3-Wire RTD

[Cold Fusion]

Was I correct with my understanding of 2-wire vs 3-wire rtd's?

...For a 2-wire RTD then the device (in this case the relay) sends a small current through two wires to the device and then measures the voltage dropped across this device and lead resistance to calculate the resistance? The lead resistance shows up as the rtd being a higher temperature than it really is

For a 3-wire RTD the device sends our a current on the + wire and then takes two voltage measurement one between the "+" and "-", and the other between the comm and "-". The meausrement between the comm and "-" will calculate the resistance dropped in the wire leads and this value will be subtracted from the overall resistance value calculated from the measurement between "+" and "-"? ...

I added a bit in red. But it's close enough. I think you got it.

cf

 

[ELA]

Your diagram is consistent with some other RTD conditioners I have used such as this one:

http://www.automationdirect.com/static/specs/f204rtd.pdf

I did not take your common symbol to mean earth ground but instead the common point in the system, is that correct?
The common terminal is usually meant to mean the signal return or (-) of the DC power supply used.

In the link I pointed to they refer to the common as 0V in one diagram. If you search this site for the actual manual there they show a "ground" symbol like you used in your diagram. Either way it does not necessarily mean earth ground but instead power supply common. Different companies often use this symbol for different intents.
In some systems they do bond earth to the power supply return so they could be one in the same, but not always.