Common Thermocouple Types
Thermocouple Types and Standards
Many combinations of materials have been used to produce acceptable thermocouples, each with its own particular application spectrum. However, the value of interchangeability and the economics of mass production have led to standardisation, with a few specific types now being easily available, covering by far the majority of the temperature and environmental applications.
These thermocouples are made to conform to an EMF/temperature relationship specified in the form of tabulated values of EMFs resolved normally to 1µV against temperature in 1°C intervals and vice versa. Internationally, these reference tables are published as IEC 60584-1 (BS EN 60584-1). It is worth noting here, that the standards do not address the construction or insulation of the cables themselves or other performance criteria. With the diversity to be found, manufacturers’ own standards must be relied upon in this respect.
The standards cover the eight specified and most commonly used thermocouples, referring to their internationally recognised alpha character type designations and providing the full reference tables for each. See the reference tables published in this guide. At this point, it’s worth looking at each in turn, assessing its value, its properties and its applicational spread. Note that the positive element is always referred to first. Note also that, especially for base metal thermocouples, the maximum operating temperature specified is not the be all and end all. It has to be related to the wire diameter - as well as the environment and the thermocouple life requirements.
As a brief summary, thermocouple temperature ranges and material combinations are given in tables 3.1 and 3.2. The former comprise rare metal, platinum-based devices; the latter are base metal types.
Table 3.1: Commonly used Platinum Metal Thermocouples
Table 3.1: Commonly used Base Metal Thermocouples
Type K Nickel-Chromium vs Nickel-Aluminium
Generally referred to as Chromel-Alumel, is still the most common thermocouple in industrial use today. This thermocouple also defined in ASTM E-230, is designed primarily for oxidizing atmospheres. In fact, great care must be taken to protect the sensor in anything else! Maximum continuous temperature is about 1,100°C, although above 800°C oxidation increasingly causes drift and decalibration. For short term exposure, however, there is a small extension to 1,200°C. The device is also suitable for cryogenic applications down to -250°C.
Although Type K is widely used because of its range and cheapness, it is not as stable as other base metal sensors in common use. At temperatures between 250°C and 600°C, but especially 300°C and 550°C, temperature cycling hysteresis can result in errors of several degrees. Again, although Type K is popular for nuclear applications because of its relative radiation hardness, Type N is now becoming a better choice. For detailed information on Type K Thermocouples please click here.
Type J Iron vs Copper-Nickel
Commonly referred to as Iron/Constantan, this thermocouple also defined in ASTM E-230, is one of the few thermocouples that can be used safely in reducing atmospheres. However, in oxidizing atmospheres above 550°C, degradation is rapid. Maximum continuous operating temperature is around 800°C, although for short term use, temperatures up to 1,000°C can be handled. Minimum temperature is -210°C, but beware of condensation at temperatures below ambient because rusting of the iron leg can result, as well as low temperature embrittlement. For detailed information on Type J Thermocouples please click here.
Type T Copper vs Copper-Nickel
Copper-Constantan, this thermocouple also defined in ASTM E-230, has found quite a niche for itself in laboratory temperature measurements over the range -250° to 400°C. Beyond 400°C, the copper leg rapidly oxidizes. Repeatability is excellent in the range -200°C to 200°C (±0.1°C). Points to watch out for include the high thermal conductivity of the copper leg, and the fact that the copper/nickel alloy used in the negative leg is not the same as that in Type J - so they’re not interchangeable. For detailed information on Type T Thermocouples please click here.
Type E Nickel-Chromium vs Copper-Nickel
Chromel-Constantan, this thermocouple also defined in ASTM E-230, is known for its high emf output and is the highest of the commonly used devices. Having a high output is less significant in these days of stable solid state amplifiers. The usable temperature range extends from about -250°C (cryogenic) to 900°C in oxidizing or inert atmospheres. Recognized as more stable than Type K, it is therefore more suitable for accurate measurement. However, Type N still scores higher marks because of its stability and range. For detailed information on Type E Thermocouples please click here.
Type N Nickel-Chromium-Silicon vs Nickel-Silicon
Type N (Nicrosil-Nisil), termed as the revolutionary replacement for the Type K thermocouple, also defined in ASTM E-230, exhibits a much greater resistance to oxidation related drift at high temperatures and other common instabilities compared to Type K and other base metal type thermocouples. (See Part 1, Section 2.4) This thermocouple can also withstand higher temperatures than Type K, (1,280°C and higher for short periods).
