Are You Doing Your RTD Calibration Correctly?

Published: 11th March 2011
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Resistance Temperature Detectors (RTDs) are electrical instruments that are used to measure temperature. An RTD is essentially a wound wire or a thin film device which uses the physical principle of positive temperature coefficient of electrical resistance of metals to measure their temperature. Electrically, its functioning is simple wherein a current is passed through a material with stable and defined change in resistance as against the change in temperature. The temperature is indicated when the RTDs are introduced into a circuit as their electrical resistance increases proportionally with the temperature. RTDs are typically used for industrial and laboratory measurements where accuracy is of utmost importance.

Like all measuring equipment and instruments RTD calibration is also essential. RTDs are extremely versatile and can be incorporated in innumerable ways into the design of the device posing a serious challenge to in accurate calibration.

Some of the most common effects of poorly calibrated RTD devices low accuracy or repeatability. In such cases transmitters, receivers, etc. will have to be replaced as the inconsistencies are discovered and troubleshooting the problem begins. There are also linearity errors and singular polarity in case of bidirectional currents. Some field devices requiring RTD calibration may not specify that they use pulsed currents. Lack of this crucial piece of information manifests in noisy readings, inconsistent reading or even as a temperature offset.

Although, several calibrators are available today, they are not designed for operation with all and expect detailed knowledge from the users in case of a mismatch. Some might argue that instead of the complex route of mapping the calibration technique with the application and then performing RTD calibration, it would be simpler to use devices that can be calibrated using thermocouple calibrators or loop calibrators.

Thermocouple calibrators use a controlled temperature source to be taken as a reference for comparison. The NIST accepts the thermocouple as an accurate reference. Some advanced thermocouple calibrators also use microprocessors and computer software for calibration. Still simpler to use are loop calibrators, which can simulate and measure DC mA currents for calibration of process devices. And a high end loop calibrator can drive for sustained periods into a process loop. These higher end units also feature DC volt measurement and sourcing along with certain amount of programmable operation.

Despite the potential issues RTDs are preferred over the many other methods of measuring temperature as RTD remains the most practical for the high accuracy, mid-range process temperatures ranging from about -100C to 800C, which fit the bill for most industrial and lab applications.

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