13 What will happen if the displacer hasfallen downwhile in line?
The output will be minimum. (wrong)
The output will be maximum (100 %) true.
14 What will happen if the displacer has a hole in it while in line?
The output will be maximum.
15 What is the used of Suppression and elevation?
Suppression and elevation are used on Level applications where (1) transmitters are not mountedon some level (2) Wetleq. i.e. condesable vapars are present.
16 Whatare the limitations of level trol ?
The limitations of a level control that it cannot be used for lengths more than 72".
17 How will you commission D.P.transmitter in field in pressurized vessel.
1. Close both the isolationvalves, Vent the H.P. side.
2. Fill it with the sealing liquid.
3. Open the L.P. side vent valve.
4. Adjust zero with suprestion spring.
5. Close the L.P. side vent valve.
6. Open both the isolation valves.
18 How will you check zero of a level D.P. transmitter while is line?
1. Close both the isolationvalves.
2. Open the vent valve on L.P. leg and H.P. leg drain.
3. Check and adjust zero if necessary.
19 Explain the working of an Enraf level gauge?
The Enraf level preciselevel gauges are based on ser powerednull balance technique. A displacer serves as continous level censingelement.
Principle : A displacer A with a relative density higher than that of a product to be measured, is suspended from a stainless steal wire B tat is attached to a measming drum. A two phase ser motor controlled by a capacitive balance systemwinds unwinds the measuring wire untilthe tension un the weight springs is in balance with the wt. ofthe displace part immersed in the liquid.The sensing system in principle measures the two capacitance formed by the moving centralsensing rod E providedwith two capaciter platesand the si plates. In balance positionthe capacitence are of equip value. A level variation will cause a differencein buyoan of the displacer. The centre sensing rod will move in to direction of one of the side capaciter plates. This causesdifference in value of this capacitance. By an electrolyte rotation of the servo motors the can driventransmitter continously change the veltagepattern to remote indicate of which the receivermotor drives a counter indicating low variation.
TEMPRETURE
1 What are the different methods oftempreture measurement? Explain.
The differentmethods of tempreture measurement are :
1. MECHANICAL 2. ELECTRICAL.
MECHANICAL METHOD:
1. Mercury in glass thermometers: This consistsof a glass tube of very fine bore joined to a veservoier at the bottom and sealed at the top. A measuredquantity of mercury is the enclosed. When the thermometer is heated the mercuryexpands much more than the glass and is therforeforced to rise up in the tubing A scale is fixed at the side.
2. Bimettalic Thermometer : Two metals whosecoefficient of linear expansion is different are welded and rolled together to the desirethickness. The actual movement of a bimetal is its flexivity with one and fixed,a straight bimetal stripdeflects in propertion to its tempreature, to the square of its lengthand inversely with its thikness.
Pressure Spring Thermometers:
There are four classes of pressure spring thermometers.
1. Liquid filled. 2. Vapour pressure. 3. Gas filled. 4. mercury filled.
Liquid filled & Mercury filled: Both type, operate on the principleof thermal expansion. Where the bulb isimmersed in a heated substance. The liquid expands causing the pressure spring to unwind. The indicating, recording or controlling mechanisms are attachedto pressure spring.
Compensated Thermometer System : Compensations are provided inorder to nulity the effect of changes in ambient tempreature. The compensation in liquid filled expansions thermal system consists of the second tubing and belical element, both liquid filled. The two elements are so constructed that the measuring belical floatson a movable base the position of which is governedby the compensating belical. The two tubing and belicals are matched in volume so that variationin tempreature at the instrumentcase and along the capillary tubingproduce equal motion from both belicals. Such motion nullity each other so that only motion producedby varying the bulb tempreature actuals the recorderpen.
Gas filled Thermometers: This type depends upon the increasein pressure of a confirm gas (constant volume) due to temp. increase. The relate between temp. and pressure in thiskind of system follow charles low and may be expressed. P/P 2
The system is filled under high pressure. The increase pressure for each degree of tempreature rise is therefore greater than if thefilling pressure were low. Nitrogenthe gas most after used for such systems, because it chemically insert and posseses a favourable coefficient thermal expansion.
