Sensoray 417 - Manual (Page 33)

Chapter
6
Thermocouples 6
Overview
A thermocouple (TC) consists of a conductor pair constructed from dissimilar metals. At one end
of the pair, the two conductors are electrically shorted together. This end of the TC—the hot junc-
tion—is thermally attached to the point to be measured. The other end of the pair—the cold, or ref-
erence junction—is connected to a measuring device. In this discussion, the measuring device is
assumed to be a termination board—any of Sensoray Models 7409TB, 7409TC or 7409TDIN—in
conjunction with a Model 417 thermocouple interface board.
Thermocouples generate an open-circuit voltage that is proportional to the thermal gradient
between hot and cold junctions. Since this voltage is a function of the temperature difference
between junctions, it is necessary to know the temperature at the cold junction in order to deter-
mine the temperature at the hot junction.
Reference Sensor
An integrated circuit temperature transducer resides on Sensoray termination boards. This trans-
ducer, because of its close physical proximity to the TC terminations, represents an accurate mea-
surement of the TC cold junction. Periodically, the 417 measures the transducer signal and
computes the corresponding TC voltage, known as the compensation voltage.
When a TC is measured, the 417 adds the TC voltage to the compensation voltage to obtain the
corrected voltage. This is the voltage one would measure if the cold junction were maintained at
zero degrees C. Finally, the corrected voltage is converted into the appropriate temperature units by
means of a non-linear mapping function.
Ensuring Accuracy
To get the very best accuracy from TCs, the reference sensor must have the same temperature as
the TC cold junction. Some manufacturers attempt to impose a constant temperature on both cold
junction and reference sensor by means of an “isothermal” heat conductor. While this may appear
to be a reasonable approach, it often fails to perform adequately in practice. The principal failure
of the Isothermal Conductor approach is this: “isothermal” conductors are rarely isothermal. Any
air flow across the heat conductor has the potential to produce a thermal gradient.
So what is the solution to this problem? Force the termination system to be isothermal. Make sure
your termination system is insulated from outside air flows, and avoid exposing the termination
system to sudden thermal transients. Example: enclose the termination system in a sealed box.