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How can we say that accuracy of measurement increases by using 3 or 4 wire RTD?
because for example:
If resistance of RTD element is say r ohms and
Resistance of the cable from transmitter to RTD is R ohms
And if 4 wire RTD is used,
____._____________________ R ohms
|
|_____._____________________ R ohms
/
\
/ r ohms
\
\
|
|______.__________________ R Ohms
|
|_______._________________ R ohms
Then the actual resistance measured will be:
Total = r + R/2 + R/2 = r +R Ohms
Note: r+R instead of r ohms. hence error
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Answer Posted / 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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