cFieldPoint RTD/Thermistor Measurements?

[pittpanther]

I am back with yet another question (primarily because Aaron keeps answering all of them - thanks!)

In this episode, I am looking to acquire data from several types of temeperature sensors - specifically, 1k RTDs as well as 10k, 20k and 100k Thermistors. As I look at the RTD cards for cFieldPoint, it looks as if the maximum resistance rating is 4k - am I reading this correctly? If so, what options (if any) do I have? I certainly wouldn't mind acquiring the signal(s) as raw resistances and linearizing them via software, but it doesn't look like that is even possible.

Any ideas? Thanks!

[Aaron_G]

Well, may as well extend my streak...

The [c]FP-RTD-122 (3-wire RTD modules) can support resistance measurements up to 4000 Ohms. The [c]FP-RTD-124 (4-wire RTD modules) will support resistance measurements up to 400 Ohms.

If you need to measure larger resistances with FieldPoint/Compact Fieldpoint, you can hook an additional resistor in parallel to the 10-100K resistors to bring the total resistance down to the range that the module supports (4K). This will allow you to measure the parallel equivalent resistance and back-calculate the resistance of the thermistor. Unfortunately, in doing this, you will lose both accuracy and resolution since you are now looking at the parallel resistance. Alternatively, placing a known voltage across the thermistor and
measuring the current through the thermistor would allow you to calculate the resistance, but you may run into ground loop issues with this approach since there currently are no channel to channel isolated Compact FieldPoint modules.

Regards,
Aaron

[pittpanther]

Aaron...

Thanks for the input. I hadn't thought of the parallel resistance - that may be the way I go with this because accuracy is not that critical (essentially we want to ensure the device in question is functioning and wired properly, not necessarily verifying its accuracy).

Thanks again :)...