Sensoray 7429 - Manual (Page 40)

THERMOCOUPLE THEORY
Thermocouples are typically formed from two wires having dissimilar metals. The
wires are built into a cable having a heat-resistant sheath and often have a metallic
shield. At one end of the cable, the wires are electrically shorted together by crimping,
welding, etc. This end of the thermocouple — the hot junction — is thermally attached
to the measurement point. The other end — the cold junction — is connected to a mea-
suring device. In this case, the measuring device is a 7429 coprocessor board.
A thermocouple generates an open-circuit voltage which is proportional to the temper-
ature gradient between the hot and cold junctions (Re: the Seebeck effect). Since the
thermocouple voltage is a function of the temperature difference between junctions, it
is necessary to know the cold junction temperature in order to determine the tempera-
ture at the hot junction.
The sensor coprocessor periodically measures the temperature at the 7409TB termina-
tion board (optional accessory). Since all thermocouples are terminated at this board,
thermocouple cold junctions will be very near to the temperature of the termination
board. A correction voltage is computed for all supported thermocouple types based on
the measured temperature. This correction voltage is the cold junction compensation
voltage.
The cold junction compensation voltage is added to the differential voltage measured
at the thermocouple cold junction. The result of this addition — the corrected thermo-
couple voltage — is the voltage one would measure if the cold junction were main-
tained at zero degrees C. The corrected thermocouple voltage is then converted to the
appropriate temperature units using a non-linear mapping function.