Temperature sensors are essential measuring instruments for ensuring the safety and efficiency of countless industrial processes. While RTDs and thermocouples both provide accuracy and reliability, each has pros and cons that make them better suited for particular applications.
Resistance thermometers (RTDs) and thermocouples are the two most common types of electronic temperature sensors used in industrial processes. The choice of which one to use depends on a variety of factors. First let’s take a look at what RTDs and thermocouples are, and how they differ from one another.
RTD
What is an RTD, and how does it work?
RTD stands for resistance temperature detector. This instrument is also called a resistance thermometer and, redundantly, an RTD probe or RTD sensor.
Within an RTD is a sensing element (resistor) that uses the change in electrical resistance of metal to determine the temperature. The most common metal in RTDs is platinum (Pt), as it is very chemically inert and has an almost linear temperature vs. resistance relationship. Platinum RTDs are often referred to as Pt100 sensors or Pt1000 sensors; the number refers to platinum’s nominal resistance (ohm Ω) at 0°C. Other metals used in RTDs are copper, nickel, and tungsten, but WIKA’s RTDs are made of platinum primarily because this metal has excellent stability, resists contamination, and its electrical resistance does not degrade over time.
Regardless of the metal used, its electrical resistance at specific temperatures is a known constant. As the temperature changes, so does the metal wire’s resistance. So, by comparing the known resistance to the measured resistance, one can calculate the temperature.
Types of RTDs
Thin-film (left) and wire-wound resistors
- Thin-film resistors are made up of a very fine layer of platinum deposited on ceramic and sealed by glass.
- Wire-wound resistors consist of a wire wrapped around and embedded inside a glass or ceramic casing.
RTDs also come with different numbers of wires in the cable.
- 2-wire RTDs are the most basic type, with the two lead wires creating a circuit. This circuit adds the resistance of the lead wires to the resistance of the RTD, which reduces its accuracy.
- 3-wire RTDs have an extra wire to compensate for the lead wire resistance, resulting in more accurate temperature measurements.
2-Wire, 3-Wire, and 4-Wire RTDs: Which Configuration Do You Need?
RTDs are available in three wiring configurations — 2-wire, 3-wire, and 4-wire. The difference comes down to how lead wire resistance is handled, which directly affects measurement accuracy. Choosing the right configuration is a tradeoff between cost and precision.
2-Wire RTD
The simplest and least expensive configuration. The two lead wires connect the RTD to the measuring instrument, but their resistance is added to the RTD’s reading, introducing a measurement error. Best suited for short lead lengths or applications where high accuracy is not critical.
3-Wire RTD
The most common industrial configuration. A third wire allows the measuring instrument to calculate and subtract lead wire resistance, significantly improving accuracy over a 2-wire setup. For compensation to work correctly, all three wires must have the same resistance — meaning matched wire material, gauge, and length. This configuration covers the vast majority of industrial process applications and is the standard choice where ±0.3–0.5°C accuracy is acceptable.
4-Wire RTD
The most accurate configuration, used when measurement precision is critical. Two wires carry excitation current through the RTD; the other two measure voltage drop across the sensing element only, completely eliminating lead wire resistance from the reading. This true Kelvin measurement method means accuracy is not affected by wire length, gauge, or material mismatches. The 4-wire configuration is required for Class AA accuracy per IEC 60751 (the sensor element must also be Class AA rated) and is the standard for laboratory calibration and high-precision process applications.
Which Wiring Configuration Should You Choose?
| Configuration | Accuracy | Cost | Best For |
|---|---|---|---|
| 2-Wire | Lowest | Lowest | Short cable runs; non-critical accuracy requirements |
| 3-Wire | Good | Moderate | Most industrial process applications; standard choice |
| 4-Wire | Highest (Class AA) | Highest | Laboratory calibration; critical process measurement; long cable runs |
The Pros and Cons of RTDs
Resistance thermometers are popular for many reasons:
- High accuracy, up to class AA (4-wire RTD)
- High repeatability
- Wide compatibility with instruments and processes due to their widespread use
- Excellent long-term stability
- Easy installation, as no extension wires are required
- Ease of calibration
- Suitable for temperatures between −321°F (−196°C) to 1,112°F (600°C).
On the other hand, RTDs cannot withstand extremely high temperatures, such as those found in chemical, petrochemical, and refinery applications. Pt100 and Pt1000 sensors can be expensive, due to the high cost of platinum. There’s also the possibility of self-heating errors, and compared to thermocouples, RTDs have a slower response time and are more susceptible to extreme shock and vibration.
Thermocouple with connection cable
What is a thermocouple, and how does it work?
Thermocouples are temperature sensors with a pair of dissimilar wires, each with a different electrical property at different temperatures. The working principle of thermocouples is that thermal energy is converted to electrical energy. At one end of the thermocouple, the two wires are welded or otherwise connected; this is the measuring point. When the temperature changes at this point, so does the electron density of each metal. The difference in temperature between the two metals creates a thermoelectric voltage. Since the relationship between temperature and voltage is known, this measured voltage is used to determine the temperature reading.
