Understanding Infrared Thermography Reading 3

RTD Materials
Different materials used in the construction of RTD offer a different relationship
between resistance and temperature. Temperature sensitive materials used in the
construction of RTD include platinum, nickel, and copper; platinum being the most
commonly used. Important characteristics of an RTD include the temperature
coefficient of resistance (TCR), the nominal resistance at 0 degrees Celsius, and the
tolerance classes. The TCR determines the relationship between the resistance and
the temperature. There are no limits to the TCR that is achievable, but the most
common industry standard is the platinum 3850 ppm/K. This means that the
resistance of the sensor will increase 0.385 ohms per one degree Celsius increase in
temperature. The nominal resistance of the sensor is the resistance that the sensor
will have at 0 degrees Celsius. Although almost any value can be achieved for
nominal resistance, the most common is the platinum 100 ohm (pt100). Finally, the
tolerance class determines the accuracy of the sensor, usually specified at the
nominal point of 0 degrees Celsius. There are different industry standards that have
been set for accuracy including the ASTM and the European DIN. Using the values of
TCR, nominal resistance, and tolerance, the functional characteristics of the sensor
are defined.
Charlie Chong/ Fion Zhang
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