Bimetallic Thermometers
About bimetallic thermometers
The bimetallic principle is probably
more commonly known in connection with its use in thermostats. It is
based on the fact that if two strips of different metals are bonded
together, any temperature change will cause the strips to bend, as
this is the only way in which the differing rates of change of length
of each metal in the bonded strip can be accommodated. In the
bimetallic thermostat, this is used as a switch in control
applications.
If the magnitude of bending is measured, the bimetallic device becomes
a thermometer. For such purposes, the strip is often arranged in a
spiral configuration, as this gives a relatively large displacement of
the free end for any given temperature change. Strips in a helical
shape are an alternative for this purpose. The measurement sensitivity
is increased further by choosing the pair of materials carefully such
that the degree of bending is maximized, with Invar (a nickel-steel
alloy) and brass being commonly used.
The system used to measure the displacement of the strip must be
carefully designed. Very little resistance must be offered to the end
of the strip, otherwise the spiral or helix will distort and cause a
false reading in the measurement of the displacement. For visual
indication purposes, the end of the strip can be made to turn a
pointer mounted in low-friction bearings which moves against a
calibrated scale. If an electrical output is required, the linear
variable differential transformer (LVDT) is a suitable form of
translational displacement transducer. Alternatively, a fiber optic
device known as the shutter sensor can be used.
Bimetallic thermometers are used to measure temperatures between -75°C
and + 1500°C. The accuracy of the best instruments can be as good as
±0.5% but such devices are quite expensive. Many instrument
applications do not require this degree of accuracy in temperature
measurements, and in such cases much cheaper bimetallic thermometers
with substantially inferior accuracy are used.
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Temperature Measurements
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