Types of Thermocouples
Thermocouples come in many different types, based on the pairing of dissimilar metals. Some of the common metal pairings are:
Thermocouple Types: Full Reference Guide
Thermocouples are classified by letter type, each defined by a specific alloy combination, temperature range, and optimal application environment. Base metal types (J, K, T, E, N) use less expensive alloys and cover most industrial applications. Noble metal types (B, R, S) use platinum-rhodium alloys for high-temperature and precision applications.
Base Metal Thermocouples
| Type | Alloy Combination | Temperature Range | Common Applications | Key Characteristic |
|---|---|---|---|---|
| J | Iron / Constantan | −210°C to 760°C (−346°F to 1,400°F) | Boilers, furnaces, general industrial | One of the few types suitable for reducing atmospheres; iron leg susceptible to rust above 550°C in oxidizing environments |
| K | Chromel / Alumel | −270°C to 1,260°C (−454°F to 2,300°F) | Most general industrial applications, nuclear, HVAC | Most widely used industrial thermocouple; wide range, low cost; susceptible to sulphur and “green rot” between 816–1,038°C |
| T | Copper / Constantan | −270°C to 370°C (−454°F to 700°F) | Cryogenics, laboratory, food processing | Excellent stability at low and cryogenic temperatures; suitable for oxidizing, reducing, and inert atmospheres; copper leg oxidizes quickly above 370°C |
| E | Chromel / Constantan | −270°C to 870°C (−454°F to 1,600°F) | Cryogenics, aviation, flow chambers | Highest thermoelectric output of all base metal types; non-magnetic; preferred over K and J at temperatures below 1,000°F |
| N | Nicrosil / Nisil | −270°C to 1,260°C (−454°F to 2,300°F) | High-temperature industrial processes where Type K instability is a concern | Superior oxidation and sulphur resistance vs. Type K; better repeatability between 300–500°C; preferred alternative to K at high temperatures |
Noble Metal Thermocouples
Noble metal thermocouples use platinum-rhodium alloys. They offer higher accuracy and stability at elevated temperatures but are significantly more expensive than base metal types and cannot be used in reducing atmospheres.
| Type | Alloy Combination | Temperature Range | Common Applications | Key Characteristic |
|---|---|---|---|---|
| R | Pt-13% Rh / Platinum | 0°C to 1,450°C (32°F to 2,642°F) | Steel industry, high-temperature furnaces, kilns | Slightly higher output and stability than Type S due to higher rhodium content; requires protective sheath to avoid contamination |
| S | Pt-10% Rh / Platinum | 0°C to 1,450°C (32°F to 2,642°F) | Medical industry, pharmaceutical, high-temperature lab and process | High accuracy and oxidation resistance; historically used as the international temperature standard; must be protected from metallic and non-metallic vapors |
| B | Pt-30% Rh / Pt-6% Rh | 800°C to 1,800°C (1,472°F to 3,272°F) | Powder metallurgy, sintering furnaces, vacuum furnaces, molten metal | Highest temperature range of all noble metal types; near-zero output below 50°C (no compensation wire required); both legs contain rhodium |
Thermocouples are made of two metals with different electron densities.
Most thermocouples are made of relatively inexpensive base metals, although some have metal pairings containing more expensive platinum, rhodium, rhenium, and tungsten.
The Pros and Cons of Thermocouples
In addition to their ruggedness in extreme conditions, thermocouples are also less expensive than RTDs and have a faster response time with their smaller diameter.
How to Choose between RTD and Thermocouple
In some applications, it doesn’t matter very much whether you use a resistance thermometer or a thermocouple. Other times, one type is definitely better than the other. In general, thermocouples are better for high-temperature and high-vibration processes, applications that require fast response times, and those with limited space. RTDs offer better accuracy, repeatability, and stability.
RTD | Thermocouple | |
Operating temperature range | −321°F (−196°C) to | −328°F (−200°C) to |
Higher accuracy | ✓ | |
Higher repeatability | ✓ | |
Better performance in high-vibration environments | ✓ | |
Better reliability in high-pressure environments | ✓ | |
Higher cost | ✓ | |
Faster response time | ✓ | |
Higher stability | ✓ | |
Better linearity | ✓ | |
Easier installation | ✓ |
Ultimately, when choosing a temperature sensor, you need to consider the application’s
- Temperature range
- Pressure range
- Humidity
- Shock and vibration
- Media (solid, liquid, or gaseous; corrosive; hazardous)
- Flow rate
In addition, some applications require the use of a thermowell to protect the temperature sensor from the process media, which affects the response time.
Choosing the right temperature sensor can be complex. For best results, contact the temperature specialists at WIKA USA for personalized advice on which instrument is the best fit your application and budget.