Wiktor Wise
Resistance thermometers, also known as resistance temperature detectors (RTDs), are sensors used to measure temperature. They are instruments for measuring electrical resistance that are calibrated in units of temperature. The RTD wire is made of a pure material, typically platinum, nickel, or copper, and its resistance/temperature relationship is used to indicate temperature. The accuracy of resistance thermometers is specified as the tolerance for measured temperatures, and they are widely used due to their accuracy, precision, and stability in high temperatures over prolonged periods.
| Characteristics | Values |
|---|---|
| Other names | Resistance temperature detectors (RTDs) |
| Function | Measures temperature by measuring electrical resistance |
| Construction | Fine wire wrapped around a heat-resistant ceramic or glass core |
| Wire material | Platinum (most common), nickel, or copper |
| Accuracy | High |
| Precision | High |
| Stability | High |
| Repeatability | High |
| Temperature range | Up to 400°C for nickel or nickel alloys, higher for platinum and its alloys |
| Wire connection | Two-, three-, or four-wire |
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What You'll Learn
How resistance thermometers work
Resistance thermometers, also called resistance temperature detectors (RTDs), are sensors used to measure temperature. They are widely used, simple, and accurate. They are designed to be less prone to electrical interference, which is common in industrial environments.
The RTD wire is made of a pure material, typically platinum (Pt), nickel (Ni), or copper (Cu). The material has an accurate resistance-temperature relationship, which is used to indicate the temperature. The RTD's resistance increases linearly when the temperature increases. The RTD wire is wrapped around a heat-resistant glass or ceramic core. The coil diameter provides a compromise between mechanical stability and allowing the expansion of the wire to minimize strain and consequential drift.
The simplest resistance-thermometer configuration uses two wires. However, RTDs can have three or four wires to increase accuracy by eliminating connection-lead resistance errors. The three-wire connection is sufficient for most purposes and is an almost universal industrial practice. Four-wire connections are used for the most precise applications.
The resistance value of the resistive element of a resistance thermometer changes at a fixed ratio with temperature changes. The temperature is derived by passing a certain current through the resistive element, measuring the voltage at both ends of it with an instrument, and calculating the resistance value through Ohm's Law (E = IR).
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The materials used to make resistance thermometers
Resistance thermometers, also called resistance temperature detectors (RTDs), are sensors used to measure temperature. They are made of a pure metal, typically platinum, nickel, or copper, whose electrical resistance varies with temperature. Platinum resistance thermometers (PRTs) are the most common type, with a standard resistance of 100 ohms at 0 °C, known as Pt100 sensors.
Platinum is the most common material used in RTD sensing elements due to its high resistance to oxidation at high temperatures. For temperatures up to 400 °C, nickel or nickel alloys can be used, but above this, platinum is preferred due to its superior oxidation resistance. Platinum sensors can be obtained in thin-film or thick-film forms, offering greater design flexibility than older platinum-wire types.
The sensing wire of an RTD is wrapped around an insulating mandrel or core, which must be an electrical insulator. At low temperatures, fibre or glass is used, while at higher temperatures, the RTD is housed in protective probes made of ceramic or glass. The mandrel is typically made of a hard-fired ceramic oxide, with the sensing wire inserted into bores and packed with finely ground ceramic powder, allowing the wire to move while remaining in thermal contact.
For industrial applications, RTDs may use thin-film or coil-wound elements, with internal lead wires ranging from PTFE-insulated stranded nickel-plated copper to silver wire, depending on the sensor size and application. Sheath material is typically stainless steel, with higher-temperature applications requiring Inconel.
Overall, the materials used in resistance thermometers are chosen to provide stability, accuracy, and durability, making them suitable for a wide range of temperature measurement applications.
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The advantages of resistance thermometers
Resistance thermometers, also called resistance temperature detectors (RTDs), are sensors used to measure temperature. They are widely used, simple, and accurate. They are constructed in a number of forms and offer greater stability, accuracy, and repeatability in some cases than thermocouples. Here are some advantages of resistance thermometers:
Accuracy and Precision
Resistance thermometers are extremely accurate sensors. They are designed to withstand the rigors of industrial environments and are less prone to electrical interference. They are also suitable for precision applications. Their accuracy and precision make them a reliable choice for industrial process applications with critical temperature readings.
Stability
Resistance thermometers offer stability in high temperatures over prolonged periods, ensuring that the readings remain consistent over time. This makes them ideal for long-term monitoring. The linear relationship between temperature and electrical resistance also simplifies the interpretation of readings.
Repeatability
Resistance thermometers are capable of producing consistent results under identical conditions, which is invaluable for processes that rely on repeatability. This is especially useful in industrial applications where temperature readings are critical.
