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The device carries out a temperature measurement at an adjustable interval for each connected PT100 sensor, stores the values and, if possible, forwards them to the server.
The device has two integrated analogue digital converters (ADC) for connecting two PT100 sensors. The PT100 can be connected to the ADC using 2, 3 or 4wire technology.
Connection 
Description 
4wire technology 
The 4wire technology provides by far the most accurate measurement results, it completely eliminates the influence of the connecting cables on the measurement result, since any differences in the cable resistances of the connecting cable are also compensated 
3wire technology 
The influence of the line resistance is largely compensated with a 3wire technology. The prerequisite for this, however, is that the line resistances are the same. 
2wire technology 
In 2wire technology, the resistance of the supply lines to the sensor flows into the measurement as an error. The 2wire technology therefore provides the most inaccurate measurement results of the 3 different connection techniques. 
Number 
Marking 
Function 


1 
V+ 
Power supply + 

2 
V 
Power supply  

3 
1F+ 
RTD 1 Force+ 

4 
1S+ 
RTD 1 Sensor+ 

5 
1S 
RTD 1 Sensor 

6 
1F 
RTD 1 Force 

7 
2F+ 
RTD 2 Force+ 

8 
2S+ 
RTD 2 Sensor+ 

9 
2S 
RTD 2 Sensor 

10 
2F 
RTD 2 Force 
The properties of a platinum resistance thermometer are defined in the IEC751 standard (Europe: EN60751). The current standard is EN60751+A2:1995 and is based on ITS90, the International Temperature Scale of 1990.
According to this standard, a platinum resistance is converted into the corresponding temperature using the following formulas.
Temperature range 
Formula 

200 °C to 0 °C 
R(T) = R0 ( 1 + AT + BT2 + C( T – 100°C ) T3 ) 
0 °C to 850 °C 
R(T) = R0 ( 1 + AT + BT2 ) 
R(T) currently measured resistance at a temperature T 

R0 => PT100 nominal resistance and is 100 Ω at 0 °C (ITS90) 

Callendar van Dusen Coefficient

A = 3.9083 * 103 °C1 
B = 5.775 * 107 °C2 

C = 4.183 * 1012 °C3 
With these formulas and the corresponding calculation tables between 200 °C and +850 °C, each temperature can be calculated or interpolated for the respective measured PT100 resistance.
Example for a PT100 table: https://www.omega.de/temperature/Z/pdf/z252254de.pdf
For highly accurate PT100 measurements, the Callendar van Dusen coefficients can be adapted to the respective PT100 sensor.
Characteristic curve
This is the characteristic curve of a Pt100 with a temperature coefficient of α = 3.851 x 103 °C1:
Conversion of the resistance value into a temperature value
With the formulas for temperature calculation described above, a table is calculated internally, which is used to convert the measured resistance value into the corresponding temperature. This table also takes into account the possible change in the Callendar van Dusen coefficients and is calculated for each change and each restart.
Description 
Calculation 

Temperature 
T(R) = Tlow + ((R  Ilow) * ((Thigh  Tlow) / (Ihigh  Ilow))) 
R = currently measured resistance value (factory adjusted) 

Ilow = next table value (index) lower than RMeas 

Ihighw = next table value (index) that is higher than RMeas 

Tlow = To Ilow related table value 

Thigh = To Ihigh related table value 
If the currently measured resistance is outside the table, the measurement is invalid.
Example of a resistancetemperature table:
Index Ω 
Temperature °C 

185.200800 
200.0 
270.964328 
180.0 
… 
... 
1000.000000 
0.0 
… 
... 
3831.286563 
825.0 
3904.811250 
850.0 