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Two Wire
When accuracy is not critical, a two-wire RTD is the least expensive;
offering. Using lead wires to place any distance between a two wire RTD and
a receiving device will further compromise its accuracy. The potential for
poor
accuracy from a two-wire RTD stems from its inability to compensate for lead
length, resistance that changes the ohm value of the original signal. A
two-wire RTD should be used only in applications where the receiving device
connects directly to the sensor |
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Three Wire RTD
Three-wire RTD's compensate
for resistance resulting from length differences by adding a third lead to the
RTD. To accomplish this requires that the wires match exactly. Any difference
in resistance between the lead wires will cause an imbalance, which will
compromise the accuracy of the RTD. Lead length variance, work hardening or
corrosion, and manufacturing irregularities are errors to avoid. Quality
manufacturing is critical to insure balance of all three leads. |
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Four Wire RTD
Errors caused by resistance imbalance between
leads are cancelled out in a four-wire RTD circuit. Four-wire RTD's are
used where superior accuracy is critical or if the sensor is installed far
from the receiving device. In a four-wire RTD one pair of wires carries
the current through the RTD the other pair senses the voltage across the RTD.
2- and three-wire RTD's require heavier lead wire because thicker wire, by
creating less resistance to the measured signal, reduces measurement
distortion. Therefore lighter gauge wire, less expensive, may be used in
four-wire RTD applications. RTD's are limited to temperatures of 1200 ° F
and because of the construction of the sensing element, RTD's do not do well
in high-vibration and severe mechanical shock environments. When selecting a
temperature sensor for an application you should consult your temperature
sensor manufacturer for recommendations. |
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