Vapour - Pressure Thermometers: Vapour pressurethermometers depend upon vapour pressureofliquid which onlypartially fillsthe system. At low tempreatures the vapour pressure increase for each unit tempreature charge is small, at higher tempreature the vapour pressurechange is much greater.
ELECTRICAL METHOD OF TEMPERATURE MEASUREMENT:
1. THERMOCOUPLES: It is a simpledevice consisting of a dissimilar metal wires joined at their ends. when an of each wire is connected to a measuring instrument thermocouples becomes an accurateand sentive tempreture measuringdevice.
Type of Thermocouples :
1. Iron - Constantan (J) -- 0 - 800'C.
2. ChromelAlumel (K) -- 0 - 1000'C.
3. Platinum - Platinum % Rhadium -- 0 - l500'C.
4. Platinum - Rhadium 13% (S) -- 0 - l700'C.
5. Chromel - Constantan (E) -- 0 -700'C.
6. Copper - Constantan (T) -- 0 - 400'C.
RESISTANCE - TEMPERATURE DETECTERS (RTD) :
RTD's are generally used for precise temperature measurement. It consists of a five wire wrappedaround an insulator and enclosed in a metal. The most shea of a resistance thermometer resembles that of bimettalic thermometer bulb.
PRINCIPLE: "Resistance increases as temperature increase" Rt. = Ro (1 + at)
Rt. = Resistance of Temperature to measured.
Ro. = Resistance of zero temperature. a = Co. off of thermal (expansion). t = Temperature to be measured.
These metals have a positivetemperature co-officient of expansion. Therefore resistance increases as the
temperature increases.
CALCULATION OF RESISTANCEFORPtlOO.
R, = 100
a for platinum = 0.00385 Ic.
To calculateResistance at 100'c.
RI00 =100[1+(38.5 xl0 4 xl00)]
= 100 + (100 x 0.385) RI00 = 138.5
Resistance at lOO°C= 138.5
2 What is Pt 100 mean. ?
Ptl00 means 100 OHMS at O'C for a platinum resistance bulb.
3 What is two wire and three wire R. T.D. system?
Two wire R .T .D. system: Two wire RTD systemuse for short distance coke a ampressor field local panel.
Three wire System: Three wire system use for long distance coke a field to control Run. The third wire is used for compensation of lead wire resistance.
4 Drawa polentiometric temp. measuring circuits and explain its. ?
OPERATION: The input to the instrument is a measurement of some in the procers using a sensing element ( such as thermocol ) or a deviceto produce direct voltage, which is the voltage (sinnal).This voltage is subtracted from a voltage developed by a known constant voltagein a poteneiome measuring circuit. The subtraction occurs by connecting two voltages in series with theopposing potarity, differance between these two voltagesproduces signal, the voltage going to the amplifier. The error will positive or negative depending on which of the two voltages greater. When amplied, the error signal will drive servo balancing motor in appropriate direction to adjust circuit ( actually drive the slide wire ) until the difference between thefeedback voltage and the input voltage is balance out. An error signalequal to zero results( null point) the balancing ( servo motor is be longer driven)
5 Whatis the constant voltage unit?
The constant voltagecircuit consists of a rectifier, CR, a filter capaciter Cl , followedby two stages of zener regulation. Abridge configuration is provided to lamp linevoltage regulation zener CR3, Rl and R2 combine provide relatively constantcurrent to zener CR4, Thus variations. Resisters R2 and R3 form a bridge that any remoment line voltage effects.
6 Explain the working of a balancing motor.
Signal in control win appears as due tank circuit formed by win and capaciter of amp. board.
+ve signal: It lags 90' from due to line phase capacitor amp. board.
-ve signal : Leads 90' from line due to line phase capacitor of amp. board.
The servo (balancing) motor is an induction motorthat functions bu creating a roting magneticfield in the stator.The rotor (armature) turns by following this field. The field is developed by the use of two windings in the stator.
It has got two windings, one of which is continously energized by the line voltage.The other winding is energized by the power emplifier, with a current whose phase with respect to line current determines the direction of rotation of motor.
