Mechanical Engineering Principles

THE MEASUREMENT OF TEMPERATURE 275

the amount of expansion and thus the temperature.
The advantages of this instrument are that it is robust
and, by increasing the length of the capillary tube,
the gauge can be placed some distance from the
bulb and can thus be used to monitor temperatures
in positions which are inaccessible to the liquid-in-
glass thermometer. Such thermometers may be used
to measure temperatures up to 600°C.

24.10 Gas thermometers

The gas thermometer consists of a flexible U-tube
of mercury connected by a capillary tube to a ves-
sel containing gas. The change in the volume of
a fixed mass of gas at constant pressure, or the
change in pressure of a fixed mass of gas at con-
stant volume, may be used to measure temperature.
This thermometer is cumbersome and rarely used
to measure temperature directly, but it is often used
as a standard with which to calibrate other types of
thermometer. With pure hydrogen the range of the
instrument extends from− 240 °C to 1500°Cand
measurements can be made with extreme accuracy.

24.11 Choice of measuring device

Problem 3. State which device would be

most suitable to measure the following:

(a) metal in a furnace, in the range 50°Cto

1600 °C

(b) the air in an office in the range 0°Cto

40 °C

(c) boiler flue gas in the range 15°Cto

300 °C

(d) a metal surface, where a visual

indication is required when it

reaches 425°C

(e) materials in a high-temperature furnace

in the range 2000°C to 2800°C

(f) to calibrate a thermocouple in the range

− 100 °C to 500°C

(g) brick in a kiln up to 900°C

(h) an inexpensive method for food

processing applications in the range

− 25 °Cto− 75 °C

(a) Radiation pyrometer

(b) Mercury-in-glass thermometer

(c) Copper-constantan thermocouple

(d) Temperature sensitive crayon

(e) Optical pyrometer

(f) Platinum resistance thermometer or gas ther-

mometer

(g) Chromel-alumel thermocouple

(h) Alcohol-in-glass thermometer

Now try the following exercise

Exercise 129 Short answer questions on

the measurement of temper-

ature

For each of the temperature measuring devices

listed in 1 to 10, state very briefly its principle

of operation and the range of temperatures that

it is capable of measuring.

1. Mercury-in-glass thermometer


2. Alcohol-in-glass thermometer


3. Thermocouple


4. Platinum resistance thermometer


5. Total radiation pyrometer


6. Optical pyrometer


7. Temperature sensitive crayons


8. Bimetallic thermometer


9. Mercury-in-steel thermometer


10. Gas thermometer

Now try the following exercise

Exercise 130 Multi-choice questions on

the measurement of temper-

ature (Answers on page 285)

1. The most suitable device for measuring
   very small temperature changes is a

(a) thermopile (b) thermocouple

(c) thermistor

2. When two wires of different metals are
   twisted together and heat applied to the