Thermocouples

Thermocouples are temperature sensors that use voltage as a means of calculating a given temperature. They contain two metal wires of differing material that are typically welded together at one end, which is known as a junction. When a junction is heated or cooled, it creates a voltage according to the different physical properties of the two metal wires. Being that thermocouples cannot determine absolute temperatures but can solely indicate a difference in temperature, a known reference (a cold junction or reference junction) is needed in order to calculate the sample temperature.

Through the use of a reference table that accounts for the characteristics of each metal wire, the temperature of interest can be acquired by measuring the voltage output and knowing the temperature of the cold junction. Thermocouples often only need fractions of seconds to provide a reading due to the low surface area of the entire apparatus. There are a wide variety of thermocouples to choose from that differ in temperature range, accuracy, reliability, and durability.

Types of Thermocouples

Type Metals Used Approximate Range Accuracy Notes
C Tungsten + 5% Rhenium Tungsten + 26% Rhenium 0 to
2300°C
  Highest temperature limit
E Chromel
(Nickel + Chromium Alloy)
Constantan
(Copper + Nickel Alloy)
-200 to
850°C
±1.7°C or
±0.5%
Higher accuracy
J Iron Constantan
(Copper + Nickel Alloy)
-100 to
750°C
±2.2°C or
±0.75%
Higher sensitivity but iron may oxidize
K Chromel
(Nickel + Chromium Alloy)
Alumel
(Nickel + Manganese + Aluminum + Silicon Alloy)
-200 to
1300°C
±2.2°C or
±0.75%
Most common, better oxidation resistance than E, J, and T
N Nicrosil
(Nickel + Chromium + Silicon Alloy)
Nisil
(Nickel + Silicon Alloy)
-270 to
1300°C
±2.2°C or
±0.75%
Similar to K but even less likely to oxidize
T Copper Constantan
(Copper + Nickel Alloy)
-200 to
350°C
±1.0°C or
±0.75%
Very stable and suitable for low yemperatures

Platinum Thermocouples

B Platinum + 30% Rhodium Platinum + 6% Rhodium 0 to
1700°C
±0.5% Suitable for very high temperatures, high accuracy and stability
R Platinum + 13% Rhodium Platinum 0 to
1450°C
±1.5°C or
±0.25%
Suitable for very high temperatures, high accuracy and stability
S Platinum + 10% Rhodium Platinum 0 to
1450°C
±1.5°C or
±0.25%
Suitable for very high temperatures, high accuracy and stability

RTDs

Resistance temperature detectors, also known as resistive thermal devices, (RTDs) are temperature sensors that function on the premise of a positive correlation between resistance and temperature. An RTD is typically a metal wire that is wrapped around a core, which can be made of various materials. As the RTD is heated, the whole sensor expands, which thus increases the resistance. As the material of the wire and core are known, a given resistance can be calculated to a specific temperature based on the specifications of the particular RTD used. The wires utilized in RTDs can be made of materials such as Copper and Nickel, however, Platinum is the most widely used and preferred metal.

Platinum resistance thermometers (PRTs) give the highest accuracy and repeatability of all of the RTDs.

While RTDs offer a greater amount of accuracy in their measurements, they have a limited range, typically peaking at around 600°C. Although RTDs can be small enough to give temperature readings in fractions of a second, generally, RTDs take up to a few seconds to provide an accurate reading. There are a variety of RTDs that are commonly used such as thin film, coiled, and wire wound elements.

The Bottom Line

In comparison with RTDs, thermocouples are not only typically more affordable, but also have broader temperature ranges, higher temperature limits, and faster response times. However, thermocouples are less accurate and less constant than RTDs, requiring additional maintenance and calibration due to corrosion of the wires. Because of this decreased corrosion, RTDs tend to provide more repeatability and stability than their counterparts. With the added repeatability, stability, and accuracy at lower temperatures, RTDs are becoming more prevalent in applications that remain below 600°C.

In environments with large amounts of vibration, thermocouples may last longer due to the sheer size of the wires used. The vibrations may cause either sensor to break; however, RTDs have thinner wires that are more prone to breaking. Selecting between RTDs and thermocouples really comes down to a case-by-case basis depending on the specifications of the given application.

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