On Tue, 16 Apr 2013 09:23:06 -0500, Pete Keillor
wrote:


If the spec is +/- 0.1F, then a 2-wire RTD ain't going to get it.
4-wire preferred, and that's pushing it, but I seem to recall the
Omrons taking 3. I always specified 4-wire and put two leads from one
end under the + terminal, the other two under the measurement and -
terminals. I also always used ungrounded sheathed elements. Don't
recall ever seeing a grounded RTD, but you could get t/c's that way.
A/C heater leakage played havoc with grounded t/c's.

Pete Keillor

He's got a 3-wire cable going to two-wire very near the end. Probably
okay provided the end is reasonably well controlled.

Perhaps worth stating that 3-wire RTD compensation requires that the
three wires (well, usually it's only two of the three, but you may not
know which ones) be as close to identical as possible, since one wire
is used to compensate for the resistance of another.

Four wire is theoretically independent of that kind of matching
requirement, but nobody outside of laboratories uses it much.