Page 290 - Mechanical Engineer's Data Handbook

278 MECHANICAL ENGINEER'S DATA HANDBOOK
FP onto the filament the brightness of which is varied by

means of a calibrated variable resistor until the
filament appears to vanish. A red filter protects the eye.

7.4. I I Bimetallic thermometer

Thermistors
The deflection of a bimetallic strip or coil may be used
to indicate temperature. This type is not very accurate
but is simple and cheap. These thermometers are used
Most metals have a positive temperature coefficient of for alarms and temperature controllers when connec-
resistance, i.e. resistance increases with temperature. ted to a mechanical system.
Semi-conductors may have a very large negative
coefficient which is non-linear. A 'thermistor' is a bead
of such material, e.g. oxides of copper, manganese and
cobalt, with leads connected to a measuring circuit.
They are extremely sensitive; for example, a change
from 4OOQ at 0°C to l00Q at 140°C. They are
inexpensive and suitable for very small changes in
temperature. The graph shows curves of resistivity for
three thermistor materials compared with platinum. Heat
Bimetallic thermometei
7.4. IO Pyrometers
7.4. I2 Temperature-sensitive paints
Total radiation pyrometer
Kits are available of paints and crayons made of
At very high temperatures where thermometers and chemicals which change colour at definite tempera-
thermocouples are unsuitable, temperature can be tures. The range is from about 30 "C to 700 "C, with an
deduced from the measurement of radiant energy from accuracy of about 5%. Several paints are required to
a hot source. The radiation is passed down a tube and cover the range. Crayons are the easiest to use. The
focused, using a mirror, onto a thermocouple or method is suitable for inaccessible places.
thermopile which is shielded from direct radiation.
7.4. I3 Fixed-point temperatures
Disappearing-filament pyrometer
The table below gives fixed-point temperatures known
The brightness and colour of a hot body varies with to a high degree of accuracy from which instruments
temperature and in the case of the disappearing can be calibrated.
filament pyrometer it is compared with the appearance
of a heated lamp filament. The radiation is focused
Temperature
I ("C)
I
I Boiling point of liquid oxygen - 182.97
I lThermocouple
Melting point of ice 0.00
Total radiation pyrometer Triple point of water 0.01
Boiling point of water 100.00
Lamp
Freezing point of zinc 419.505
Boiling point of liquid sulphur 444.60
Freezing point of liquid antimony 630.50
Melting point of silver 960.80
Melting point of gold 1063 .OO
DisapQearing-filament pyrometer