Answer / guest

Resistanc Temperature Detector(RTD)encorporates two basic
concepts

1)Variation of temperature varies the resistance of Pt wire.
2)Variation in resistance affects the balancing of DC
bridge circuit.

For measurement purposes lead wires are brought out to make
external connections.These lead wires have some resistances
which affects the bridge balancing.

Since we can not reduces the resistances of lead wire to
zero.So, either 3 wire or 4 wire connection is employed to
ELIMINATES THE IMBALANCING TO THE BRIDGE CIRCUIT caused by
resistances of lead wires so that an accurate measurement
can be made.These wires(3 or 4) are called compensating
leads.

Author:arbelectbit@yahoo.com

Answer / sam

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 / maswood alam

Actuly there is little diffrence b/w 3 wire nd 4wire, 3wire
has 95% accurcy nd 4 wire has 100% but 4 wire more costly nd
both are use as long distance so we genarally prefared 3
wired RTD System......

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