RTD（ Thermal resistance temperature detector) Is a temperature change when change the heat resistance of the metal.
for precision thermal resistance measurement is one of the most common metal platinum. Choose platinum because it is a precious metal, it is stable, corrosion resistance, not easy oxidation, and is easy to be processed into fine lines.
copper is used as the RTD. Due to its low electrical resistance, RTD copper ohm value is very low, and the need to accurately measure resistance when used as RTD.
because of the copper wire thermal resistance change with temperature and is used to connect the RTD transmitter, therefore must consider RTD and wire length between 2 w transmitter.

RTD sensor three versions of RTD compensated the RTD and length of wire between the transmitter.
signals from the RTD are usually by making a constant current through the RTD to produce, and measuring the voltage at the ends of the RTD to determine the resistance of the RTD.
= voltage/current resistance.

RTD wiring diagram
2 WIRE - Current flows through the RTD. The signal is at the ends of the RTD voltage drop. The resistance of the Cu wire must be short enough, so they change will not have serious error in the measurement.
- 3 line Same as 2 line, just at the bottom of the copper wire from the RTD transmission to the TX. It allows the measurement of Common Cu wire voltage drop. The double voltage and subtract from the signal voltage. This offset the drop in the public and current Cu wire.
4线- Cu signal leads directly into the RTD. They allow measuring RTD at the ends of the voltage and current and common copper wire does not have any effect. This is the RTD Settings, for the most accurate measurement, when the copper wires become problems due to its length.
transmitter must elaborate design, so it does not have a current source, can stimulate the RTD, drift with the change of environmental temperature. Signal processing circuit must be stable.
RTD's relationship with thermal resistance and the temperature is not linear. Three chang RTD can be compensated. If the sender is analog design by a small amount of RTD signal feedback to inspire RTD current source to complete linearization. Each for a temperature change, this kind of feedback can make a slight increase in current. Through the appropriate compensation, can achieve very high accuracy.
the transmitter based on microprocessor can Cu lead to similar way to simulate version management, but the plus/minus mathematical operation performed by the microprocessor.
using value lookup table of the microprocessor can be done more precise linearization.