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UNIT - 4
1. Explain filled system thermometers with its types
Filled system thermometers are classified into 4 types.
1. Gas filled thermometers
2. liquid filled thermometers
3. mercury filled thermometers
4. vapor pressure thermometers
A thermometer consists of thermometer bulb, capillary tube, bourdon tube. All are
interconnected. The fluids used are methyl alcohol, mercury, and toluene. When we
measure it thermometer bulb should be placed inside the fluid. This causes the filling
fluid inside the bulb to heat or cool until the temperature matches the temperature of the
measured substance. This change in temperature causes the bourdon tube to expand or
contract. The movement of the bourdon tube becomes the measure of temperature.
1. Gas filled thermometers:
Here a certain amount of inert gases is enclosed in the bulb, capillary and bourdon tube.
Due to this the pressure indicated by the bourdon tube may be calibrated in terms of the
temperature of the bulb.
The most commonly used gas is nitrogen because of its inexpensive. In certain cases the
gas will react with the bulb in such conditions helium will be used.
2. Liquid filled thermometers:
The filling fluid used here is inert hydrocarbon such as xylene. The thermometer is
completely filled with water. The principle used is liquid expansion with an increase of
temperature.
3. Mercury filled thermometers:
Mercury is a liquid and in this respect a mercury filled thermometers is similar to the
liquid filled thermometers. Mercury provides rapid response, accuracy, plenty of power
for operating control system.
4. Vapor pressure thermometers:
In this system the bulb is partially filled with liquid , while the capillary and bourdon
tubes are filled with vapor. When we measure it thermometer bulb should be placed
inside the fluid. This causes the filling fluid inside the bulb to heat or cool until the
temperature matches the temperature of the measured substance. This change in
temperature causes the bourdon tube to expand or contract. The movement of the
bourdon tube becomes the measure of temperature.
2. What are the possible sources of errors in filled system thermometers and how it is
compensated?
Errors are,
1. Ambient temperature effect:
2. Head or Elevation effect
3. Barometric effect:
4. Immersion effect
5. Radiation effect:
1. Ambient temperature effect:
The change of ambient temperature causes volume changes in the capillary tube and
bourdon tube thereby causing error in measurement.
Compensation:
Compensation is done by using bimetallic material in which bimetallic strip deflects to
compensate for movement of the bourdon tube due to change of filling fluid temperature
2. Head or Elevation effect:
If the thermometer bulb is placed at a different height compared to bourdon tube
elevation error occurs.
Compensation:
In order to avoid error the filling of fluid is done at a high pressure compared with the
height of the bulb.
3. Barometric effect:
The effect due to the change in atmospheric pressure is called as barometric effect.
Compensation:
This error may be avoided by keeping the filled system at a pressure sufficiently larger
than atmospheric pressure.
4. Immersion effect:
If the bulb is not properly immersed or fully immersed and the head is not properly
insulated, heat from the bulb is lost due to conduction. This causes the immersion error.
Compensation:
Correct temperature is measured if the free surface of liquid in the bulb is in good thermal
contact with fluid whose temperature is to be measured.
5. Radiation effect:
Radiation errors occur due to temperature difference between the bulb and other solid
bodies around.
Compensation:
A radiation shield is used around the bulb to minimize this error.
3. Explain in detail about RTD and its characteristics
RTD:
RTD means resistance thermometer detector. It is also called as electrical resistance
thermometer. It is a semiconductor device which has positive temperature coefficient of
resistance.
Principle:
When RTD is subjected to temperature change its resistance changes and it can be
measured using bridge circuit. This change in resistance becomes a measure of
temperature.
Characteristics:
Variation of resistance R with temperature T is given by,
R = Ro (1 + α1T + α2T2 + . . . .)
Ro = resistance at temperature T =0.
α = constants.
For most metals, quadratic relation gives good accuracy
R = Ro (1 + α1T + α2T2 )
4. Explain in detail about thermistors and its characteristics
Thermistors:
It is a semiconductor device which has negative temperature coefficient of resistance.
Resistance decreases with increase in temperature. It is made up of specific mixture of
pure oxides of nickel, cobalt, copper, iron. The thermistors may be in the form of beads,
rods, discs, and probe.
Principle:
When thermistors are subjected to temperature change its resistance changes and it can be
measured using bridge circuit. This change in resistance becomes a measure of
temperature.
Characteristics:
The relationship is given by,
RT1 = RT2 exp ( β (1/ T1 – 1 /T2) )
Where RT1 = resistance at temperature T1
RT2 = resistance at temperature T2.
β = temperature coefficient of resistance
Resistance temperature characteristics:
Here the resistance will decrease with increase in temperature because it is a negative
temperature coefficient of resistance.
Voltage current characteristics:
Here the current will increase with increase in voltage upto a certain limit and after that
it will decrease.
Current time characteristics:
It is the time taken for the current to reach a maximum value with increase in voltage.
5. How temperature scale has been standardized? What are the fixed points and how they
are used in temperature standards?
Temperature of a substance is a measure of hotness or coldness of that substance.
It is the thermal state of a body which determines whether it will give heat or receive heat
from other bodies
Temperatures scales are based on some fixed points.2 fixed points are required
1. the lower fixed point (or) ice point
2. the upper fixed point (or)steam point
The ice point is the temperature of ice, prepared from distilled water when melting under
a pressure of 760 mm of mercury.
The steam point is the temperature of steam, prepared from distilled water when boiling
under a pressure of 760 mm of mercury.
The temperature interval between the ice point and steam point is known as the
fundamental interval.
Boiling point:
Boiling point is the temperature at which the substance changes from physical state and
becomes a gas.
Freezing point:
Freezing point is the temperature at which the substance changes from physical state and
becomes a solid.
Triple point:
At a particular temperature and pressure at which three different phases of one substance
can exist in equilibrium is known as triple point.
6. Explain bimetallic thermometer and its types
Principle:
 All metals expand or contract with change in temperature
 The temperature coefficient of expansion is not same for all metals and therefore
their rates of expansion or contraction are different
It consists of a bimetallic strip which is constructed by bonding together 2 thin strips of 2
different metals such that they cannot move relative to each other. Bimetallic strip is in
the form of a straight cantilever beam. With one end fixed the temperature changes
because the free end to deflect.
The deflection of the free end is directly proportional to the temperature change and
square of the length of strip and inversely proportional to the thickness.
Types:
1. Spiral
2. Helix
Figure shows the bimetallic strip wound in the form of a helix and spiral. One end of the
helix is fastened to the causing of the bulb and other end is connected to the pointer. The
pointer sweeps over a circular dial to indicate the temperature. Usually it is mounted in
wells to provide protection against wear and corrosion.
Advantages:



