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Cold Junction Compensation
Thermocouples can only measure relative, not absolute, temperature. To measure the true
temperature using a thermocouple, its value must be compensated. For the TCM, either a
temperature measuring IC (mounted inside), a resistive temperature device - RTD (mounted
externally), or an external temperature value as the source of the compensation temperature can
be used. The combination of both the relative thermocouple reading and the compensation
temperature give the true temperature measurement.
A thermocouple being used to measure temperature always has a reference thermocouple
associated with it (commonly called the cold junction). All transitions or connections between
dissimilar metals along the two wires from the measuring thermocouple junction and the TCM
terminals will create additional thermocouple junctions. All but one of these junctions will be
common to both of the signal paths with their effects canceling out. The remaining junction is
called the cold junction. The cold junction is always present since the metals used in the
measuring thermocouple will always transition to the copper leads used inside the TCM
somewhere along the signal path. The location where this occurs is on the TCM terminal block
(if the thermocouple leads are connected there) or on the isothermal thermocouple transition
block (where the thermocouple leads are connected to the copper wires feeding the signal to the
TCM terminal block).
If the thermocouple leads are connected directly to the TCM, the internal temperature measuring
IC in contact with the connector is selected to provide the cold junction compensation to obtain
the true temperature of the measuring thermocouple. The input terminal block is not a true
isothermal connector and any temperature difference between the thermocouple connection and
the IC will result in a similar difference or error in the temperature reading.
If the thermocouple leads are connected to an external connector block (with the connections
from this block to the TCM made with a standard copper cable), the connector should be an
isothermal block with an RTD to minimize measurement errors. The RTD allows measurement
of the temperature of the cold junction for optimum cold junction compensation. In this case the
TCM should be set to use the RTD temperature as the cold junction compensation source. This
solution provides the best accuracy for TCM operation.
GFK-1466 Chapter 4 Operation and Field Wiring Information 4-5
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