Mechanical Engineering Principles

24 The measurement of temperature

At the end of this chapter you should be

able to:

• describe the construction, principle of
  operation and practical applications of the
  following temperature measuring devices:

(a) liquid-in-glass thermometer (includ-

ing advantages of mercury, and

sources of error)

(b) thermocouples (including advantages

and sources of error)

(c) resistance thermometer (including

limitations and advantages of plat-

inum coil)

(d) thermistors

(e) pyrometers (total radiation and opti-

cal types, including advantages and

disadvantages

• describe the principle of operation of

(a) temperature indicating paints and

crayons

(b) bimetallic thermometers

(c) mercury-in-steel thermometer

(d) gas thermometer

• select the appropriate temperature measur-
  ing device for a particular application

24.1 Introduction

A change in temperature of a substance can often
result in a change in one or more of its physical
properties. Thus, although temperature cannot be

measured directly, its effects can be measured. Some

properties of substances used to determine changes

in temperature include changes in dimensions, elec-

trical resistance, state, type and volume of radiation

and colour.

Temperature measuring devices available are many

and varied. Those described in sections 24.2 to

24.10 are those most often used in science and

industry.

24.2 Liquid-in-glass thermometer

Aliquid-in-glass thermometeruses the expansion

of a liquid with increase in temperature as its prin-

ciple of operation.

Construction

A typical liquid-in-glass thermometer is shown in

Figure 24.1 and consists of a sealed stem of uniform

small-bore tubing, called a capillary tube, made of

glass, with a cylindrical glass bulb formed at one

end. The bulb and part of the stem are filled with a

liquid such as mercury or alcohol and the remaining

part of the tube is evacuated. A temperature scale

is formed by etching graduations on the stem. A

safety reservoir is usually provided, into which the

liquid can expand without bursting the glass if the

temperature is raised beyond the upper limit of the

scale.

0 102030405060708090100

Liquid

Bulb

Scale

Capillary tube

Safety

reservoir

Figure 24.1

Principle of operation

The operation of a liquid-in-glass thermometer

depends on the liquid expanding with increase

in temperature and contracting with decrease in