In most cases, a computer is included
in the temperature measurement
loop to handle functions such as data
gathering and transmission, bulk data
storage, display, alarms and control.
This has given rise to the manufacture of
temperature measurement instruments
that are computerized and intelligent.
to 50°C; toluene: -70 to 100°C; mercury
(vacuum): -35 to 280°C; mercury (gas-
filled): -35 to 750°C. Occasionally the liquid
is pressurized to increase the measuring
limit. The thickness of the material used
in manufacturing the liquid-filled glass
thermometer usually affects the time
of response. To achieve high accuracy,
requirements such as high thermal
conductivity, high coefficient of thermal
expansion for the fluids, and linear
expansion must be considered.
B imetallic
A bimetallic thermometer consists of a
strip of two dissimilar metals with different
coefficient heat of expansion joined
together and attached to an indicator. As
a result of the metals’ different rates of
expansion, the bimetal is caused to warp,
indicating a temperature change. One
end of the metal strip is attached to the
housing and the other end to an indicator.
The metal strips can be riveted together
but they are usually rolled or soldered
jointly. In order attain a high deflection
degree, a bimetallic strip is wound to
form a spiral. Bimetalic thermometers can
come in different design: cone-shaped
spiral, cylindrical spiral, flat spiral, and
the combination of flat and cylindrical
spiral. For high accuracy, bimetallic
thermometers must be calibrated at
or close to the conventional operating
temperature being monitored.
While these devices are portable and they
do not require a power supply, they are
usually not as accurate as thermocouples
or RTDs. Bimetallic thermometers tend to
be used where relative changes need to
be monitored, and applications include
use in oil refineries, in hot work wire
heaters, and they also work effectively in
tempering tanks.
T hermocouple
This is a type of temperature measurement
technique that allows the direct
electronic monitoring of temperature. A
thermocouple consists of two different
kinds of wires (dissimilar metals) joined
together at one end.
Issue 32 PECM
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