How RTD’s Work
A resistance temperature detector (RTD) operates on the principle of change in electrical resistance in wire as a function of temperature. All metals produce a positive change in resistance for a positive change in temperature, but the most commonly used as detectors are Nickel and Platinum.

Simulation of output voltage from heating an
RTD connected to a constant current source

Wire Resistance
A typical RTD consists of a fine platinum wire wrapped around a mandrel and covered with a protective coating. Usually, the mandrel and coating are glass or ceramic.

Variation of the Platinum wire resistance with temperature can be expressed by:

R0 is the wire resistance at 0ºC, A = 3.9x10^-3 and B = -5.8x10^-7

Alpha Value
The mean slope of the resistance vs. temperature curve for the RTD is often referred to as the alpha value, alpha standing for the temperature coefficient. The slope of the curve for a given sensor depends somewhat on purity of the platinum in it.

Alpha can be calculated from:

where:

Alpha for the Platinum 100 Ohms sensor is:

Accuracy and Stability
Platinum RTD elements can be supplied in two classes of accuracy defined as:

Class             A ƒ¢T = +/- ( 0.15 + 0.002 x T)
Class             B ƒ¢T = +/- (0.3 + 0.005 x T)

Where:

Platinum RTD’s offer very good long term stability: e.g. better than 0.1°C per year.

Although the Platinum temperature – resistance curve is much more linear than the thermocouple’s temperature – voltage curve, for high accuracy temperature readings linearisation of this curve is still needed.

Metal Film RTD’s
In the latest RTD’s the Platinum wire has been replaced by a Platinum thin film deposited onto a small flat ceramic substrate, etched with a laser-trimming system and sealed. Due to its small size the device can respond quickly to temperature changes, and is more robust and therefore more suitable for aerospace applications.

Standard RTD’s Types