Mechanical Engineering Principles

THE MEASUREMENT OF TEMPERATURE 271

Protective sheath

(made of glass, quartz, porcelain

or metal)

Coil (of platinum, or nickel,

or copper)

Former

(made of

mica or

ceramic)

Insulation spacers

(such as mica or

ceramic tubing)

A

B

Copper

dummy leads

Copper

extension

leads

Figure 24.4

θ°CisRθ,thenRθ=R 0 ( 1 +αθ),whereαis the
temperature coefficient of resistance of the material
(see Chapter 20).
Rearranging gives:

temperature,θ=

Rθ−R 0

αR 0

Values ofR 0 andαmay be determined experimen-
tally or obtained from existing data. Thus, ifRθ
can be measured, temperatureθcan be calculated.
This is the principle of operation of a resistance
thermometer. Although a sensitive ohmmeter can be
used to measureRθ, for more accurate determina-
tions aWheatstone bridge circuitis used as shown
in Figure 24.5. This circuit compares an unknown
resistanceRθwith others of known values,R 1 and
R 2 being fixed values andR 3 being variable. Gal-
vanometer G is a sensitive centre-zero microamme-
ter.R 3 is varied until zero deflection is obtained on
the galvanometer, i.e. no current flows through G
and the bridge is said to be ‘balanced’.
At balance:
R 2 Rθ=R 1 R 3

from which,

Rθ=

R 1 R 3

R 2

and ifR 1 andR 2 are of equal value, thenRθ=R 3
A resistance thermometer may be connected between
pointsAandBin Figure 24.5 and its resistanceRθ
at any temperatureθaccurately measured. Dummy
leads included in armBC help to eliminate errors
caused by the extension leads which are normally
necessary in such a thermometer.

R 1 R 2

R 3

ACG

Rq

B

Dummy

leads

Figure 24.5

Limitations

Resistance thermometers using a nickel coil are used

mainly in the range− 100 °C to 300°C, whereas

platinum resistance thermometers are capable of

measuring with greater accuracy temperatures in the

range− 200 °C to about 800°C. This upper range

may be extended to about 1500°C if high melting

point materials are used for the sheath and coil con-

struction.

Advantages and disadvantages of a platinum coil

Platinum is commonly used in resistance thermome-

ters since it is chemically inert, i.e. un-reactive,

resists corrosion and oxidation and has a high melt-

ing point of 1769°C. A disadvantage of platinum is

its slow response to temperature variation.