Durability
Resistance thermometers, especially those with a ""strain-free" design, provide the durability necessary for industrial use. This design allows the sensing wire to expand and contract freely, without influence from other materials. This feature also helps to minimize mechanical strain.
Versatility
Resistance thermometers can be constructed from a variety of materials, including platinum, nickel, or copper, and can be designed for different temperature ranges. They can also be used in a range of applications, from general-purpose temperature measurements to precision fast response measurements.
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The disadvantages of resistance thermometers
Resistance thermometers, also known as resistance temperature detectors (RTDs), are sensors used to measure temperature. They are based on the principle that the electrical resistance of a material changes with temperature. The advantages of resistance thermometers include accuracy, precision, and stability. However, there are several disadvantages to using resistance thermometers, which are detailed below:
Limited temperature range
Resistance thermometers can only be used over a limited temperature range due to the different expansion rates of the substrate and resistive deposit. This creates a "strain gauge" effect that impacts the resistive temperature coefficient. While the elements can work with temperatures up to 300 °C (572 °F) without additional packaging, they require encapsulation in glass or ceramic to operate at higher temperatures. Special high-temperature RTD elements can be used up to 900 °C (1,652 °F) with suitable encapsulation.
Long response time
Resistance thermometers have a longer response time compared to other devices, such as thermocouples. This refers to the time it takes for the device to respond to a given input or stimulus.
Complexity of wiring systems
Resistance thermometers typically use a two-, three-, or four-wire wiring system. While the two-wire system is inexpensive, it is not practical as it requires checking and adjusting the resistance of the conductor beforehand. The three- and four-wire systems are more precise but require careful consideration of the conductor's material, outer diameter, length, and electric resistance to minimize errors.
Conductor resistance
Even with the appropriate wiring system, it is essential to minimize the effect of conductor resistance as much as possible. This involves ensuring that the conductors have the same material, outer diameter, length, and electric resistance, and that there is no temperature gradient. For long conductors, checking the resistance value per meter is necessary to avoid errors.
Contamination at high temperatures
At temperatures above 660 °C, it becomes challenging to prevent platinum, a commonly used material in resistance thermometers, from becoming contaminated by impurities from the metal sheath of the thermometer. This is why laboratory standard thermometers often replace the metal sheath with a glass construction.
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Resistance thermometers vs thermocouples
Resistance thermometers, also called resistance temperature detectors (RTDs), are sensors used to measure temperature. They are constructed in a number of forms and offer greater stability, accuracy, and repeatability in some cases than thermocouples. RTDs are temperature sensors that contain a resistor that changes resistance value as its temperature changes. The most popular RTD is the Pt100, which has been used for many years to measure temperature in laboratory and industrial processes. RTD elements consist of a length of fine wire wrapped around a heat-resistant ceramic or glass core. The RTD wire is a pure material, typically platinum (Pt), nickel (Ni), or copper (Cu). The material has an accurate resistance-temperature relationship, which is used to provide an indication of temperature.
Thermocouples, on the other hand, rely on the Seebeck effect, where a voltage is generated when two dissimilar metals are joined at one end. The voltage produced is directly proportional to the temperature difference between the two ends. Thermocouples consist of two different types of metals joined together at one end, creating a junction. When the junction of the two metals is heated or cooled, a voltage is created that can be correlated back to the temperature. Thermocouples are commonly used in many applications due to their wide range of models and technical specifications. They can also be used at higher temperatures and have a better response time.
RTDs are generally more accurate than thermocouples. RTDs have typically an accuracy of 0.1 °C, compared to 1 °C for most thermocouples. RTD probe readings stay stable and repeatable for a long time. Thermocouple readings tend to drift due to chemical changes in the sensor. RTDs' linearity and lack of drift make them more stable in the long term. Thermocouples are more economical than RTDs due to their cheaper manufacturing process. RTDs, on the other hand, provide a more reliable output.
The choice between RTD sensors and thermocouples depends on the specific needs of the temperature measurement application. Consider factors such as accuracy, temperature range, response time, cost, and maintenance requirements to make an informed decision.
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Frequently asked questions
A resistance thermometer is an instrument for measuring electrical resistance that is calibrated in units of temperature.
The electrical conductivity of a metal depends on the movement of electrons through its crystal lattice. Due to thermal excitation, the electrical resistance of a conductor varies according to its temperature. This forms the basis of resistance thermometry.
The materials most used for resistance thermometers are platinum, copper, and nickel. Platinum is the most dominant material internationally.
The typical range of a resistance thermometer depends on the material used. For example, the resistance change of nickel can be used for comparatively small temperature ranges, up to around 400°C, while platinum and its alloys are more suitable for higher temperature ranges due to their greater resistance to oxidation.
Resistance thermometers offer greater stability, accuracy, and precision in temperature measurements. They are also less prone to electrical interference and can be designed to handle shock and vibration.