7 What is burnout feature ?Explain.
Burnout provides the warnsug feature of driving indicator the end of scale if the input circuit should open.
A burnout resistor is provided which develops a voltage dropbetwen the measuring circuit and the emplifier. The polarityof the signal determines the direction of the servo drive upon an open circuit in the input.
Upscale burnout : R value 10M Downscale burnout: R value 2.2 M
8 Why is a converter used in a temp. recorder?
The converter is designed to convert D. C. input voltage intoan A. C. input voltage propertional in amplitube to the input.
9 Why are thermowells used?
In numerous application it is neitherdesirable nor proacticalto expase a temperature sensor directly to a process i = material Wells are thereforeused toprotect against damage corression, arosion, abrosion and
high pressureprocesses. A thermowell is also usefulin protecting a sensor from physical damageduring handling and normal operation.
Selecting a thermowell :
The significant properties considered in nelecting a material for the well are as follows:
1. Resistance to corrossion and oxidation.
2. Resistanceto mechanical and thermal shock.
3. Low permeability ( Resistance to gas leakage ).
4. Mechanical strength.
5. Thermal conductivity. Material for Wells:
1. Stainless steal.
2.Inconel.
3. Monel.
4. Alloy steal.
5. Hastelloy'C'.
9 How will you calibrate a temp. recorder using a potentiometer ?
Connect the potentiometer output the input of temp.recorder.
1. Connect the ( +ve ) to the ( +ve ) and ( -ve ) to the ( -ve ).
2. If ambient conpensation is provided in potentiometer set it to the correct ambienttemp.
3. If no ambient compensation is provided take a thermometer and measure the correctambient temp .. Findout the corrosponding m v s for that temp. for the given input type of thermocouple.
4. While feedingsubstract the ambienttemp. m v s from the corrosponding temp. everytime.
5. Adjust the necessaryadjustments.
MEASURING TEMPERATURE WITH A POTENTIOMETER:
1. Connect the input of the potentiometer to the thermocouple.
2. If no ambient compensation is provided find out the corrosponding millivolts for that ambienttemp. for the type of thermocouple used.
3. Add the ambienttemp. millivolts to the corr. input millivolts measured. Find out from the chart the corrosponding temperature.
10 Whattype of sensing element would you use to measure verylow temperature?
The sensingelement used for measuring very low temperature is R. T. D.
11 What are skin temperature thermocouples?
Skin thermocouples are those which are directlyconnected to the process withoutany thermowell. Used for measuring the skin temperature of heaters furnaces, flue gas etc.
12 Whatis the speciality of thermocouples lead wires?
They should be of the same material as the thermocouple.
13 What is the difference the a wheatstone bridge and a potentiometer?
The difference between a potentiometer and a wheatstone bridge measureing instrument is that potentiameter is a voltage measuring instrument and wheatstone bridgeis a current measuring instrument.
14 Explain the continous balance potentiometer system using R T. D. 'so
In a balance wheatstone bridge resistance thermometer a resistance bulb is connectedinto one branch of a d.c. bridge circuit; in another branch is a variable resistence in the form of a calibrated slidewire. Variations in temp. of the measured meduimcause a changein resistance of the bulb and a consequent unbalance of the bridge circuit. A self balancing wheatstone bridge recognises the condition of unbalance, determines its directionand magnitude and position the slidewire contractor to rebalance the bridge and indicate the temp. on the scale.
The D. C. potentioal appearing at AA is converted by the converting stage to an A. C.voltage appearing at BB and is maltiplied by the voltageamplifier to a large value at cc. It is then used to control the power amplifier output DD which drives the balancing motor in the proper directionto balance the bridge.
The polarity of the signal at AA determines the phase of the alternating voltage at BB which in turn determines the directionof rotation of the balancingmotor.
15 Howis automatic Referencejunction compensation carried out in temp. ? Recorders ect.
For automatic reference junction compensation a variable nickel resister is used. As the temperature changes, so does its resistance. This reference junction compensator is located, so that it will be at the tempreature of the reference junction. The reference junction is at the poset where the dissimilar wire of the termocouple is rejoined, which invariably is at the terminal strip of the instrument.
