274 MECHANICAL ENGINEERING PRINCIPLESthe hot source and the ammeter can be calibrated
to read the temperature directly. Optical pyrometers
maybeusedtomeasuretemperaturesupto,and
even in excess of, 3000°C.
Advantages of pyrometers
(i) There is no practical limit to the temperature
that a pyrometer can measure.(ii) A pyrometer need not be brought directly into
the hot zone and so is free from the effects of
heat and chemical attack that can often cause
other measuring devices to deteriorate in use.(iii) Very fast rates of change of temperature can
be followed by a pyrometer.
(iv) The temperature of moving bodies can be
measured.
(v) The lens system makes the pyrometer virtually
independent of its distance from the source.Disadvantages of pyrometers
(i) A pyrometer is often more expensive than
other temperature measuring devices.(ii) A direct view of the heat process is necessary.(iii) Manual adjustment is necessary.
(iv) A reasonable amount of skill and care is
required in calibrating and using a pyrometer.
For each new measuring situation the pyrom-
eter must be re-calibrated.
(v) The temperature of the surroundings may
affect the reading of the pyrometer and such
errors are difficult to eliminate.24.7 Temperature indicating paints
and crayons
Temperature indicating paintscontain substances
which change their colour when heated to certain
temperatures. This change is usually due to chemical
decomposition, such as loss of water, in which the
change in colour of the paint after having reached
the particular temperature will be a permanent one.
However, in some types the original colour returns
after cooling. Temperature indicating paints are used
where the temperature of inaccessible parts of appa-
ratus and machines is required. They are particularly
useful in heat-treatment processes where the temper-
ature of the component needs to be known before a
quenching operation. There are several such paints
available and most have only a small temperature
range so that different paints have to be used for
different temperatures. The usual range of temper-
atures covered by these paints is from about 30°C
to 700°C.
Temperature sensitive crayonsconsist of fusible
solids compressed into the form of a stick. The melt-
ing point of such crayons is used to determine when
a given temperature has been reached. The crayons
are simple to use but indicate a single temperature
only, i.e. its melting point temperature. There are
over 100 different crayons available, each cover-
ing a particular range of temperature. Crayons are
available for temperatures within the range of 50°C
to 1400°C. Such crayons are used in metallurgi-
cal applications such as preheating before welding,
hardening, annealing or tempering, or in monitor-
ing the temperature of critical parts of machines or
for checking mould temperatures in the rubber and
plastics industries.24.8 Bimetallic thermometers
Bimetallic thermometersdepend on the expansion
of metal strips which operate an indicating pointer.
Two thin metal strips of differing thermal expansion
are welded or riveted together and the curvature
of the bimetallic strip changes with temperature
change. For greater sensitivity the strips may be
coiled into a flat spiral or helix, one end being
fixed and the other being made to rotate a pointer
over a scale. Bimetallic thermometers are useful
for alarm and over-temperature applications where
extreme accuracy is not essential. If the whole
is placed in a sheath, protection from corrosive
environments is achieved but with a reduction in
response characteristics. The normal upper limit of
temperature measurement by this thermometer is
about 200°C, although with special metals the range
can be extended to about 400°C.24.9 Mercury-in-steel thermometer
Themercury-in-steel thermometeris an extension
of the principle of the mercury-in-glass thermome-
ter. Mercury in a steel bulb expands via a small bore
capillary tube into a pressure indicating device, say a
Bourdon gauge, the position of the pointer indicating