed for controlling large amounts of load power in motors, heaters, lightingsystems etc.Epa$atio$ :Aecause of the unusual connection we have a Kve feedback also calledregeneration. 0 change in current at any point in the loop is amplified and returned to thestarting point with the same phase. or instance if the 5& base current increases, the 5&collector current increases. This force base current through 58. In turn this produces alarge 58 collector current which drives the 5& base harder. This build up in currents willcontinue until both transistors are driven in saturation. In this case the latch acts like aclosed switch.

%n the other hand , if something causes the 5& base current to decrease, the 5& collector

current will decrease. This reduces the 58 base current. In turn, there is less 58 collectorcurrent, which reduces the 58 base current even more. This regeneration continues untilboth transistors are driven into cut off. 0t this time the latches like a open switch. Thislatch will always stay in open or close position.

Qe. : What are o0ic 0ates ! Epai$ )ith trth tabe.0ns. :0T= : 0 gate is a logic circuit with one output and one or more inputs . 0n output signaloccurs only for control combination of input signals.

1. OR @

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TRUT2TA5+E :

0 A N

5 5 85 8 88 5 88 8 5

.Qe. : Epai$ the )ori$0 o# TI+ @ NAND