KROHNE OPTITEMP TRA-TCA Plus EN - Manual (Page 43)

TECHNICAL DATA
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OPTITEMP TRA/TCA PLUS
www.krohne.com 01/2012 - 4000630301 - MA OPTITEMP TRA/TCA Plus R01
Technical data
7.1 Measuring principle
All of the thermometers described here belong to the class known as "contact thermometers".
Unlike "radiation thermometers", these thermometers come into direct contact with the
medium whose temperature they are to measure.
The type of measuring principle depends on the measuring insert sensor that you combine with
the transmitter. Two different sensor types are available. Their respective measuring principles
are described in the following subsections.
7.1.1 Resistance thermometer
The measuring insert with a resistance thermometer features 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 thermometers
to measure temperature. The "Pt100" thermometer 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 thermometer is 100 Ω (100 mV /
1 mA = 100 Ω).
Figure 7-1: Pt100 resistance thermometer in 4-wire connection at 0°C / +32°F, schematic.
1 Pt100 RTD
2 Voltage meter
3 Current source
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