KROHNE OPTITEMP Compact sensors - Manual (Page 32)

7
TECHNICAL DATA
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OPTITEMP TRA-CXX
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Technical data
7.1 Measuring principle
All of the temperature assemblies described here belong to the class known as "contact
temperature assemblies". Unlike "radiation temperature assemblies", these temperature
assemblies come into direct contact with the medium whose temperature they are to measure.
The type of measuring principle depends on the sensor that you combine with the transmitter.
Two different sensor types are available for transmitters, RTD and thermocouple. The RTD's
measuring principle are used in the compact sensors and described in the following subsection.
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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