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Entry type: FAQ Entry ID: 109739448, Entry date: 01/05/2017
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How do you calculate for the worst case the measuring error when measuring temperatures with an S7-1500 analog input module?

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This calculation is for worst case values. In practice, the measuring errors of single components compensate each other and the actual measuring error is less.

Description
The table below gives you an overview of frequently used types of measurement and gives concrete examples of resulting measuring errors. We take the example of the analog input moduleof the S7-1500 AI 8xU/R/RTD/TC HF (article no.: 6ES7 531-7PF00-0AB0).

  • A thermocouple (TC) connected directly to the module is used for temperature measurement. The calculation is made with internal and external (RTD) compensation.
  • Measurement via Pt100 in the climate range is used for the temperature measurement with a resistance thermometer or resistance temperature detector (RTD).

Calculation of measuring errors
The measuring errors of the thermocouples and resistance thermometers used are to be taken from the relevant data sheet and the accuracy class contained in it in each case. Typical values of the selected module:

  • Thermocouple Type E +/- 1.5K
  • Resistance thermometer RTD Pt100 +/- 0.5K

Note also that thermocouples are subject to aging, which is why in some sectors and applications there are regulations stipulating that these thermocouples have to be renewed or calibrated after a specific time.

Temperature measurement (TC) internal compensationTemperature measurement (TC)
External compensation (RTD)
Temperature measurement (RTD)
Relevant for the calculationExample 1)Relevant for the calculationExample 2)Relevant for the calculationExample 3)
TC sensorX+/- 1.5KX+/- 1.5K-
Measurement
Temp. TC
X+/- 1.0KX+/- 1.0K-
RTD sensor--X+/- 0.5KX+/- 0.5K
Measurement
Temp. RTD
--X+/- 0.5KX+/- 0.5K
Error internal
compensation
X+/- 1.5K-0-
Measuring error
(worst case)
+/- 4.0K+/- 3.5K+/- 1.0K
Properties
Little wiring (only TC)
Low accuracy
For high temperatures

Much wiring (TC + RTD)

Better accuracy
External reference junction required

Much wiring (only RTD)

Very high accuracy


1)Analog input module; thermocouple Type E; direct connection to the module (no compensating cables)
2)Analog input module; thermocouple Type E; connection of the external reference junction to the module (no compensating cables); RTD compensation with RTD Pt100 climate
3)Analog input module; RTD Pt100 climate

The examples in the table above The examples show that with the same analog input module the measuring error with TC measurement is in the worst case always greater than with the RTD measurement.

  • An additional measuring error occurs with the TC measurements caused by the required compensation.
  • Resistance thermometers require 2- 4 wires (depending on the required accuracy), which results in more wiring.
  • Thermocouples are suitable for high temperatures but are more expensive with regard to material because of the compensation cables.
  • With external compensation the thermocouples are not connected directly, but via the copper wire from the reference junction to the module. With this compensation wire additional errors might occur from the thermocouple to the reference junction.

Note on potential jumpers in front connectors
Do not use any potential jumpers in the front connectors of analog modules. The potential jumpers for the front connectors are provided for connecting together load groups (jumpers for L+ and M) of digital outputs modules in the S7-1500. If potential jumpers are slotted in front connectors for using with analog modules, this might cause the channels to influence each other and lead to corrupted measured values or incorrect outputs.

Further Information
More information about the SIMATIC S7-1500 analog input module is available in the device manual under Technical Data.