TITLE.PM5

BASIC CONCEPTS OF THERMODYNAMICS 27

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M-therm/th2-1.pm5

thermometers. For good performance the volume of the bulb should be made at least 8 times than

that of the rest of the system.

These thermometers are generally used for pressures below 35 bar.

3. Thermocouple Thermometers :
   For higher range of temperature i.e., above 650°C, filled thermometers are unsuitable. For
   higher range of temperature, thermocouples and pyrometers are used.

Copper

leads

Millivoltmeter or

temperature recorder

Reference

junction

Metal-1

Metal-2

Measuring

junction

Hot body

Fig. 2.11. Thermocouple.

In its simplest form a thermocouple consists of two dissimilar metals or alloys which develop

e.m.f. when the reference and measuring junctions are at different temperatures. The reference

junction or cold junction is usually maintained at some constant temperature, such as 0°C. Fig.

2.11, shows a simple circuit of a thermocouple and the temperature measuring device. In many

industrial installations the instruments are equipped with automatic compensating devices for

temperature changes of the reference junction, thus eliminating the necessity of maintaining this

junction at constant temperature.

Table 2.1 gives the composition, useful temperatures range and temperature versus e.m.f.

relationship for some commercial thermocouples.

Table 2.1. Composition, useful temperature range and

e.m.f. produced for some thermocouples

Temperature (°C) Thermoelectric

S.No. Thermocouple Composition power

Useful Max. °C Millivolt Remarks

range

1. Platinum vs Pure platinum 400 to 1700 0 0.0 Used for high
   Platinum- vs Pt + 10 or 1450 500 4.219 temperature
   rhodium 13% Rh 1000 9.569 measurements
   1500 15.498


2. Chromel vs 90% Ni + 10% – 200 to 1450 – 200 – 5.75 High resistance
   alumel Cr vs 95% 1200 0 0.0 to oxidation
   Ni + 5% 300 12.21
   (Al + Sn) Mn 600 24.90
   900 37.36
   1200 48.85