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Field Instrumentation

Two main classes of thermocouples are Noble metal thermocouples
Base metal thermocouples

1.6 THERMOCOUPLE:
It works on the principle that if two wires made of different metals are joined atone end and joined ends are at different temperatures than the open ends, a small voltage is produced across the open ends. Joined ends are called hot junction while open ends are called cold junction. To keep the wires apart, porcelain insulator is used for higher temperature. For accurate temperature measurement, reference junction temperature must remain constant. Indicating or recording instruments use internal and automatic cold junction compensation. Either a thermistor directly measures mV injunction box and is compensated for net temperature output or a thermocouple added to actual thermocouple can give net temperature. TEMPERATURE CURVES OF
RTD,
T
HERMOCOUPLES AND
T
HERMISTORS

22 Field Instrumentation GEN Rev)

WORKING
PRINCIPLE
OF
A
THERMOCOUPLE

INTERNAL
CONSTRUCTION
OF
A
THERMOCOUPLE

23 Field Instrumentation GEN Rev)

Two main classes of thermocouples are

Noble metal thermocouples: platinum or gold is used as one wire. Highly resistant to corrosion, low electric resistivity, good repeatability. Type SR and Bare all noble metal thermocouples.

Base metal thermocouples: type J, TE, and K are all base metal thermocouples. Thermal emf developed is dependant on metals of thermocouple. Common types of thermocouples areas follows.

A).COPPER-CONSTANTAN (T TYPE) Copper element used as positive conductor and constantan element for negative conductor. Constantan contains 55% copper and 45% Nickel. This thermocouple is used in Oxidizing or reducing atmosphere. Range -300 o
F to +600 o
F

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