Mechanical Engineering Principles

THE MEASUREMENT OF TEMPERATURE 269

MetalA MetalB

X

Y

I

I

Vx

Vy

G

Figure 24.2

is usually called the ‘hot junction’ and the lower
temperature junction the ‘cold junction’. If the cold
junction is kept at a constant known temperature,
the galvanometer can be calibrated to indicate the
temperature of the hot junction directly. The cold
junction is then known as the reference junction.
In many instrumentation situations, the measuring
instrument needs to be located far from the point at
which the measurements are to be made. Extension
leads are then used, usually made of the same
material as the thermocouple but of smaller gauge.
The reference junction is then effectively moved to
their ends. The thermocouple is used by positioning
the hot junction where the temperature is required.
The meter will indicate the temperature of the hot
junction only if the reference junction is at 0°Cfor:

(temperature of hot junction)

=(temperature of the cold junction)

+(temperature difference)

In a laboratory the reference junction is often placed
in melting ice, but in industry it is often positioned in
a thermostatically controlled oven or buried under-
ground where the temperature is constant.

Construction

Thermocouple junctions are made by twisting

together the ends of two wires of dissimilar metals

before welding them. The construction of a typical

copper-constantan thermocouple for industrial use

is shown in Figure 24.3. Apart from the actual

junction the two conductors used must be insulated

electrically from each other with appropriate

insulation and is shown in Figure 24.3 as twin-holed

tubing. The wires and insulation are usually inserted

into a sheath for protection from environments in

which they might be damaged or corroded.

Applications

A copper-constantan thermocouple can measure

temperature from− 250 °C up to about 400°C, and

is used typically with boiler flue gases, food pro-

cessing and with sub-zero temperature measure-

ment. An iron-constantan thermocouple can mea-

sure temperature from− 200 °C to about 850°C, and

is used typically in paper and pulp mills, re-heat

and annealing furnaces and in chemical reactors. A

chromel-alumel thermocouple can measure temper-

atures from− 200 °C to about 1100°C and is used

typically with blast furnace gases, brick kilns and in

glass manufacture.

For the measurement of temperatures above 1100°C

radiation pyrometers are normally used. However,

thermocouples are available made of platinum-

platinum/rhodium, capable of measuring tempera-

tures up to 1400°C, or tungsten-molybdenum which

can measure up to 2600°C.

Advantages

A thermocouple:

(i) has a very simple, relatively inexpensive con-

struction,

Hot junction

Copper

Protective

sheath

(such as silica,

porcelain or mild

steel)

Insulation

(such as p.v.c.,

glass fibre, asbestos,

or ceramic tubing)

Constantan

Cold

junction

Extension leads

G

Figure 24.3