Application note

PD 642 / 643 AnalogIn scaling; Thermocouple temperature

This document applies to the PD 642 as well as the PD 643, for both modules when used for thermocouple temperature measurement.

The AnalogIn register shows the temperature in °C.

PD 642 / 643 (in Thermocouple mode) Analogue Input modules need to be configured for the type of thermocouple sensor being used.

This is configured in the ChConfig.SignalType register.

Thermocouple types (see datasheet):

– Type R. Platinum-13% Rhodium/Platinum (-50 °C – 1768 °C)
– Type S. Platinum-10% Rhodium/Platinum (-50 °C – 1768 °C)
– Type B. Platinum-30% Rhodium/Platinum (0 °C – 1820 °C)
– Type J. Iron/Copper-Nickel (-109.1 °C – 1200 °C)
– Type T. Copper/Copper-Nickel (-166.5 °C – 400 °C)
– Type E. Nickel-Chromium/Copper-Nickel (-94.8 °C – 1000 °C)
– Type K. Nickel-Chromium/Nickel-Aluminium (-153.7 °C – 1372 °C)
– Type N. Nickel-Chromium-Silicon/Nickel-Silicon (-270 °C – 1300 °C)

The thermocouple sensor measures the temperature difference between two points. One point is the actual process and the other point is at the junction of the sensor cable and the installation cable going to the PD 642 / 643 module.

The temperature measured by the PD 642 / 643 is the Process temperature minus the temperature at the point of cable connection. To automatically remove the influence of the cable junction temperature, the PD 642 / 643 adds the value of OffsetTemp register to the measured temperature before presenting the measurement in AnalogIn.

If the sensor cable / installation cable junction is situated in a place having a constant temperature, the value of OffsetTemp can be set as a constant.

If the temperature at the sensor / cable junction varies, the temperature of the junction has to be measured by an absolute temperature sensor (e.g. Pt100) and stored in the OffsetTemp register.
This is also known as “cold-junction compensation”.

Thermocouple sensors are available with different cable lengths allowing several thermocouple connections to be made at the same place and thus share a common reference temperature sensor.

(See also: AnalogIn)

[sta_anchor id="test"] Data sheet for thermocouple measurements (PD 642 / 643)

Related Topics

Pt100 / Pt1000 / Thermocouple Offset adjustment
Limit alarms
Application Notes for PD 640 (mA)
Application Notes for PD 641 / 643 (Pt100 / Pt 1000)
Application Notes for PD 642 / 643 (mV / thermocouple)
Analogue modules