TEMPERATURE

Que. :  What are the different methods of temperature measurement ? Explain.

Ans. :  The different methods of temperature measurement are :

1. MECHANICAL     2. ELECTRICAL.

MECHANICAL METHOD :

1. Mercury in glass thermometers : This consists of a glass tube of very fine bore joined to a reservoir at the bottom and sealed at the top. A measured quantity of mercury is the enclosed. When the thermometer is heated the mercury expands much more than the glass and is therefore forced to rise up in the tubing A scale is fixed at the side.

2. Bimetallic Thermometer : Two metals whose coefficient of linear expansion is different are welded and rolled together to the desire thickness. The actual movement of a bimetal is its flexivity with one end fixed, a straight bimetal strip deflects in proportion to its temperature, to the square of its length and inversely with its thickens.            

TEMPERATURE GAUGE

Pressure Spring Thermometers :

There are four classes of pressure spring thermometers.

1. Liquid filled.

2. Vapor pressure.

3. Gas filled.

4. mercury filled.

Liquid filled & Mercury filled :

Both type, operate on the principle of thermal expansion. Where the bulb is immersed in a heated substance. The liquid expands causing the pressure spring to unwind. The indicating, recording or controlling mechanisms are attached to pressure spring.

TEMPERATURE SWITCH WITH GAUGE

Compensated Thermometer System :

Compensations are provided in order to nullify the effect of changes in ambient temperature. The compensation in liquid filled expansions thermal system consists of the second tubing and helical element, both liquid filled. The two elements are so constructed that the measuring helical floats on a movable base the position of which is governed by the compensating helical. The two tubing and helicals are matched in volume so that variation in temperature at the instrument case and along the capillary tubing produce equal motion from both helicals. Such motion nullity each other so that only motion produced by varying the bulb temperature actuates the recorder pen.

Gas filled Thermometers :

This type depends upon the increase in pressure of a confirm gas (constant volume) due to temp. increase. The relate between temp. and pressure in this kind of system follow Charles law and may be expressed.

P1

P2

The system is filled under high pressure. The increase pressure for each degree of temperature rise is therefore greater than if the filling pressure were low. Nitrogen the gas most after used for such systems, because it chemically insert and possesses a favorable coefficient thermal expansion.

Vapor - Pressure Thermometers :

Vapor pressure thermometers depend upon vapor pressure of liquid which only partially fills the system. At low temperatures the vapor pressure increase for each unit temperature charge is small, at higher temperature the vapor pressure change is much greater.

ELECTRICAL METHOD OF TEMPERATURE MEASUREMENT :

1. THERMOCOUPLES : It is a simple device consisting of a dissimilar metal wires joined at their ends. when an of each wire is connected to a measuring instrument thermocouples becomes an accurate and sensitive temperature measuring device.

Type of Thermocouples :

1. Iron - Constantan (J) ---- 0 - 800'C.

2. Chromel Alumel ( K ) ---- 0 - 1000'C.

3. Platinum - Platinum % Rhodium ---- 0 - 1500'C.

4. Platinum - Rhodium 13% (S) ---- 0 - 1700'C.

5. Chromel - Constantan (E) ---- 0 -700'C.

6. Copper - Constantan (T) ---- 0 - 400'C.

THERMOCOUPLES

RESISTANCE - TEMPERATURE DETECTERS (RTD) :

RTD's 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 resistance thermometer resembles that of bimetallic thermometer bulb.

PRINCIPLE : "Resistance increases as temperature increase"

Rt. = Ro (1 + xt)

Rt. = Resistance of Temperature to measured.

Ro. = Resistance of zero temperature.

x     = Co. off  of thermal (expansion).

t      = Temperature to be measured.

These metals have a positive temperature co-efficient of expansion. Therefore resistance increases as the temperature  increases.

TEMPERATURE SWITCH

CALCULATION OF RESISTANCE FOR Pt100.

 Ro.      = 100

x for platinum  = 0.00385 /c.

To calculate Resistance at 100'c.

R100  = 100 [ 1+ ( 38.5  x 10  4  x 100 ) ]

           = 100 + (100  x  0.385)

R100  = 138.5

Resistance at 100'c = 138.5 

Que. :  What is Pt 100 mean. ?

Ans. :  Pt100 means 100 OHMS at 0'C for a platinum resistance bulb.

Que. :  What is two wire and three wire R.T.D. system ?

Ans. :  Two wire R .T .D. system :

Two wire RTD system use for short distance like a compressor 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.

Que. :Draw a potentiometric temp. measuring circuits and explain its . ?

