Kipp&Zonen COMBILOG Data Logger - Manual (Page 43)

Temperature Measurement with Thermocouple
Thermocouples consist of two “thermoelectric wires” made of
different materials (e.g. platinum and platinum rhodium) that
are welded to each other at one end. If the contact and the oth-
er ends of the thermoelectric wires have different temperatures,
a “thermoelectric voltage” U
th
appears at the contact of both
thermoelectric wires. This voltage is largely proportional to the
temperature difference. It can be measured and used for
temperature measurement purposes. With data logger
COMBILOG 1022 the thermoelectric voltage is measured
differentially.
Since thermocouples can only measure a temperature differ-
ence (difference between temperature to be measured and
temperature at the connecting terminals on the sensor
module), a terminal temperature (internal cold junction com-
pensation, TC
int
) or a known temperature reference (external
cold junction compensation, TC
ext
) also have to be determined.
With measurement of temperature with internal cold junction
compensation an additional temperature sensor is necessary to
measure the temperature
ϑ
k
at the “cold” terminal. A special
cold junction terminal is available, where a Pt100 temperature
sensor is integrated directly in the terminal block. The temper-
ature of the test point is determined on basis of linearization
trace to
ϑ
x
= Lin(U
x
+Lin -1 ϑ
k
).
Hardware Manual COMBILOG 1022
43