Mechanical Engineering Principles

272 MECHANICAL ENGINEERING PRINCIPLES

Applications

Platinum resistance thermometers may be used as
calibrating devices or in applications such as heat-
treating and annealing processes and can be adapted
easily for use with automatic recording or control
systems. Resistance thermometers tend to be fragile
and easily damaged especially when subjected to
excessive vibration or shock.

Problem 2. A platinum resistance

thermometer has a resistance of 25at

0 °C. When measuring the temperature of an

annealing process a resistance value of 60

is recorded. To what temperature does this

correspond? Take the temperature coefficient

of resistance of platinum as 0.0038/°C

Rθ=R 0 ( 1 +αθ),whereR 0 = 25 ,Rθ = 60 
andα= 0. 0038 /°C. Rearranging gives:

temperature,θ=

Rθ−R 0

αR 0

=

60 − 25

( 0. 0038 )( 25 )

= 368. 4 °C

Now try the following exercise

Exercise 128 Further problem on the

resistance thermometer

1. A platinum resistance thermometer has a
   resistance of 100at 0°C. When mea-
   suring the temperature of a heat process
   a resistance value of 177is measured
   using a Wheatstone bridge. Given that
   the temperature coefficient of resistance
   of platinum is 0.0038/°C, determine the
   temperature of the heat process, correct
   to the nearest degree. [203°C]

24.5 Thermistors

A thermistor is a semi-conducting material — such
as mixtures of oxides of copper, manganese, cobalt,
etc. — in the form of a fused bead connected to two
leads. As its temperature is increased its resistance
rapidly decreases. Typical resistance/temperature
curves for a thermistor and common metals are
shown in Figure 24.6. The resistance of a typical

Semiconductor

Temperature

Nickel

Platinum

Copper

Resistance

Figure 24.6

thermistor can vary from 400at 0°C to 100

at 140°C.

Advantages

The main advantages of a thermistor are its high

sensitivity and small size. It provides an inexpensive

method of measuring and detecting small changes in

temperature.

24.6 Pyrometers

A pyrometer is a device for measuring very high

temperatures and uses the principle that all sub-

stances emit radiant energy when hot, the rate of

emission depending on their temperature. The mea-

surement of thermal radiation is therefore a con-

venient method of determining the temperature of

hot sources and is particularly useful in industrial

processes. There are two main types of pyrometer,

namely the total radiation pyrometer and the optical

pyrometer.

Pyrometers are very convenient instruments since

they can be used at a safe and comfortable distance

from the hot source. Thus applications of pyrometers

are found in measuring the temperature of molten

metals, the interiors of furnaces or the interiors of

volcanoes. Total radiation pyrometers can also be

used in conjunction with devices which record and

control temperature continuously.

Total radiation pyrometer

A typical arrangement of a total radiation pyrome-

ter is shown in Figure 24.7. Radiant energy from a