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RTD Accuracy and Tolerances
Understanding the temperature accuracy of Class A and Class B RTDs

Standard, off-the-shelf, RTDs are not themselves perfectly accurate devices, so you don’t need an equation that fits their characteristic perfectly. RTDs commonly come in two classes of accuracy. Class A devices have a tolerance of ±0.2°C at 0°C increasing linearly to ±1°C at 500°C. Class B devices have a tolerance of ±0.35°C at 0°C increasing linearly to ±2°C at 400°C. See this Excel table of RTD tolerances.

Off-the-shelf RTD tolerances determine the temperature errors you'll see when you replace RTDs. Consequently, their baseline tolerances are often called the interchangeability tolerance. The following chart provides the errors you can expect to see when you interchange RTDs, as a function of temperature and for different RTD standards.

RTD interchangeability tolerance chart, accuracy chart showing temperature errors of Class A and Class B RTDs
Fig. 1  RTD interchangeability tolerances and accuracy chart for Class A and Class B RTDs

This chart is reproduced from the Omega web site where you can find further technical information on RTDs.

FIXME Include a note about the greater accuracy attained if you calibrate after replacing RTDs.

See also → Measuring Temperature

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