RTD Probe Basics And Types

By Marissa Velazquez

The RTD probe is an acronym for what's better known as a resistance temperature detector. The concept is based on the fact that the resistance of a wire made of a specific metal rises and falls in direct correlation to its temperature. Measuring the change in resistance of these probes when they come in contact with another object or environment therefore provides a highly precise measurement of the temperature.

This obviously requires highly calibrated probes. This aspect is discussed in more detail below, but the simple fact is that all the measurements are relative to a base value. In the case of RTDs, the change in resistance is measured in ohms, with the base value being the resistance at a zero degree temperature in Celsius. As an example, consider that 100 ohm probes are expected to have a zero-degree resistance of 100 ohms.

RTDs are able to measure temperatures across a wide range, although it may vary depending on the metal used and other factors such as the type and construction of the probes. For example, platinum is the preferred choice of metal on account of its stability. Not to mention the fact that it is highly resistant to oxidation and corrosion. Other metals such as copper and nickel that are not so stable may also be used, as can alloys such as nickel-iron.

For applications where the temperature may vary from 50 degrees below zero to 500 above, it's possible to make do with thin-film probes. A wire-wound one will give a slightly higher range of up to 660 degrees. Coiled-element probes work at even higher temperatures of around 850 degrees, and the range can be further increased by using higher-grade metal with bigger diameters.

The most common RTD is a Pt100 sensor made of platinum with a 100 ohm base resistance and a sensitivity of 0.385 ohm per degree Celsius. Highly accurate platinum resistance thermometers (PRTs) use expensive large diameter reference-grade platinum wires with lower resistance of around 25.5 ohms. These are called Standard PRTs or SPRTs, and can measure temperature with an accuracy of 0.03 degrees over a range that extends from -200 to 1000 degrees.

Laboratories often buy more affordable secondary SPRTs which use the same high-grade platinum, but with smaller diameters. Either way, the enhanced accuracy of an SPRT comes by sacrificing some of its durability. Even so, RTDs that are as durable as a thermocouple may be built for industrial applications that do not need extremely precise measurements.

The accuracy of the reading is also dependent on the calibration. The most accurate method is fixed-point calibration, which makes use of the melting or freezing point of water or other pure substances to generate known temperatures. Another method commonly used to calibrate industrial RTDs is comparison calibration, where one thermometer is calibrated in comparison with another one.

The RTD probe has several advantages over sensors such as thermocouples and thermistors. As explained above, the stability makes them desirable when the applications involve a wide range of temperatures. RTDs also offer better repeatability and are more accurate than thermocouples, which makes them the best choice for applications that require high-precision measurements.

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