Editing Resistance thermometer (section)

==Wiring configurations==

===Two-wire configuration===
[[File:twowire.gif|Two-wire resistance thermometer]]

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 added to that of the sensor, leading to errors of measurement. This configuration allows use of 100&nbsp;meters of cable. This applies equally to balanced bridge and fixed bridge system.

For a balanced bridge usual setting is with R2 = R1, and R3 around the middle of the range of the RTD. So for example, if we are going to measure between {{convert|0|and|100|C|F|0}}, RTD resistance will range from 100 Ω to 138.5 Ω. We would choose R3 = 120 Ω. In that way we get a small measured voltage in the bridge.

===Three-wire configuration===
[[File:RTD 3Wire.svg|Three-wire RTD Wheatstone bridge]]

In order to minimize the effects of the lead resistances, a three-wire configuration can be used. The suggested setting for the configuration shown, is with R1 = R2, and R3 around the middle of the range of the RTD. Looking at the [[Wheatstone bridge]] circuit shown, the voltage drop on the lower left hand side is V_rtd + V_lead, and on the lower righthand side is V_R3 + V_lead, therefore the bridge voltage (V_b) is the difference, V_rtd &minus; V_R3. The voltage drop due to the lead resistance has been cancelled out. This always applies if R1=R2, and R1, R2 >> RTD, R3. R1 and R2 can serve the use of limiting the current through the RTD, for example for a Pt100, limiting to 1&nbsp;mA, and 5&nbsp;V, would suggest a limiting resistance of approximately R1 = R2 = 5/0.001 = 5,000 Ohms.

===Four-wire configuration===
[[File:4wire2.svg|400px|Four-wire resistance thermometer]]

The four-wire resistance configuration increases the accuracy of measurement of resistance. [[Four-terminal sensing]] eliminates voltage drop in the measuring leads as a contribution to error. To increase accuracy further, any residual thermoelectric voltages generated by different wire types or screwed connections are eliminated by reversal of the direction of the 1&nbsp;mA current and the leads to the DVM (digital voltmeter). The thermoelectric voltages will be produced in one direction only. By averaging the reversed measurements, the thermoelectric error voltages are cancelled out.{{citation needed|date=December 2014}}