7
TECHNICAL DATA
40
TT 51 SERIES
www.krohne.com
08/2015 - 4000754203 - MA TT 51 R03 en
Technical data
7.1 Measuring principle
7.1.1 Resistance temperature sensor
The measuring insert with a temperature-sensitive sensor made from a platinum RTD, whose
value at 0
°
C / +32
°
F is 100
Ω
. That is where the name "Pt100" comes from.
It is generally valid that the electric resistance of metals increases according to a mathematical
function as the temperature rises. This effect is taken advantage of by resistance temperature
sensors to measure temperature. The "Pt100" temperature sensors features a measuring
resistance with defined characteristics, standardised in IEC 60751. The same is true for the
tolerances. The average temperature coefficient of a Pt100 is 3.85 x 10
-3
K
-1
in the range from
0...+100
°
C / +32...+212
°
F.
During operation, a constant current I (
≤
1 mA) flows through the Pt100 RTD, which brings about
a voltage drop U. The resistance R is calculated using Ohm's Law (R=U/I). As the voltage drop U
at 0
°
C / +32
°
F is 100 mV, the resulting resistance of the Pt100 temperature assembly is
100
Ω
(100 mV / 1 mA = 100
Ω
).
Figure 7-1: Pt100 resistance temperature sensor in 4-wire connection at 0
°
C / +32
°
F, schematic.
1
Pt100 RTD
2
Voltage meter
3
Current source
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