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.
Calibration issues are discussed in detail below, but it's simple enough to figure out that the change must obviously be measured relative to a known base value at a certain temperature. For RTDs, this base figure is the resistance in ohms measured at a temperature of zero degrees Celsius. For example, 100 ohm probes have a resistance of 100 ohms at zero degrees.
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.
Thin-film RTDs work in the -50 to 500 degree Celsius range. Wire-wound RTDs have an even wider range that extends up to 660 degrees. Coiled-element RTDs offer the highest range, and can be used to measure temperatures as high as 850 degrees. The range may be further increased by using high-grade material that remains stable at extreme temperatures.
A good example of a common RTD is the Pt100. It is a platinum probe with a 100 ohm resistance at zero degrees and sensitivity in ohm per degree Celsius of 0.385. For applications that need highly accurate probes, the platinum resistance thermometer or PRT used has a resistance of 25.5 ohms. This is possible because the PRT has a high-grade platinum wire with a much larger diameter. This SPRT (standard PRT) easily functions at temperatures varying from -200 to +1000, and the readings have an accuracy of 0.03 degrees.
Some labs use secondary SPRTs which have the same high-grade platinum, but are cheaper because they have lower diameter wires. SPRTs provide greater accuracy at the expense of durability. It's still possible to get RTDs that are just as durable as thermocouples for industrial use, but they won't be as accurate as SPRTs.
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.
An RTD probe is better in many ways as compared to other temperature sensors such as a thermocouple or thermistors. Apart from the stability they provide and the wide range over which they can be used, RTDs are also more accurate than thermocouples and offer excellent repeatability. All this makes them much more suitable for high-precision applications.
Calibration issues are discussed in detail below, but it's simple enough to figure out that the change must obviously be measured relative to a known base value at a certain temperature. For RTDs, this base figure is the resistance in ohms measured at a temperature of zero degrees Celsius. For example, 100 ohm probes have a resistance of 100 ohms at zero degrees.
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.
Thin-film RTDs work in the -50 to 500 degree Celsius range. Wire-wound RTDs have an even wider range that extends up to 660 degrees. Coiled-element RTDs offer the highest range, and can be used to measure temperatures as high as 850 degrees. The range may be further increased by using high-grade material that remains stable at extreme temperatures.
A good example of a common RTD is the Pt100. It is a platinum probe with a 100 ohm resistance at zero degrees and sensitivity in ohm per degree Celsius of 0.385. For applications that need highly accurate probes, the platinum resistance thermometer or PRT used has a resistance of 25.5 ohms. This is possible because the PRT has a high-grade platinum wire with a much larger diameter. This SPRT (standard PRT) easily functions at temperatures varying from -200 to +1000, and the readings have an accuracy of 0.03 degrees.
Some labs use secondary SPRTs which have the same high-grade platinum, but are cheaper because they have lower diameter wires. SPRTs provide greater accuracy at the expense of durability. It's still possible to get RTDs that are just as durable as thermocouples for industrial use, but they won't be as accurate as SPRTs.
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.
An RTD probe is better in many ways as compared to other temperature sensors such as a thermocouple or thermistors. Apart from the stability they provide and the wide range over which they can be used, RTDs are also more accurate than thermocouples and offer excellent repeatability. All this makes them much more suitable for high-precision applications.
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