Accuracy in degrees C for X20AT4222 input card

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1

Hi Community,
I am using some X20AT4222 RTD input cards that are connected to a Siemens PLC. I having difficulty interpreting the accuracy statements in the data sheet (V 3.01), as they are based on the resistance values rather than degrees C, and refer to the percentages as relating to:

  1. based on the current resistance value.
  2. based on the entire resistance range (0.1 - 4500, or 0.05 - 2250 Ohms)
    The maximum offset and gain drift are also ahown as percentages per degree. Can someone help me with the formula to calculate the accuracy in degrees C for a 0 - 400 deg C PT100 input?
    Thanks.
2

Hi,

i think this would be difficult for us.

According to the datasheet the hardware uses the formula given in EN60751. Unfortunately there are no info how accurate the calculation would be.
(There are some online calculators for this formula).

As an alternative we could use the resistance measurement. The accuracy is given in the datasheet. But the electronic is not aware of the sensor accuracy.
And we also have to take into account the self-heating caused by the measuring current. And thirdly, there is the line resistance.

So in my opinion you should discuss this with the sensor manufacturer, it’s their professional business.

3

Hello Robert,

I’m going to give this my best try to explain, and I’m pretty sure this is accurate. I’m still waiting on a bit of clarification, but I’ll post this now and edit it later with any corrections.

If the X20AT module is operating at 25 C, then the equation for the accuracy of the module only is as follows:

+/- AccuracyResistance = (MeasuredResistanceValue * Gain) + (FullResistanceRange * Offset)

For the X20AT4222, operating at 25 C, and reading a value of 80 C (130.897 Ohms), I get the following:
Accuracy = (130.897 * 0.00037) + (4500 * 0.000015)
Accuracy = +/- 0.115 Ohms
(which is approx. +/- 0.2 C if I’m interpreting the chart correctly)

If the module is not operating at 25 C, then the equation expands slightly.

+/- Accuracy = MeasuredResistanceValue * (Gain + GainDrift * (ModuleTemp-25)) + FullResistanceRange * (Offset + OffsetDrift * (ModuleTemp-25))

Note that this only takes into account the module accuracy, and not the sensor, or cable resistance to the RTD.

I’m still clarifying the FullResistanceRange, to see if that value shrinks to the min/max resistance of the PT or is the full measurement possible range of the module.

Best regards,
Austin

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