KROHNE PTITEMP TT 20 C EN - Manual (Page 21)

TECHNICAL DATA
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OPTITEMP TT 20
www.krohne.com 10/2011 - 4000753102 - MA OPTITEMP TT 20 R02 en
Technical data
7.1 Measuring principles
The kind of the measuring principle depends on the measuring insert that you combine with the
transmitter. In matters of the thermometer type the manufacturer offers two different
measuring inserts, either with a resistance thermometer or with a thermocouple. This
transmitter only supports resistance thermometer.
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
.book Page 21 Friday, October 14, 2011 11:22 AM