49
Subject to change without notice
7.2 Platinum temperature sensor Pt100
The platinum temperature sensor PT 100 is a resistance sensor.
Due to the stability of the resistance over time and because it
stands up well against aggressive media, platinum is a good re-
sistive material for temperature sensors. A change in tempera-
ture causes a change in the resistance. The nominal value R
0
is:
R
0
= 100 Ω at T
0
= 0 °C
The temperature range for the PT100 extends from –200 °C
bis +850 °C.
there are more Pt resistance sensors like Pt10,
Pt25, Pt500, Pt1000. the nominal resistance
values at To = 0 °C are here: 10, 25, 500 and 1000 Ω
respectively. the types Pt10, Pt25, Pt500 can not
be used with the Hm8112-3.
7.3 temperature measurement with the Pt100 /
Pt1000
The most used and most accurate method of temperature
measurement is in a 4-wire circuit. From the SOURCE
26
ter-
minals of the measuring instrument a constant current flows
to the PT100. The change of PT100 resistance depends on the
change of temperature at the PT100. A change of temperature
also causes a change of the resistance of the connecting cables
R
L
. As the measuring voltage is directly taken from the PT100
and applied to SENSE
24
, and because the input resistance of
the input amplifier is very high, a neglegible current will flow in
the SENSE cables (I
meas
appr. 0). Hence the voltage drop across
the SENSE cables caused by the current in them does not (or
only to a neglegible extent) influence the measurement. Also
any change of resistance R
L
in the SENSE cables has hardly any
influence. As the measuring voltage is taken from the PT100
at the ends of the SOURCE cables, only the resistance of the
PT100 is measured. Any change of resistance of the SOURCE
cables has no influence on the measurement.
If utmost accuracy is not required, a 2-wire measurement set-
up may suffice. Due to the fact that the measurement point
with the PT100 and the measuring instrument are mostly at
different temperatures, a temperature change of the cables to
the PT100 causes a change of resistance R
L
. This temperature
dependence of the cables, thermoelectric voltages and the
voltage drop across the cable resistances influence the PT100
measurement.
7.4 nicr-ni thermocouple (K-type)
The application range of a NiCr – Ni thermocouple of the K type
is from –270 °C bis +1,300 °C.
As the name implies, the themocouple delivers a voltage. This
temperature-dependent voltage is generated at the contact
junction of two dissimilar metals. It is called contact or thermal
voltage. Due to the steady thermal movement of the electrons
in the metal’s lattice; some electrons at the surface can leave
the lattice. This requires energy to break loose from the lattice
and surmount the bonding forces. If now two metals are joined
which have different bonding forces, electrons will leave the
metal with the lower bonding forces and flow to the one with
the higher bonding forces. If two such junctions are arranged
in a circuit, and if both junctions are at different temperatures,
a current will flow.
Temperature measurement with the NiCr – Ni thernocouple:
– The NiCr wire and the Ni wire are connected by junctions
at both ends.
– The junction 1 (KS1) , in our case, is assumed to have the
higher temperature with respect to junction 2 (KS2).
– Due to thermal movement at junction 1, electrons will break
loose in the NiCr wire from the metal lattice.
– The electrons will flow to the Ni wire and constitute the drift
current I1
drift
.
– The drift current I1
drift
flows through the junction 2 (KS2)
and there constitutes the diffusion current I
diffusion
.
– At the junction 2 (KS2), due to the thermal movement, also
a drift current I2
drift
is generated.
– I2
drift
opposes the drift current I1drift at junction 1 (KS1).
– I2
drift
also causes a diffusion current at junction 1 (KS1).
– The total current I
therm
follows from the addition of the
currents, observing their polarities: I
therm
= I1
drift
+ I2
drift
– If the temperature at junction 1 (KS1) is lower than that
at junction 2 (KS2), the direction of current flow I
therm
will
reverse.
V
A
SENSE
SOURCE
LO
HI
max.
250Vrms
max.
850
Vpk
max.
850
Vpk
Ω
,
ϑ
FUSE
1A
F250V
CAT
II
max. INPUT
600Vrms / 1Arms
PT100
Measurement voltage with I
meas
≅
0
Measurement current I
PT100
= const
V
A
SENSE
SOURCE
LO
HI
max.
250Vrms
max.
850
Vpk
max.
850
Vpk
,
FUSE
1A
F250V
CAT
II
max. INPUT
600Vrms / 1Arms
PT100
Measurement voltage
U
PT100
R
L
R
L
I1
drift
I
diffusion
Elektrons in
the metal’s
lattice
Wire NiCr
+2,2 mV/100K
Wire Ni
–1,9 mV/100K
Contact junction KS2
Temperature T
KS2
<T
KS1
Contact junction KS1
Temperature T
KS2
>T
KS1
I2
drift
I
therm
KS2
I
drift
A c m e a s u r e m e n t s
t e m p e r a t u r e m e a s u r e m e n t
