Vers. No. 6.1
148 Released: 25/04/2017
6 Functional Procedures → Measuring with thermocouples
6.6 Measuring with thermocouples
Thermocouples consist of two “thermoelectric wires” which are
formed from different materials, e.g. platinum and platinum/rho-
dium, and are joined together at one end, usually by welding. If
this c
ontact point and the other ends of the thermoelectric cables
have different temperatures, a “thermoelectric voltage” is pro-
duced at the contact point. This volt
age is essentially propor-
tional to the temperature difference between the contact point
and the ends o
f the cables.
Since thermocouples only measure a temperature difference (dif-
ference between the temperature at
the contact point and the
measured temperature at the terminal strip on the Q.bloxx mod-
ule), the terminal temperature must be known or the “transition”
from
the thermocouple cable or compensating cable to the cop-
per cable must occur at a known temperature level. The first case
is
known as internal cold junction compensation (TC
int
) and the
second case as external cold junction compensation, TC
ext
.
To acquire the temperature with internal cold junction compen-
sation an additional te
mperature probe is used which measures
the reference temperature. For the Q.bloxx modules a cold junc-
tion compensation terminal with an integrated Pt1000 tempera-
ture probe is used. In this way, the temperature at the “transition
point” is det
ermined and the voltage produced by the thermocou-
ple is corrected depending on the
type of thermocouple.
To measure the temperature using
external cold junction com-
pensation, a second ther
mocouple of the same type is needed
which is connected in series with the first one. The polarity is
chosen such that the thermoelectric voltages subtract. The sec-
ond thermocouple is located at a fixed reference temperature
(usually 0
°C). Then, the Q.bloxx module calculates the tempera-
ture at the measuring point based on the linearization curve.
Howeve
r, the Q.bloxx module requires the information of which
reference temperature (cold point temperature) is being used.
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