KROHNE OPTITEMP TT 51 EN - Manual (Page 38)

7
TECHNICAL DATA
38
TT 51 SERIES
www.krohne.com 09/2010 - 4000754201 - MA TT 51 R01 en
7.1 Measuring principle
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