Low RTD measurements

[Wayne Marsh]

I am measuring 16 3-wire 100 ohm RTDS with two FP-122 modules and then transmitting them to a computer via an FP-1001 module.  The computer has FP 2.0 and NIDaq 6.6 on it.  Labview then puts the values on the screen.  My problem is that one set of 8 RTDs has 5 readings that show low on the screen as compared to the field values.  I have changed modules out and recieve the same results.  Is there something that I'm missing here?  This system has been in operation for 15 years.  The RTDs are measuring sea water temperature.  Is there a possibility of "stray currents"?  Thank you. Wayne Marsh

[Matthew_Williams]

Hi Wayne.  Have you tried swapping sensors?  If a bad reading stays with the sensor when swapped to a channel that was previously good, you can eliminate the FP side of the measurement.

Can you replace any of the sensors?  First guess is that the bad sensors were infiltrated with salt water, which could have the effect of lowering the reading via stray currents within the sensors.

Matt

[Wayne Marsh]

We removed the worst RTD on the intake (#7) and swapped it with the first one on the outfall module.  The temperature did not change.  Therefore, we believe that it is in the RTD.  We will have to look at the RTDs.  This won't happen any time soon.  Thank you for your help.

Wayne Marsh

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