WHAT IS DIFFERENCE BETWEEN TWO WIRE RTD AND THREE WIRE RTD?
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Answer Posted / 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.
| Is This Answer Correct ? | 35 Yes | 8 No |
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