Tuesday, March 12, 2019

Theory of RTD Operation

Theory of RTD OperationAn RTD is a temperature measuring device that changes resistance with temperature change, rather than changing voltage, as with a thermocouple.

Most commonly used is the platinum 100 ohm RTD because of their stability in air and linearity. Their resistance is 100 ohms @ 0 Deg.C and increases with temperature.

Theory of RTD OperationCommon terms associated with RTD’s are temperature coefficient or alpha, and tolerance class.

Alpha is ohms per ohm per Deg.C.
The average resistance change per unit of temperature from boiling point to ice point of water:

  • Rboiling – Rice point/100deg/100ohms
  • 138.5 – 100.0/100/100 = .00385

Tolerance class is the amount an RTD will differ from the standard resistance curve per Deg.C.

  • Class A (+/- .15 + .002*t)
  • @ temp of 100DegC = +/- .35DegC

When ordering an RTD, a tolerance class will be part of the order, dependent on the application. IEC 751 stipulates that the RTD be marked with their nominal R0 value, their tolerance class, the wiring configuration and the temperature range.
Theory of RTD Operation
3-wire configuration

  • Pt100 / A / 3 / -100/+200  = Platinum 100 Ohm / Class A / 3-Wire / -100 to +200 Deg.C
The most common RTD configuration is the 3-wire type. This configuration is more than adequate for 99.9% of applications. If absolute accuracy is needed, a fourth wire can be introduced, but rarely is it worth the added cost.
Theory of RTD Operation
2-wire configuration

Another configuration is a two wire RTD with a stand-alone loop. (Probably rarely used today).

Since the RTD is a resistance device, the resistance of the wires used to connect the RTD to the measurement meter introduces errors and must be known. This is the reason a third (or fourth), wire is used.

Theory of RTD Operation
3rd wire used to cancel wire error
First the meter reads the resistance of the two common wires to determine the value of Rwire. For a three wire RTD, it is assumed that this resistance is the same as that of one common and one non-common wire.

Then the meter reads the resistance of one of the common wires, the RTD, and the non-common wire to determine Rtotal

Theory of RTD Operation
Meter reading 2 common wires
Meter electronics and software then subtract Rwire from Rtotal to get Rt which is then converted to a temperature.

Theory of RTD Operation

Rt = Rtotal – Rwire