Mechanical Engineering Principles

268 MECHANICAL ENGINEERING PRINCIPLES

temperature. The position of the end of the column
of liquid in the tube is a measure of the tempera-
ture of the liquid in the bulb — shown as 15°Cin
Figure 24.1, which is about room temperature. Two
fixed points are needed to calibrate the thermometer,
with the interval between these points being divided
into ‘degrees’. In the first thermometer, made by
Celsius, the fixed points chosen were the tempera-
ture of melting ice (0°C) and that of boiling water at
standard atmospheric pressure (100°C), in each case
the blank stem being marked at the liquid level. The
distance between these two points, called the funda-
mental interval, was divided into 100 equal parts,
each equivalent to 1°C, thus forming the scale.
Theclinical thermometer, with a limited scale
around body temperature, the maximum and/or
minimum thermometer, recording the maximum
day temperature and minimum night temperature,
and theBeckman thermometer, which is used only
in accurate measurement of temperature change and
has no fixed points, are particular types of liquid-
in-glass thermometer which all operate on the same
principle.

Advantages

The liquid-in-glass thermometer is simple in con-
struction, relatively inexpensive, easy to use and
portable, and is the most widely used method of
temperature measurement having industrial, chemi-
cal, clinical and meteorological applications.

Disadvantages

Liquid-in-glass thermometers tend to be fragile and
hence easily broken, can only be used where the
liquid column is visible, cannot be used for surface
temperature measurements, cannot be read from a
distance and are unsuitable for high temperature
measurements.

Advantages of mercury

The use of mercury in a thermometer has many
advantages, for mercury:

(i) is clearly visible,

(ii) has a fairly uniform rate of expansion,

(iii) is readily obtainable in the pure state,

(iv) does not ‘wet’ the glass,

(v) is a good conductor of heat.

Mercury has a freezing point of− 39 °C and cannot

be used in a thermometer below this temperature. Its

boiling point is 357°C but before this temperature

is reached some distillation of the mercury occurs

if the space above the mercury is a vacuum. To

prevent this, and to extend the upper temperature

limits to over 500°C, an inert gas such as nitrogen

under pressure is used to fill the remainder of the

capillary tube. Alcohol, often dyed red to be seen

in the capillary tube, is considerably cheaper than

mercury and has a freezing point of− 113 °C, which

is considerably lower than for mercury. However it

has a low boiling point at about 79°C.

Errors

Typical errors in liquid-in-glass thermometers may

occur due to:

(i) the slow cooling rate of glass,

(ii) incorrect positioning of the thermometer,

(iii) a delay in the thermometer becoming steady

(i.e. slow response time),

(iv) non-uniformity of the bore of the capillary

tube, which means that equal intervals marked

on the stem do not correspond to equal tem-

perature intervals.

24.3 Thermocouples

Thermocouples use the e.m.f. set up when the junc-

tion of two dissimilar metals is heated.

Principle of operation

At the junction between two different metals, say,

copper and constantan, there exists a difference in

electrical potential, which varies with the tempera-

ture of the junction. This is known as the ‘thermo-

electric effect’. If the circuit is completed with a

second junction at a different temperature, a current

will flow round the circuit. This principle is used

in the thermocouple. Two different metal conduc-

tors having their ends twisted together are shown

in Figure 24.2. If the two junctions are at dif-

ferent temperatures, a current I flows round the

circuit.

The deflection on the galvanometer G depends on

the difference in temperature between junctionsX

andY and is caused by the difference between

voltagesVxandVy. The higher temperature junction