RTD Scaling

[Ryan@Bearcat]

Can someone help me with the best way to use a RTD sensor with Plus+1? I have a couple examples that my predecessor has done but I am a bit confused as to why he set them up the way he did and I want to get my head wrapped around it better. He using an analog input and has the pin config set to 1 for  temp/rheostat mode, but he is using the AnIn signal and not the Ohm signal which seems odd to me?

I have found in my searching online that 0 C. should equal 100 Ohms but I haven't been able to find what the Ohms should be at max temp which 480 F. with the RTDs that I have on hand. I also read on a different thread here that Guide has an "Ohms to Temp" function block but I don't see it in any of the libraries that I currently have.

Also, while I have your attention (hopefully), the sensors that I have are 3 wire. 2 red wires and a white wire. We currently connect the white wire to sensor ground and one of the red wires to the input and do NOT use the other red wire. How would one wire it if they were using 3 wires and what would the benefit of that be?

Thank you in advance!

[spittet]

If you could post the manufacturer/model number of the sensor, it would help us to guide you. The term RTD is widely used and can means multiple types of sensors. Having the correct resistance vs temperature curve is critical, so the datasheet is needed.

Sam

[Ryan@Bearcat]

Here is a link to the data sheet. It is the PR-10L.


https://assets.omega.com/spec/PR-10_Spec.pdf

[acmall]

Only guessing but your predecessor may have used AnIn instead of Resistance get better a bit better resolution. AnIn is the raw value from the A/D convertor whereas Resistance is the A/D convertor value scaled to Ohms.

There is some information on RTD's here: -
http://www.mosaic-industries.com/embedded-systems/microcontroller-projects/temperature-measurement/platinum-rtd-sensors/resistance-calibration-table

[spittet]

Here's some info on the 2-wires, 3-wires, 4 wires configurations and their differences.

https://instrumentationtools.com/difference-between-2-3-and-4-wire-rtds/#:~:text=2%2Dwire%20RTD's%20are%20mostly,measure%20true%203%2Dwire%20resistance.

If you are only using a single analog input in your Danfoss controller, then the 2-wires configuration must be used. If the wires aren't really long and if they are #18AWG, I don't think you should have much of an issue.


About the temperature vs resistance curve, please take a look at the following link : https://www.instrumentationtoolbox.com/2011/01/sensors-used-in-industrial_23.html

In your case, with 100ohm at 0°C and a TCR of 0.00385, at 482°F (250°C), you should have a resistance of 100*(1+0.00385*(250-0)) = 196.25ohm