RTD Wire Configuration
RTD is available in 3 different types of wire configurations. The choice of such lead wire configuration is entirely dependent on the expected accuracy as well as on the instrumentation that is being utilized for measurement purpose.
Two Wire RTD
Of the 2 wires, the RTD has the simplest circuit configuration. 1 of the wires is attached to each of the sides of the element. The measurement can be made with any device that has been equipped for measuring resistance, which includes the basic VOMs (i.e. the volt-ohmmeters). This is the least accurate method of measuring temperature because the resistance of the lead wire is in serial conjugation with the sensing element. The lead wire is at a temperature different from the sensing element, and is also having varied resistance & temperature characteristics. More the length of the wire, the bigger is its effect over the measurement.
Three Wire RTD
Three-wire RTDs happen to be the most popular configurations for use in industrial applications. Three wires can be used to minimize the effect of wire resistance. Making use of this way, the 2 wires leading to the sensor are on adjacent branches. Each leg of the bridge has a lead resistor, so the impedance cancels as long as the two lead resistors are exactly the same. This type of configuration allows up to 600 meters of cable. When utilized in the correct fashion, the three-wire configuration removes any series resistance, which allows accurate measurement of the sensor element. Two wires are connected to one side of the sensor element and one wire to the other side. The resistance of L1 & L3 must be very close as much as possible; which will cause the resistance of the wire to decrease. The color code for a three-wire RTD is 2 red and 1 white.
Four Wire RTD
Four-wire RTDs are the most accurate ways of RTD measurement. They are mostly used in labs & are rarely seen in any kind of industrial applications. The 4-wire resistance thermometer configuration augments the accuracy as well as the reliability of the measured resistance: the resistance error is zero due to the resistance of the lead wire. Four-wire RTD circuits eliminate the effects of mismatched resistors in the wires. Constant current passes through L1 & L4, L2 & L3 measure the voltage drop across the RTD elements. Four-wire RTDs are mostly color-coded as 2 red and 2 white wires.
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