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
IEC 60584-1 - Type S Platinum-10% Rhodium vs Platinum
This thermocouple can be used in oxidising 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 recrystallised 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 vapour 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 - leading to a reduction in output.
3.2 IEC 60584-1 - Type R Platinum-13% Rhodium vs Platinum
Similar to the Type S combination, this thermocouple has the advantage of slightly higher output and improved stability. In general Type R thermocouples are preferred over Type S, and applications covered are broadly identical.
3.3 IEC 60584-1 - Type J Iron vs Copper-Nickel
Commonly referred to as Iron/Constantan, this is one of the few thermocouples that can be used safely in reducing atmospheres. However, in oxidising 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 - rusting of the iron arm can result, as well as low temperature embrittlement.
3.4 IEC 60584-1 - Type K Nickel-Chromium vs Nickel-Aluminium
Generally referred to as Chromel-Alumel it is still the most common thermocouple in industrial use today. It is designed primarily for oxidising 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 a far better choice.
3.5 IEC 60584-1 - Type T Copper vs Copper-Nickel
Copper-Constantan, its original name, has found quite a niche for itself in laboratory temperature measurement over the range -250°C to 400°C - although above this the copper arm rapidly oxidises. 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 arm, and the fact that the copper/nickel alloy used in the negative arm is not the same as that in Type J - so they’re not interchangeable.
3.6 IEC 60584-1 - Type E Nickel-Chromium vs Copper-Nickel
Also known as Chromel-Constantan, this thermocouple is known for its high output - the highest of the commonly used devices, although this is less significant in these days of ultra stable solid state amplifiers. The usable temperature range extends from about -250°C (cryogenic) to 900°C in oxidising or inert atmospheres. Recognised as more stable than Type K, it is therefore more suitable for accurate measurement. However, Type N still ranks higher because of its stability and range.
3.7 IEC 60584-1 - Type B Platinum-30% Rhodium vs Platinum-6% Rhodium
Type B, developed in the 1950’s, and 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.
3.8 IEC 60584-1 - Type N Nickel-Chromium-Silicon vs Nickel-Silicon
Billed as the revolutionary replacement for the Type K thermocouple (the most common in industrial use), but without its drawbacks - Type N (Nicrosil-Nisil) exhibits a much greater resistance to oxidation-related drift at high temperatures than its rival, and to the other common instabilities of Type K in particular, but also the other base metal thermocouples to a degree (see Part 1, Section 2.4). It can thus handle 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 vapour pressure elements in mineral insulated thermocouple assemblies (mainly manganese and aluminium 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 aluminium. Finally, since manganese, aluminium and copper are not used in the NN conductor, stability against nuclear bombardment is much better.
Standardised in 1986 as BS EN 60584-1 Part 8 and subsequently published in IEC 60584, this relative newcomer to thermocouple thermometry has even been said to 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 (which are between 10 and 20 times the price).
Although adoption of this sensor was slower than many anticipated, 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.
3.9 IEC 60584-1 - Type C Tungsten-5% Rhenium vs Tungsten-26% Rhenium
Formerly known as W5, Type C thermocouples (and all Tungesten/Rhenium alloy combinations in general) offer reasonably high and relatively linear EMF outputs for high temperature measurement. These types of thermocouples should be used in vacuum, inert atmospheres or dry hydrogen applications. Above 1200°C tungsten can become brittle due to recrystallisation.
3.10 IEC 60584-1 - Type A Tungsten-5% Rhenium vs Tungsten-20% Rhenium
Similar to Type C above, Type A thermocouple have a slightly extended temperature range, up to 2500°C.
3.11 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 specialised applications, or for historical reasons). There are, however, four main non-standard types which continue to have their place in thermocouple thermometry.
3.12 Other Tungsten – Rhenium Thermocouple
There are two other primary combinations of this thermocouple: Type G (Tungsten vs Tungsten-26% Rhenium) and and D (Tungsten-3% Rhenium vs Tungsten-25% Rhenium). Both 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 vaporisation. As for insulators, beryllia and thoria are generally recommended, although again problems can occur at elevated temperatures, with wire and insulators potentially reacting.
3.11 Iridium-40% Rhodium vs Iridium
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, embrittlement after use at high temperatures is possible.
3.12 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.
3.13 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.