Ans. : 

OPERATION :

The input to the instrument is a measurement of some in the processes using a sensing element ( such as thermocol ) or a device to produce direct voltage, which is the voltage (signal). This voltage is subtracted from a voltage developed by a known constant voltage in a potentiometric measuring circuit. The subtraction occurs by connecting two voltages in series with the opposing polarity, difference between these two voltages produces signal, the voltage going to the amplifier. The error will positive or negative depending on which of the two voltages greater. When amplified, the error signal will drive servo balancing motor in appropriate direction to adjust circuit ( actually drive the slide wire ) until the difference between the feedback voltage and the input voltage is balance out. An error signal equal to zero results ( null point ) the balancing ( servo motor is be longer driven )

Que. :  What is the constant voltage unit ?

Ans. :  The constant voltage circuit consists of a rectifier, CR, a filter capacitor C1, followed by two stages of zener regulation. Abridge configuration is provided to 1amp line voltage regulation zener CR3, R1 and R2 combine provide relatively constant current to zener CR4, Thus variations. Resisters R2 and R3 form a bridge that any remoment line voltage effects.

Que. :  Explain the working of a balancing motor.

Ans. :  Signal in control win appears as due tank circuit formed by win  and capacitor 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 motor that functions by creating a rotating magnetic field 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 continuously energized by the line voltage. The other winding is energized by the power amplifier, with a current whose phase with respect to line current determines the direction of rotation of motor.

Que. :  What is burnout feature ? Explain.

Ans. :  Burnout provides the warning feature of driving indicator the end of scale if the input circuit should open.

A burnout resistor is provided which develops a voltage drop between the measuring circuit and the amplifier. The polarity of the signal determines the direction of the servo drive upon an open circuit in the input.

Upscale burnout         : R value 10 M

Downscale burnout : R value 2.2 M

Que. :   Why is a converter used in a temp. recorder ?

Ans. :   The converter is designed to convert D. C. input voltage into an A. C. input voltage proportional in amplitude to the input.

Que. :   Why are thermowells used ?

Ans. :   In numerous application it is neither desirable nor practical to expose a temperature sensor directly to a process material. Wells are therefore used to protect against damage corrosion, errosion, abrasion and high pressure processes. A thermowell is also useful in protecting a sensor from physical damage during handling and normal operation.

Selecting a thermowell :

The significant properties considered in selecting a material for the well are as follows :

1. Resistance to corrosion and oxidation.

2. Resistance to 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'.

Que. :  How will you calibrate a temp. recorder using a potentiometer ?

Ans. :  Connect the potentiometer output the input of temp. recorder.

1. Connect the ( +ve ) to the ( +ve ) and ( -ve ) to the ( -ve ).

2. If ambient compensation is provided in potentiometer set it to the correct ambient temp.

3. If no ambient compensation is provided take a thermometer and measure the correct ambient temp.. Find out the corresponding mV s for that temp. for the given input type of thermocouple.

4. While feeding subtract the ambient temp. m v s from the corresponding temp. every time.

5. Adjust the necessary adjustments.

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 corresponding millivolts for that ambient temp. for the type of thermocouple used.

3. Add the ambient temp. millivolts to the corr. input millivolts measured. Find out from the chart the corresponding temperature.

Que. :  What type of sensing element would you use to measure very low temperature ?

Ans. :  The sensing element used for measuring very low temperature is R. T. D.

Que. :  What are skin temperature thermocouples ?

Ans. :  Skin thermocouples are those which are directly connected to the process without any thermowell.

            Used for measuring the skin temperature of heaters furnaces, flue gas etc.

Que. :  What is the specialty of thermocouples lead wires ?

Ans. :  They should be of the same material as the thermocouple.

Que. :  What is the difference the a wheatstone bridge and a potentiometer ?

Ans. :  The difference between a potentiometer and a wheatstone bridge measuring instrument is that potentiometer is a voltage measuring instrument and wheatstone bridge is a current measuring instrument.

Que. :  Explain the continuos balance potentiometer system using R. T. D.'s.

Ans. :  In a balance wheatstone bridge resistance thermometer a resistance bulb is connected into one branch of a d.c. bridge circuit; in another branch is a variable resistance in the form of a calibrated slidewire. Variations in temp. of the measured medium cause a change in resistance of the bulb and a consequent unbalance of the bridge circuit. A self balancing wheatstone bridge recognizes the condition of unbalance, determines its direction and magnitude and position the slidewire contractor to rebalance the  bridge and indicate the temp. on the scale.

The D. C. potential appearing at AA is converted by the converting stage to an A. C. voltage appearing at BB and is multiplied by the voltage amplifier 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 direction to balance the bridge.

The polarity of the signal at AA determines the phase of the alternating voltage at BB which in turn determines the direction of rotation of the balancing motor.

Que. :  How is automatic Reference junction compensation carried out in temp. ? Recorders etc,.

Ans. :  For automatic reference junction compensation a variable nickel resister is used. As the temperature changes, so does its resistance. This reference junction compensatory is located, so that it will be at the temperature of the reference junction. The reference junction is at the position where the dissimilar wire of the thermocouple is rejoined, which invariably is at the terminal strip of the instrument