Learning Event 3:
DESCRIBE THERMOCOUPLE APPLICATIONS
1. There are many, many uses made of thermodynamic devices. This subcourse deals
with one specific application for thermocouples, their use in electronics.
Thermocouples used in electronic instruments are most often referred to as thermal
converters. This name is derived from the fact that heat induced in the heater by
the current to be measured is converted to DC current in the measuring circuit.
2. As with any volt-ohmmeter (VOM), it is possible to build multirange
thermocouple voltmeters, ammeters or wattmeters (figs 1-7 and 1-8). Thermocouples
are generally used in some of the better meters exhibiting accuracies of one
percent or better.
The thermocouple is more accurate than either a standard
dynmamometer or rectifier when converting an AC signal to DC. In Figures 1-7 and
1-8, the variable resistor (Rj) is the full scale adjustment.
The automatic
temperature compensation (ATC) is a resistor with a negative temperature
coefficient to compensate for any temperature changes of the reference junction.
This temperature compensation is effective over a limited range.
The frequency
response of multirange thermocouple meters is limited by the frequency response of
the input divider circuit rather than that of the thermocouple. For this reason,
the multirange meters have a much lower frequency response than single range
meters. Another consideration when using a thermocouple in a metering circuit, is
that common vacuum thermocouples have a limited current range (2 to 500 mA) and the
shunts and multiplier resistors must be chosen with this in mind.
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