Answer / sam

Your maths is not right but actually you are right, and the
temperature indication does not increase so significantly
that it is worth the effort to use a 3 or 4 wire RTD. It
might increase with something like 0,001Deg C so that is so
small you might as well say there is no difference in the
accuracy of a 2,3 and 4 wire RTD, using a local or smart
transmitter.
In the old days we use to use a 2 wire RTD in the field and
then run a cable say 200m to the temperature indicator. By
the time it gets to the indicator the temperature is
completely different from what it was in the field due to
the cable resistance that add itself the the RTD resistance.
We compensated for that by installing a 3de wire for the
sole propose to measure the resistance of the cable itself
and deduct that from the total resistance measured at the
temperature indicator.
So the actual resistance of the RTD as measured at the
temperature indicator is RTD - RLine1.(or [(Rline1/2)x2]if
you want)
With the 4 wire it makes it more accurate in that you can
now measure line one and line 2. The theory is that the one
line might have a small difference compare to the other line.
So the actual RTD resistance at the temperature indicator is
measured RTD - [(RLine1/2)+(RLine2/2)]
But since we all use the small compact local and smart
temperature transmitters these days, 3 and 4 wires are no
longer needed since the distance from the RTD to the
transmitter is only from about 50 to 500mm and but it seems
it have stayed due to some design engineer always saying,
why buy a 2 wire if you can get the 3 and 4 wire for just
about the same price. It will make the indication just more
accurate, but they never say by how much (0,001Deg C)
Good luck

Answer / kalla anil kumar,jeypore

Hi Friend
There will be small difference between 2wire,3wire and 4wire RTD, in 2wire if the Distance between RTD and Transmitter is so long then there will be adding of cable resistance so it will shows error reading for example in 2 wire RTD like PT-100 RTD and wire resistance 2ohms each 2wire x 2ohms so total resistance 104 ohms at 0deg,so to avoid the error in reading we can use 3rd wire to compensate the error,for example in 3wire RTD like PT-100 main wire 1 and reaming two wire are 2 & 3,then Tx will calculate total resistance in 1&2 wires it will be 104 ohms at 0deg, and in between 2&3 wire 4 ohms,so Transmitter will calculate 104-4=100 ohms at 0 deg, NOTE- lead cable resistance of RTD should be same from field to transmitter.

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