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PT100 measurements using CR6 and AM16/32b

ipantev Oct 27, 2020 12:02 PM

Hi everyone,

I have a monitoring system for a project which uses:

- CR6 datalogger

- AM16/32b multiplexers

- 3-wire PT100 Class A probes from Omega (thermowells for measuring temperature in a pipe network)

The PT100 probes are currently connected to the AM16/32B multiplexer (2x32 mode, e.g. 1H-1L-G) and this is hooked up to the CR6 logger (appropriate U-channels). I am using the Resistance3W routine (e.g. Resistance3W (THW_C_In,1,mV5000,U9,2500,True,0,50,1.0,0) ) to make measurements of the resistance and the PRTCalc routine to calculate the temperature. Wires connecting the PT100s to the multiplexer are all approx. 1.4m 22AWG, hence the wire resistance should be close to negligible (aiming for an accuracy of 0.2degC). 

My question is if using the AM16/32B multiplexer in between will introduce significant inaccuracies in the measurement?

Can I use the built-in 100Ohm internal resistance on the CR6 for these types of measurements or would I need an additional Terminal Input Module (e.g. 3WHB10K) to provide the reference resistance?

Many thanks!

JDavis Oct 28, 2020 01:53 PM

The resistance of the relays in the AM16/32B is low. However, you can't be certain the resistance is perfectly balanced. If you ever used the AM16/32B to run power to a sensor, the resistance is likely imbalanced. If using 4 wire PRTs, it would not be an issue at all.

Being 3 wire PRTs, the best would be to put an individual input module on the AM16/32B for each PRT. 3WHB10K is the recommended module.

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