50
Subject to change without notice
– If the temperatures at both junctions are identical, the
currents I1
drift
and I2
drift
will cancel.
in order to characterize the various metals and
their thermoelectric properties, the temperature
dependence of the metals with respect to plati-
num was determined and recorded in the ther-
moelectric voltage table, which gives the voltage
in mV/100 K relative to platinum and for the cold
junction at 0 °c.
thermoelectric voltage table
Cold junction reference temperature 0 °C
Measuring temperature 100 °C, in [mV/100 K]
Platinum Nickel
Copper Iron
Ni-Cr
(Pt) (Ni)
(Cu) (Fe) (CrNi)
0,0
-1,2 ...-1, 94 +0,75
+1,88
+2,2
If the junction 2 (KS2) is considered as the reference and kept on
a constant temperature, the other junction 1 (KS1) may be used
for temperature measurement. The thermal voltage is propor-
tional to the temperature difference between both junctions:
I
therm
proportional to
Δ
T = T
KS1
– T
KS2
(Seebeck effect)
Copper cable
Copper cable
V
therm
NiCr wire
Ni wire
Sensing element
Mesurement
location
KS1
Tempe-
rature
T
Meas
T
Ref
= const
Isothermal block
Reference junction KS2
T
Ref
= const
7.5 reference
junction
The measurement
junction 1 is connec-
ted to the measu-
rement system by
socalled extension
w ir e s w hich ar e
made of the same
materials that form
junction 1. As a rule,
the signal has to be sent over quite a distance, therefore the
extension wires have to be contacted to regular copper wires.
These contacts form a pair of junctions which constitute junc-
tion 2. In order to guarantee a decent accuracy, those contact
terminals are mounted on a socalled isothermal metal block
with a temperature sensor; a standard regulation circuit keeps
the block on 0 °C.
An early auxiliary method used melting ice to keep the block
temperature constant; this works quite well, with a deviation
of
<
1 mK, until all the ice is gone. In practice, this is quite
cumbersome. Who would like to carry a bowl of water and an
ice block around? And this only to just check the temperature
of an oven in the production line. In order to save the customer
from pushing a cart with all the utensils necessary for creating
a reference junction including a refrigerator, most measuring
instruments feature an internal reference junction. All that is
needed is the thermocouple and the appropriate measuring
instrument – the HM8112-3. Thermocouples are less expen-
sive than platinum sensors; in industrial applications there
are often hundreds which are connected to the measuring
instrument via a scanner.
t e m p e r a t u r e m e a s u r e m e n t
