RTD Calibration...

[Bluenoser337]

Trying to calibrate temperature probes using our usual expensive Fluke calibrator. The sensors are 100ohm platinum RTD probes with S-products 82800 transmitter pucks. We have the software and cable from S-products to connect a PC to the transmitter, but when we "inject" various temperature values from the Fluke into the RTD terminals on the puck (as per the manufacturer's instructions) the reading on the PC (and the 4-20mA output from the puck) is not accurate. It seems to float 1-3 degrees around what it should actually be. We've used this calibrator to test RTD transmitters many times and it works on all but these S-products units. The manufacturer is out of ideas and we have tried this on 2 different units (one brand new), with the same results. Any ideas or suggestions appreciated. Thanks!!

 

[hacksaw]

by floating, do you mean from sensor to sensor, or variablity in time?

have you recently certified the fluke?

 

[Bluenoser337]

Variability in time.

 

[hacksaw]

i'd check the calibration instrument for stability, if it is in working order, the readings should be very stable.

 

[Bluenoser337]

hacksaw....tried it out on another transmitter (different brand) and everything worked as expected. Having trouble getting info from the manufacturer of the "floating" units...and several identical units are doing the same thing. Calibrator was just professionally checked and calibrated. I'm lost.

 

[hacksaw]

rather strange, the transmitter may be sensitive to common mode noise. This can occur if the fluke is picking up ac power stray fields. It won't be a differential signal that the transmitter responds directly but it will modulate its response around the sensed input.

how to solve: you can measure common mode noise (fluke to display gnd, etc.) with a separate voltmeter (w/ dc blocking).

noticed that the manf. spec's suggest that shield grounding is rather critical, so it might be an issue in your case...good luck

 

[danw2]

1) These are the limits on excitation current that the a Fluke 712 can tolerate when it is in simulation mode.

What is the excitation current from the S-products transmitter?

2) Anecdotally similar experience - going back to the early-mid 1990's when electronic simulators were replacing decade resistance boxes, about half of the RTD analog inputs, whether transmitters or controllers or PLC AI's had problems with electronic simulation. I never sorted it out, just got out the decade box to finish the job.

 

[hacksaw]

0.1 mA excitation current, if you believe the specs

did notice the manf is quite specific about grounding in a way that does not always work in a field environment, does this suggest that the unit is unusually sensitive to common mode noise pick-up?