RTD Sensor Calibration

Accuracy checks for a resistance temperature device (RTD) is often necessary to validate the accuracy of a new RTD.

How They Work

There are three basic types of RTDs: 2-wire, 3-wire, and 4-wire units all of which are wired in a bridge configured circuit. An RTD is a metallic element whose resistance varies with temperature. By connecting it in one leg of a Wheatstone bridge, its resistance can be measured. 2-wire RTDs are susceptible to errors caused by changes in lead wire resistance. To compensate for these and other errors, 3-wire RTDs and 4-wire RTDs may be used when accuracy is required.

Input and Output Measurement Standards

The application range of the RTD determines the measurement standards to be used to check this instrument. An appropriate input standard would be a temperature bath. A standard thermometer must be placed in one of the wells to confirm the accuracy of the bath. RTDs do not require temperature compensation. A volt-ohmmeter or decade resistance box can be used as the output measurement standard, because they measure resistance. To check an RTD with a volt-ohmmeter, look-up tables that relate RTD resistance to temperature are needed.

Connections

The two red RTD leads are connected to the positive meter leads, and the black RTD lead is connected to the negative lead.

Three Point Check

Test RTDs at: ambient temperature, mid-range temperature and high end of range temperature. RTD’s cannot be calibrated. When they are not within the manufactures specifications, the unit must be replaced.

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