TEMPERATURE MEASUREMENT

TEMPERATURE

• Fundamental parameter use to control the plant
• Measure of hotness or coldness of the body
• Temperature represents the thermal state of the substance
• Indicates whether the substance gain heat or loss heat from other bodies
• Measure of degree of heat

Scales

• Fahrenheit (F)
• Celsius of centigrade (C)
• Rankine (R)
• Kelvin (K)

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Temperature and heat flow are related by second law of thermodynamics

Scale conversion

Category of Temperature Measuring Devices

• Expansion of a material to give visual indication
• Electrical resistance change
• Semiconductor characteristic change
• Voltage generated by dissimilar metals
• Radiated energy

Temperature measurement methods

• Nonelectrical methods
• Electrical methods
• Thermoelectric methods
• Pyrometers

Nonelectrical methods

• Nonelectrical methods are described as follows
• Expansion of liquids - for example mercury in glass thermometer, liquid in glass thermometer
• Expansion of gases - for example gas thermometer
• Expansion of solids - for example bimetallic thermometer

Liquid in glass thermometer

• Consists of small bore graduated glass tube
• Bulb containing mercury at the bottom
• Heat is transferred through the bulb to the mercury
• Mercury expands pushing the column of the mercury to higher value in the capillary
• Temperature scale graduated indicates the temperature of the body

Liquid in glass thermometer

• Liquid in glass thermometer has the same principle as the mercury thermometer
• Liquids used have similar properties to mercury i.e., high linear coefficient of expansion, clearly visible, nonwetting and nontoxic
• Liquid in glass thermometer is used to replace mercury thermometer and to expand its operating range
• These thermometers give accurate values
• Liquids used are: xylene, ether and alcohol

Bimetallic thermometer

• The principle is that metals are pliable and they have different coefficients of thermal expansion
• Two strips of dissimilar metals such as brass and invar (copper – nickel alloy) are welded together
• Change in length of metals as the temperature changes
• Bimetallic strips are usually configured as a helix for compactness and used with a pointer to indicate the temperature

Bimetallic thermometer

• Different metals have different coefficient of the thermal expansion
• The bimetals expands to the different lengths as the temperature rises
• The bimetallic strip bends towards the side of the low coefficient of thermal expansion
• One end of the bimetallic strip is fixed
• Displacement of the other end of the bimetallic strip is proportional to the total change in temperature

Bimetallic thermometer

• Displacement of the bimetallic strip is used to deflect the pointer over a calibrated scale to indicate the temperature
• Bimetallic thermometer is relatively slow to respond and less accurate
• It has hysteresis and drift
• It is tough and less costly

PRESSURE THERMOMETER

• The pressure spring device has a metal bulb made with low coefficient of expansion material with long metal tube
• Bulb contain the material with a high coefficient of expansion
• Metal tube is terminated with a helix bourdon tube pressure gauge
• As temperature in bulb increases, the vapor pressure in the system rises

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PRESSURE THERMOMETER

• Vapor pressure rise being proportional to the temperature change
• Change in pressure is sensed by the bourdon tube coils which deflects the pointer to indicate temperature
• There are three types of pressure spring devices
• Liquid filled
• Vapor pressure
• Gas filled

PRESSURE THERMOMETER

• Liquid filled thermometer works on the same principle as the liquid in glass thermometer
• The device has good accuracy
• Vapor pressure thermometer system is partially filled with liquid and vapor such as methyl chloride, ethyl alcohol, ether and toluene
• Lowest operating temperature must be higher than boiling point of the liquid and the maximum temperature is limited by the critical temperature of the liquid
• Temp- pressure characteristic of thermometer is non-linear

PRESSURE THERMOMETER

• Gas thermometer is filled with a gas such as nitrogen at a pressure range of 1000 to 3000 kPa at room temperature
• Gives the linear relationship between absolute temperature and pressure

Liquid in metal thermometer

• Consists of steel bulb filled with high pressure mercury
• Capillary tube of steel and the bourdon tube of steel are completely filled with mercury
• Bourdon tube is used to measure the change in volume of the mercury due to change in temperature
• The temp of the body rises consequently the mercury in the bulb expands resulting the flow of mercury through the capillary tube to the bourdon tube

Liquid in metal thermometer

• Rise in temperature of the surrounding causes the flow of mercury into the bourdon tube resulting it to bend
• The one end of the bourdon tube is fixed
• The displacement of the other end of the tube is communicated to the pointer which moves on the calibrated scale
• The lower temperature range depends upon the freezing point of the liquid

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ELECTRICAL METHODS

• Electrical temperature instruments generally used are

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• Resistance thermometer
• Thermistor

Resistance thermometers

• Resistance thermometers, which are alternatively known as resistance temperature devices (or RTDs),
• The principle that the resistance of a metal varies with temperature according to the relationship
• R₂ = R₁ (1 + a₁T + a₂T² + a₃T³ +…..+a_nTⁿ)
• This equation is non linear

Resistance thermometers

• On linearization
• R₂ = R₁ (1 + a₁T) or
• R₂ = R₁ (1 + coefficient (T₂-T₁)
• Where

R₂ = resistance at temperature T₂

R₁ = resistance at temperature T₁

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Resistance thermometers

• Temperature coefficient of resistance of metals

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Resistance thermometers

• RTD are wire wound resistors
• The devices are sealed in a glass ceramic composite material
• Electrical resistance of the pure metals increases linearly with temperature
• These devices are accurate and can be used to measure temperature from 170 ° C to 780 ° C

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Resistance thermometers (Platinum)

• Electrical resistance thermometer consists of the platinum wires enclosed in the metal sheath
• The electrical resistance platinum wire are surrounded by porcelain insulator to prevent short circuit between the electrical wire and metal sheath
• Electrical platinum wire is attached to with lead to each side of wire

Resistance thermometers (Platinum)

• Electrical resistance thermometer is placed in the medium whose temperature is to be measured
• The metal sheath of the thermometer reaches the temperature of the medium
• The change in temperature of the metal sheath causes to heat or cool the platinum wire
• Correspondingly the resistance of the wire changes with change in temperature
• The change in resistance of the wire is calibrated to indicate the temperature