Answer / moumita

The simplest resistance thermometer configuration uses two
wires. It is only used when high accuracy is not required as
the resistance of the connecting wires is always included
with that of the sensor leading to errors in the signal.
Using this configuration you will be able to use 100 meters
of cable. This applies equally to balanced bridge and fixed
bridge system.
In order to minimize the effects of the lead resistances a
three wire configuration can be used. Using this method the
two leads to the sensor are on adjoining arms, there is a
lead resistance in each arm of the bridge and therefore the
lead resistance is cancelled out.

Answer / varun krishna.m kottayam

Serious lead-wire resistance errors can occur when using a
two-wire RTD (see Fig. 3A), especially in a 100Ω sensor. In
a two-wire circuit, a current is passed through the sensor.
As the temperature of the sensor increases, the resistance
increases. This increase in resistance will be detected by
an increase in the voltage (V = I•R). The actual resistance
causing the voltage increase is the total resistance of the
sensor and the resistance introduced by the lead wires. As
long as the lead wire resistance remains constant, it can be
offset and not affect the temperature measurement. The wire
resistance will change with temperature, however, so as the
ambient conditions change, the wire resistance will also
change, introducing errors.
If the wire is very long, this source of error could be
significant. Two-wire RTDs are typically used only with very
short lead wires, or with a 1000Ω element.

In a 3-wire there are three leads coming from the RTD
instead of two. L1 and L3 carry the measuring current, while
L2 acts only as a potential lead. Ideally, the resistances
of L1 and L3 are perfectly matched and therefore canceled.
The resistance in R3 is equal to the resistance of the
sensor Rt at a given temperature—usually the begining of the
temperature range. At this point, V out = zero. As the
temperature of the sensor increases, the resistance of the
sensor increases, causing the resistance to be out of
balance and indicated at V out. Resistances L1 and L3 in
leads up to tens of feet long usually match well enough for
100 ohm three-wire RTDs. The worst case is resistance offset
equal to 10% of single-lead resistance.

The optimum form of connection for RTDs is a four-wire
circuit (see Fig. 3C). It removes the error caused by
mismatched resistance of the lead wires. A constant current
is passed through L1 and L4; L2 and L3 measure the voltage
drop across the RTD. With a constant current, the voltage is
strictly a function of the resistance and a true measurement
is achieved. This design is slightly more expensive than two
or three-wire configurations, but is the best choice when a
high degree of accuracy is required.

Answer / yugesh shah

Commonly 2 wire rtd used wher the place rtd installed and control room where indicated the temperature is not so far
But where indicator and rtd is in distance the third wire which is known a s compensation wire is used to stop of the effect of temperature which is in atmosphere and in process area...so if u wanna exactly temperature then should use 3 wire rtd

Answer / rama krishna

How to measure PROFIBUS SIGNALS,,?

Answer / sam

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

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