Set Resistance for cFP-RTD-122

[fuchur]

Hello Forum,

I use a cFP-RTD-122 to measure temperatures of PT100. My problem is, that our sensors don't have exactly 100 Ohm but rather 115-120 Ohm. How can I set this value? The only thing I found is the possibility to change the TCR value for each channel which doesn't solve the problem.

Thanks for help,

Christoph

[Matthew_Williams]

Hi Christoph.  Are your sensors at 0C when they read 115-120 ohms?  PT100 sensors
should be 100 ohms at 0C and 115-120 at 40C to 55C.  How long are your lead wires, and
what gauge?  Are you using 3 wires to the sensors?

If your sensors are really PT100 sensors, then selecting that and the proper TCR should
give you a good reading.

If your sensors are something other than PT100, my guess is that you will have to setup
the RTD-122 to read resistance and translate to temperature in your code.

Matt

[fuchur]

Hej Matt,

thnaks for your fast answer.

I have here a thermal manikin which has measurement wires below the surface. The wires used are PT100 but the resistance changes a bit because of the used length an the connectros and cables attached to them. I use a 3-wire-measurement and the measured 115-120 Ohms are at 0C.

I guess I have to calculate the temperature from the resistance then?

/Christoph

[Matthew_Williams]

Wow.  15-20 ohms is a lot of lead resistance.  Any chance you can reduce
that with bigger wires/fewer connectors?

What temperature does the RTD-122 give you if you just connect the sensors and
set to PT100 and the proper TCR?  You might get away with just a calibration offset,
i.e. RTD-122 reading - 20C = actual temperature, depending on the temperarure
range and accuracy required for your application.

Matt

[fuchur]

I can not change anything on the manikin without destroying it.
I tried the thing with the offset you mentioned as well. The problem is that it is not a constant offset but rather linear dependet on the temperature.
What I do no is calculate the offset from a calibration  and subtract this from the measured offset.

Thanks for your help,

Christoph

[Matthew_Williams]

Sounds like you will have to take readings at several temperature points
in your range and do some interpolation/calculations.  At least you are
aware of the problem.  Sorry I don't see any simpler answers.  Good luck
with it.

Matt