OK, then I would consider the following. Make a bridge about like I drew it. When choosing Rg (Gain Resistor) I would look around for a 5 K ten turn or even better 5 K twenty turn trimmer pot. With about 1.6K the Rg will get you close to 212 Deg F = 2.4 Volts. As to actually checking the temperature you can try using some crushed ice and distilled water and make a Ice Bath Slurry as best you can. While not perfect it will be close. Next place the sensor in a pot of boiling water and adjust your Rg for 2.4 Volts. Again, unless you are at sea level under ideal conditions it will not be 100 C but will be close enough. That should give you a zero and span. Do not connect to the analog input channel until Rg is adjusted. I am guessing these devices have some form of input protection?

Actually this 2K ten turn pot would be a good Rg.

Ron

Ohhh, that makes a lot of sense. Is there a reason it has to be a 2k ten turn pot, or similar? For example I know it would allow me to vary my resistance to obtain a higher gain (or lower), but couldn't I also calculate the resistance it would take to be at 100 degrees C. Then I would calculate the voltage and then apply a gain that does let me hit the voltage input limit (or near it) of the U3?

I say this because the RTD's are already firmly set on the Heat Pumps, and to get someone to uninstall one to so I could calibrate it would be difficult.

 

Oh OK. You could just use a few resistors to simulate the RTD. Use 100 Ohms for your 0 Degrees C and 138.51 Ohm for your 100 Degrees C. Yes, rather than a pot for Rg you can work it out and use a fixed resistor. I like a pot simply so I can tweak. Doing it this way you will be close and likely close enough.
When doing RTDs at work we used a precision temperature bath to calibrate the RTD and then a precision resistor box to simulate the RTD for the display unit, even taking into consideration lead resistance but if you can live with a few degrees error what you mention should work fine.

Ron

 

You have a few options. Using a RTD within a Wheatstone Bridge you are going to need a instrumentation amplifier to get the signal up to a usable level. When using a bridge or any method resulting in a current passing through the RTD there can be problems with the RTD having a "rtd self heating effect" and the link just being one example, a Google of RTD Self Heating Effect should bring up a few dozen examples and hits.

Within industry one pretty common solution to the use of RTDs or Thermocouples is the use of RTD or Thermocouple Transmitters, many of which are programmable. Here is an example of what I am talking about. They come literally in a dozen flavors with a dozen prices. Most will output a 4 to 20 mA current loop. You place a resistor in the current loop of for example 250 or 500 Ohms resulting in a 1 to 5 Volt or 2 to 10 Volt voltage drop proportional to the measured temperature. Then you scale your analog to digtal converter be it Labjack or similar for the engineering units, Degrees F or Degrees C.

Aside from a 100 Ohm RTD there are a good number of off the shelf solutions like the LM 35 Precision Centigrade Temperature Sensors. If you choose the Wheatstone Bridge option keep in mind your resistors used in the bridge should be 1% precision resistors with a good temperature coefficient. The INA you choose should be a low noise flavor and preferably with programmable gain requiring a low parts count since you will need to fabricate a circuit board for everything.

Ron

These may come in handy: https://www.pce-instruments.com/eng.../test-meters/signal-converter-kat_40575_1.htm

 

These may come in handy: https://www.pce-instruments.com/eng.../test-meters/signal-converter-kat_40575_1.htm

Modules like that can be useful. I have used the fully programmable flavors with real good results.

Ron

 

A quick and easy solution might be to connect all 8 PT100 RTDs to this display recorder, use it to collect the data, and then export the data to Excel.