RTD vs T/C solution

I have a 20ma output card and a Thermocouple card which also can be configured to 0-100mv. I need a remote temperature sense. There are control wires at the remote site 200ft away. The wires are not T/C wires.
Is it pluasible to excite a wheatstone bridge and use an RTD connected to the 0-100mv input and then scale deflection of the bridge to 0F to 212F temperature range. Are there accuracy problems, need +/-.5 degrees?
This is to avoid a high dollared RTD card and/or running T/C wire to the remote location, the conduit is packed and underground.
I have the RTD, 20ma output, 0-100mv input. Need to crunch the numbers for the wheatstone bridge.
Would like some input before I move on this?

How about a signal conditioner to convert the RTD signal to 4 - 20 mA. Then put a 5 ohm resistor across the analog input terminals to give you a 20 - 100 mV input range.

I don't like your chances

Attempting to get +/- 0.5 degF accuracy over the range 0-212 degF will not be simple.
First - RTD's do not respond linearly.
Second - RTD's are manufactured to a tolerance. i.e. within x% of the ideal curve.
Third & Fourth - what are the tolerances for your 20 mA source and 100 mV input?

If you really need the accuracy, you will need to calibrate your RTD under laboratory conditions with specialised equipment at small incremental steps over the full range.
You will also need to do a similar calibration on your input card and check it regularly.

I would suggest looking for a suitable instrument.

Is it pluasible to excite a wheatstone bridge and use an RTD connected to the 0-100mv input and then scale deflection of the bridge to 0F to 212F temperature range. Are there accuracy problems, need +/-.5 degrees?

Click to expand...

Unlikely unless your labor is worth about $0.05 an hour. You're talking about designing, from the ground up, a very stable and accurate RTD circuit, which needs a very stable constant current source. 0.5°F is quite a tight spec. That's assuming the T/C card's optional 0-100mV range is a true 0-100mV linear range, which does not apply ice point compensation or linearize to a common thermocouple type, like K or J or T.

Like Steve Bailey says, buy an RTD 2 wire, loop powered transmitter, which produces a conventional 4-20mA output, drop the 4-20mA across a precision 5.00 ohm resistor (any error in the resistance value is directly proportional error at the input) to get a 0-100mV signal for your 0-100mV input. A 2 wire, loop powered RTD transmitter uses copper wire (not T/C wire) and a conventional 24 Vdc power supply, that can be located in the control panel, not at the remote site.

Dan