A resistance temperature detector, otherwise known as an RTD probe, is based on a phenomenon seen in metals wherein the resistance changes along with temperature. Knowing the resistance is therefore as good as getting a temperature reading. Practically speaking, RTDs function as sensors or probes whose resistance changes when they come in contact with an environment or object whose temperature is to be measured.
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.
Resistance temperature detectors function in a wide range of temperatures. However, the exact extreme limits depend on several factors including the choice of metal and the type of construction. Platinum is the ideal metal for use in RTDs because of the high degree of stability it offers, and its imperviousness to oxidation and corrosion. More affordable probes may be built using commonly available metals such as copper or nickel, and also nickel-iron alloys.
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.
Regardless of the type of instrument, its accuracy is largely dependent on the calibration. The two most common methods used for RTDs are fixed-point and comparison calibration. The former works by taking the melting or freezing point of pure substances such as water as a reference. The comparison method simply uses calibrated RTDs of a higher range as a reference to calibrate another probe in a lower range.
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.
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.
Resistance temperature detectors function in a wide range of temperatures. However, the exact extreme limits depend on several factors including the choice of metal and the type of construction. Platinum is the ideal metal for use in RTDs because of the high degree of stability it offers, and its imperviousness to oxidation and corrosion. More affordable probes may be built using commonly available metals such as copper or nickel, and also nickel-iron alloys.
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.
Regardless of the type of instrument, its accuracy is largely dependent on the calibration. The two most common methods used for RTDs are fixed-point and comparison calibration. The former works by taking the melting or freezing point of pure substances such as water as a reference. The comparison method simply uses calibrated RTDs of a higher range as a reference to calibrate another probe in a lower range.
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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