RTDs - or Resistance Temperature Detectors - are temperature sensors that contain a resistor that changes resistance value as its temperature changes. They have been used for many years to measure temperature in laboratory and industrial processes, and have developed a reputation for accuracy, repeatability, and stability.
Most RTD elements consist of a length of fine coiled wire wrapped around a ceramic or glass core. The element is usually quite fragile, so it is often placed inside a sheathed probe to protect it. The RTD element is made from a pure material whose resistance at various temperatures has been documented. The material has a predictable change in resistance as the temperature changes; it is this predictable change that is used to determine temperature.
This page will help you better understand RTDs, but you can also speak to our application engineers at anytime if you have any special measurement challenges.
The discovery that resistivity of metals showed a marked temperature dependence was made by Sir Humphrey Davy the same year Seebeck made his discovery about thermoelectricity. Fifty years later, Sir William Siemens proffered the use of platinum as the element in a resistance thermometer.
The RTD is one of the most accurate temperature sensors. Not only does it provide good accuracy, it also provides excellent stability and repeatability. Most OMEGA standard RTDs comply with DIN-IEC Class B. RTDs are also relatively immune to electrical noise and therefore well suited for temperature measurement in industrial environments, especially around motors, generators and other high voltage equipment.
A probe is an assembly composed of an element, a sheath, a lead wire, and a termination or connection. Once the RTD element is selected, the wiring and packaging requirements need to be determined. There are a number of ways to wire the sensors, along with an unlimited number of probe or sensor constructions to choose from.
RTD Wiring Arrangement
In order to measure temperature, the RTD element must be connected to some sort of monitoring or control equipment. Since the temperature measurement is based on the element resistance, any other resistance (lead wire resistance, connections, etc.) added to the circuit will result in measurement error. Except for the 2-wire configuration, all other wiring arrangements allow the monitoring or control equipment to factor out the unwanted lead wire resistance and other resistances that occur in the circuit. Sensors using the 3-wire construction are the most common design, found in industrial process and monitoring applications. The lead wire resistance is factored out as long as all of the lead wires have the same resistance; otherwise, errors can result.
When specifying the lead wire materials, care should be taken to select the right lead wires for the temperature and environment the sensor will be exposed to in service. When selecting lead wires, temperature is by far the primary consideration, however, physical properties such as abrasion resistance and water submersion characteristics can also be important. The three most popular constructions are:
- PVC Insulated Probes offer a temperature range of -40 to 105°C, with good Abrasion Resistance and applicable for Water Submersion.
- PFA Insulated RTD Probes offer a temperature range of -267 to 260°C with Excellent Abrasion Resistance. They are also great for Water Submersion Applications.
- Although Fiberglass Insulated RTD Probes offer a higher temperature range of -73 to 482°C, its performance under abrasion or water submersion is considered to be not as effective.
Probes may be terminated in a connection head, quick disconnect, terminal block, or extension wire. Other termination styles are available upon special request.
Once the RTD element, wire arrangement, and wire construction are selected, the physical construction of the sensor needs to be considered. The final sensor configuration will depend upon the application. Measuring the temperature of a liquid, a surface, or a gas stream requires different sensor configurations.
1. Platinum (most popular and accurate)
4. Balco (rare)
5. Tungsten (rare)
There are two standards for platinum RTDs: the European standard (also known as the DIN or IEC standard) and the American standard. The European standard, also known as the DIN or IEC standard, is considered the world-wide standard for platinum RTDs. This standard, DIN/IEC 60751 (or simply IEC751), requires the RTD to have an electrical resistance of 100.00 Ω at 0°C and a temperature coefficient of resistance (TCR) of 0.00385 Ω/Ω/°C between 0 and 100°C.
There are two resistance tolerances specified in DIN/IEC751:
Class A = ±(0.15 + 0.002*t)°C or 100.00 ±0.06 Ω at 0ºC
Class B = ±(0.3 + 0.005*t)°C or 100.00 ±0.12 Ω at 0ºC
Two resistance tolerances used in industry are:
1⁄3 DIN = ±1⁄3* (0.3 + 0.005*t)°C or 100.00 ±0.10 Ω at 0ºC
1⁄10 DIN = ±1 ⁄10* (0.3 + 0.005*t)°C or 100.00 ±0.03 Ω at 0ºC
The combination of resistance tolerance and temperature coefficient define the resistance vs. temperature characteristics for the RTD sensor. The larger the element tolerance, the more the sensor will deviate from a generalized curve, and the more variation there will be from sensor to sensor (interchangeability). This is important to users who need to change or replace sensors and want to minimize interchangeability errors.
Each type of temperature sensor has a particular set of conditions for which it is best suited. RTDs offer several advantages:
• A wide temperature range (approximately -200 to 850°C)
• Good accuracy (better than thermocouples)
• Good interchangeability
• Long-term stability
With a temperature range up to 850°C, RTDs can be used in all but the highest-temperature industrial processes. When made using metals such as platinum, they are very stable and are not affected by corrosion or oxidation. Other materials such as nickel, copper, and nickel-iron alloy have also been used for RTDs. However, these materials are not commonly used since they have lower temperature capabilities and are not as stable or repeatable as platinum.