Mechanical Engineering Principles

THE MEASUREMENT OF TEMPERATURE 273

G

Hot

source

Radiation

from hot

source

Shield Sighting

telescope

Thermocouple

Mirror

Figure 24.7

hot source, such as a furnace, is focused on to the
hot junction of a thermocouple after reflection from
a concave mirror. The temperature rise recorded by
the thermocouple depends on the amount of radi-
ant energy received, which in turn depends on the
temperature of the hot source. The galvanometer G
shown connected to the thermocouple records the
current which results from the e.m.f. developed and
may be calibrated to give a direct reading of the
temperature of the hot source. The thermocouple
is protected from direct radiation by a shield as
shown and the hot source may be viewed through
the sighting telescope. For greater sensitivity, a ther-
mopile may be used, a thermopile being a number
of thermocouples connected in series. Total radia-
tion pyrometers are used to measure temperature in
the range 700°C to 2000°C.

Optical pyrometers

When the temperature of an object is raised suffi-
ciently two visual effects occur; the object appears

brighter and there is a change in colour of the

light emitted. These effects are used in the optical

pyrometer where a comparison or matching is made

between the brightness of the glowing hot source

and the light from a filament of known temperature.

The most frequently used optical pyrometer is

the disappearing filament pyrometer and a typical

arrangement is shown in Figure 24.8. A filament

lamp is built into a telescope arrangement which

receives radiation from a hot source, an image of

which is seen through an eyepiece. A red filter is

incorporated as a protection to the eye.

The current flowing through the lamp is controlled

by a variable resistor. As the current is increased the

temperature of the filament increases and its colour

changes. When viewed through the eyepiece the fil-

ament of the lamp appears superimposed on the

image of the radiant energy from the hot source. The

current is varied until the filament glows as brightly

as the background. It will then merge into the back-

ground and seem to disappear. The current required

to achieve this is a measure of the temperature of

Hot

source

Radiation from

hot source

(such as a furnace)

Telescope

arrangement

Variable

resistor

Filament lamp Red filter

Eyepiece

A

Figure 24.8