Basically, oxidation resistance is superior because of the combination of a higher level of chromium and silicon in the positive Nicrosil conductor. Similarly, a higher level of silicon and magnesium in the negative Nisil conductor form a protective diffusion barrier. The device also shows much improved repeatability in the 300°C to 500°C range where Type K’s stability is somewhat lacking (due to hysteresis induced by magnetic and/or structural inhomogeneities). High levels of chromium in the NP conductor, and silicon in the NN conductor provide improved magnetic stability. Beyond this, it does not suffer other long term drift problems associated with transmutation of the high vapor pressure elements in mineral insulated thermocouple assemblies (mainly manganese and aluminum from the KN wire through the magnesium oxide insulant to the KP wire). Transmutation is virtually eliminated since the conductors contain only traces of manganese and aluminum. Finally, since manganese, aluminum and copper are not used in the NN conductor, stability against nuclear bombardment is much better.
Standardized in 1986 and subsequently published in IEC 60584 and ASTM E-230, this relative newcomer to thermocouple thermometry has even been said make all other base metal thermocouples (E, J, K and T) obsolete. Another claim by the more enthusiastic manufacturers and distributors is that it provides many of the rare metal thermocouple characteristics, but at base metal costs. In fact, up to a maximum continuous temperature of 1,280°C, depending on service conditions, it can be used in place of Type R and S thermocouples - devices which are between 10 and 20 times the price.
In fact, although adoption of this sensor was slower than many anticipated, now that Nicrobell and similar alloys have been developed, tried and tested for sheathing mineral insulated and metal sheathed Type N thermocouples for higher temperatures, it is seeing ever greater use - and this can only grow. There is now no doubt that it is indeed a fundamentally better thermocouple than its base metal rivals. For detailed information on Type N Thermocouples please click here.
Type S Platinum-10% Rhodium vs Platinum
This thermocouple, also defined in ASTM E-230, can be used in oxidizing or inert atmospheres continuously at temperatures up to 1600°C and for brief periods up to 1700°C. For high temperature work, insulators and sheaths made from high purity recrystallized alumina are used. In fact, in all but the cleanest of applications, the device needs protection in the form of an impervious sheath since small quantities of metallic vapor can cause deterioration and a reduction in the emf generated.
Continuous use at high temperatures also causes degradation, and there is the possibility of diffusion of rhodium into the pure platinum conductor - again leading to a reduction in output. For detailed information on Type S Thermocouples please click here.
Type R Platinum-13% Rhodium vs Platinum
Similar to the Type S combination, this thermocouple also defined in ASTM E-230, has the advantage of a slightly higher output and improved stability. In general Type R thermocouples are preferred over Type S, and applications covered are broadly identical. For detailed information on Type R Thermocouples please click here.
Type B Platinum-30% Rhodium vs Platinum-6% Rhodium
Type B is of a more recent vintage (1950’s) and this thermocouple is also defined in ASTM E-230, can be used continuously up to 1,600°C and intermittently up to around 1,800°C. In other respects the device resembles the other rare metal based thermocouples, Types S and R, although the output is lower, and therefore it is not normally used below 600°C. An interesting practical advantage is that since the output is negligible over the range 0°C to 50°C, cold junction compensation is not normally required.
Non Standard Thermocouples
Although there have been many, many thermocouple combinations developed over the years, almost all are no longer available or in use - except for very specialized applications, or for historical reasons. There are, however, four main non-standard types which continue to have their place in thermocouple thermometry.
Tungsten Rhenium Thermocouples
There are three primary combinations of this thermocouple. These are: Type G (tungsten vs tungsten-26% rhenium); Type C (tungsten-5% rhenium vs tungsten-26% rhenium); and D (tungsten-3% rhenium vs tungsten-25% rhenium). Of these, the first is certainly the cheapest, but embrittlement can be a problem in the tungsten leg. All can be used up to 2,300°C, and for short periods up to 2,750°C in vacuum, pure hydrogen, or pure inert gases. Above 1,800°C, however, there can be problems with rhenium vaporization. As for insulators, beryllia and thoria are generally recommended, although again problems can occur at the upper end of the temperature spectrum, with wires and insulators potentially reacting.
Iridium-40% Rhodium vs Iridium
With a claim to fame of being the only rare metal thermocouple that can be used in air without protection up to 2,000°C (short term only), these devices can also be used in vacuum and inert atmospheres. However, there are no standard reference tables, and users must depend upon the manufacturer for batch calibrations. Also, watch out for embrittlement after use at high temperatures.
Platinum-40% Rhodium vs Platinum-20% Rhodium
Recommended for use instead of Type B where slightly higher temperature coverage is required, this sensor can be used continuously at up to 1,700°C, and for short term exposure up to 1,850°C. Beyond this, the application rules as described for Type S apply. There are no standard reference tables, but normally batch calibrations are available from the manufacturer.
Nickel-Chromium vs Gold-0.07% Iron
This is probably the ultimate thermocouple specifically for cryogenics, being designed to measure below 1K, although it fares better at 4K and above. Reference tables have been published by the National Bureau of Standards, but in Europe the negative leg alloy is more commonly gold-0.03% iron.