Simple ,inexpensive
Withstand about 50% over ranges in temperature.
Accuracy +1%
Disadvantages:
 Not recommended above 400 0 c
 Errors will occur
Application:



Used in control system
Used in air conditioning
Used in refineries, oil burners, tyre vulcanisers.
7. Discuss the electrical methods of measuring temperature
Temperature can be electrically measured using
1. RTD
2. Thermistor.
RTD:
RTD means resistance thermometer detector. It is also called as electrical resistance
thermometer. It is a semiconductor device which has positive temperature coefficient of
resistance.
Principle:
When RTD is subjected to temperature change its resistance changes and it can be
measured using bridge circuit. This change in resistance becomes a measure of
temperature.
Characteristics:
Variation of resistance R with temperature T is given by,
R = Ro (1 + α1T + α2T2 + . . . .)
Ro = resistance at temperature T =0.
α = constants.
For most metals, quadratic relation gives good accuracy
R = Ro (1 + α1T + α2T2 )
Thermistor:
It is a semiconductor device which has negative temperature coefficient of resistance.
Resistance decreases with increase in temperature. It is made up of specific mixture of
pure oxides of nickel, cobalt, copper, iron. The thermistor may be in the form of beads,
rods, discs, and probe.
Principle:
When thermistor is subjected to temperature change its resistance changes and it can be
measured using bridge circuit. This change in resistance becomes a measure of
temperature.
Characteristics:
The relationship is given by,
RT1 = RT2 exp ( β (1/ T1 – 1 /T2) )
Where RT1 = resistance at temperature T1
RT2 = resistance at temperature T2.
β = temperature coefficient of resistance
8. Describe the construction and working of 3 wires and 4 wire RTDs.
9. Explain the different types of expansion thermometers
It can be classified according to the nature of substance which expands.
1. Expansion of solids
(a). Bimetallic thermometers
2. Expansion of liquids
(a) Liquid in glass thermometers
(b) Liquid in metal thermometers
3. Expansion of gases:
(a) Gas thermometers.
(a) Bimetallic thermometers:
Principle:
 All metals expand or contract with change in temperature
 The temperature coefficient of expansion is not same for all metals and therefore
their rates of expansion or contraction are different
It consists of a bimetallic strip which is constructed by bonding together 2 thin strips of 2
different metals such that they cannot move relative to each other. Bimetallic strip is in
the form of a straight cantilever beam. With one end fixed the temperature changes cause
the free end to deflect.
The deflection of the free end is directly proportional to the temperature change and
square of the length of strip and inversely proportional to the thickness.
(b) Liquid in glass thermometers:
The filling fluid used here is inert hydrocarbon such as xylene. The thermometer is
completely filled with water. The principle used is liquid expansion with an increase of
temperature. A thermometer consists of thermometer bulb, capillary tube, bourdon tube.
All are interconnected. The fluids used are methyl alcohol, mercury, and toluene. When
we measure it thermometer bulb should be placed inside the fluid. This change in
temperature causes the bourdon tube to expand or contract. The movement of the
bourdon tube becomes the measure of temperature
(c) Liquid in metal thermometers:
In this system the bulb is partially filled with liquid, while the capillary and bourdon
tubes are filled with vapor. When we measure it thermometer bulb should be placed
inside the fluid. This causes the filling fluid inside the bulb to heat or cool until the
temperature matches the temperature of the measured substance. This change in
temperature causes the bourdon tube to expand or contract. The movement of the
bourdon tube becomes the measure of temperature.
(d) Gas thermometers:
Here a certain amount of inert gases is enclosed in the bulb, capillary and bourdon tube.
Due to this the pressure indicated by the bourdon tube may be calibrated in terms of the
temperature of the bulb. The most commonly used gas is nitrogen because of its
inexpensive. In certain cases the gas will react with the bulb in such conditions helium
will be used.
10. Discuss the various types of vacuum pressure measurement
Viscosity gauge:
Viscosity of a gas is directly proportional to its pressure. One way is to measure torque
required to rotate at constant speed. The rotation is produced by an electric motor rotating
at a constant speed of 3000 rpm.
Viscosity of gas is independent of pressure of the gas if the gas is greater than 1 torr.
While measuring pressure above 1 torr bladed wheels are used in place of smooth
cylinders. These wheels cause a momentum exchange which is dependent upon pressure.
Advantages:



Simple ,inexpensive
Withstand about 50% over ranges in temperature.
Accuracy +1%
Disadvantages:
 Not recommended above 400 0 c
 Errors will occur