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Types of Temperature measurement



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Field Instrumentation
1.3 Types of Temperature measurement
All temperature measurements in industry are of two basic groups.
i.
Non Electric Type

Bimetallic, Liquid in glass, Filled thermal system.
ii.
Electric Type

Thermocouple, RTD, Thermistors, Pyrometer (Radiation & Optical)

Temperature Indication
Temperature is indicated by the changes it causes in certain instruments. Changes in density ( liquid in glass thermometers) Changes in length or volume (bimetallic or filled system) Voltage generated at junction of dissimilar metals (thermocouple) Resonant frequency of crystals (quartz thermometers) Changes in electrical resistance (Resistance thermometers)


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TEMPERATURE RANGES FOR DIFFERENT TEMPERATURE MEASURING DEVICES


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BIMETALLIC THERMOMETERS
Coefficient of thermal expansion
It is the change in length or volume of a substance per degree of temperature change.





















D
IFFERENT CONDITIONS OF A BIMETAL STRIP


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If one end of strip is fixed , the distance the other end bends is directly proportional to the square of the length of the metal strip, as well as to the total change in temperature, while inversely proportional to the thickness of the metal. Invar (alloy of iron and nickel) is used as low expansion material while brass (alloy of copper and zinc) is used as high expansion material.

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