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 / centhile r mohan

the difference between is that,the third wire for 3 wire
RTD is used for lead compensation.

Answer / rajendraa

in 3 wire rtd lead wire resistance s reduced to a grt extent, lead wire error s such that t s the difference of wires A&B(A s the wire connected to rdt & B with the R in bridge circuit). in 4 wire rtd, the lead wire error s completely reduced,it s mainly used for calibrations & is very costly...

Answer / dinesh priyadarshi

RTD are used as 3 wire and 4 wire combination for following
reasons :
1) Lead wire compensation requird because this reduce the
error produce by the wire resistance
2) The distance also increae the error so to reduce the
errors we r using 3rth wire for lead compensation.
3) 3 wire and four wire lead compensation increase the
accuracy by means of resistance

Answer / ng

there is lead resistance compensation in 3 wire RTD, where as 4 wire RTD is idle of lead resistance compensation as 2 wires are used for volt measurement and 2 wire for current measurement.there is no question of lead resistance error, it gives accurate result

Answer / niranjan

RTD is inherently a 2-wire device, A 2-wire RTD can be used with either a 3 or a 4 -wire instrument by jumping the appropriate terminals.3-wire instrument, a 4-wire RTD will not provide any better accuracy. If the fourth wire is not connected, the device is only as good as the 3-wire RTD; if the fourth wire is connected, new errors will be introduced. Connecting a 3-wire RTD to a 4-wire instrument can cause serious errors or simply not work at all, depending on the instrument circuitry.

In RTU Panel we are having several Analogue & Digital Inputs /Outputs. We are recently facing some problems over Analog inputs , which are not updating properly (But Digital Input are updating normally) Can any one tell me from where the problem is creating , we have checked PLC CPU card, Analog Input card & we reconfigured ,But still the issue is there. Issue : 1. Sometimes Analog values are updating & sometimes not. 2. Analog values are not updating normally , showing some wrong Values 3. Total dead (0 value) Analog values are Level & flow measurement from the field instruments. (But in field the instruments are working normally & Displaying the values normally in local display unit) This problem we are facing in our Water Plant. If any one experienced this same problem or having any ideas related to our issue PLS send your valuable Answers/Replies...

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