Thermocouple Sensors - Reference Information
What is a Thermocouple?
A thermocouple is a type of temperature sensor used for a wide variety of temperature measurement applications. There are many types of thermocouple available in a wide range of designs and constructions making them a practical choice for almost all temperature measurement applications and temperature ranges in industry, science and beyond.
If there is a temperature gradient in an electrical conductor, the energy (heat) flow is associated with an electron flow along the conductor and an electromotive force (EMF) is then generated in that region. Both the size and direction of the EMF are dependent on the size and direction of the temperature gradient itself as well as on the material forming the conductor. The voltage is a function of the temperature difference along the conductor length. This effect was discovered by TJ Seebeck in 1822.
The voltage appearing across the ends of the conductor is the sum of all the EMFs generated along it. For a given overall temperature difference, T1-T2, the gradient distributions shown in figures 2.1 a, b and c produce the same total voltage, E. As long as the conductor has uniform thermoelectric characteristics throughout its length.
The output voltage of a single conductor as shown, is not however normally measurable since the sum of the internal EMFs around a completed circuit in any temperature situation is zero. So, in a practical thermocouple sensor, the trick is to join two materials having different thermoelectric EMF/temperature characteristics in order to produce a usable net electron flow and a detectable net output voltage.
Thus, two connected dissimilar conductors, A and B, exposed to the same temperature gradients given in figure 2.1 generate outputs as shown in figure 2.2. Basically, there is a net electron flow across the junction caused by the different thermoelectric EMFs, in turn resulting from the interaction of the gradient with the two different conductors. Hence the term, 'thermocouple'.
Figures 2.1 a,b,c: Temperature Distributions Resulting in Same Thermoelectric EMF
Figures 2.2 a,b,c: Thermocouple EMFs Generated by Temperature Gradients
It is worth noting that the thermoelectric EMF is generated in the region of the temperature gradient and not at the junction as such. This is an important point to understand since there are practical implications for thermocouple thermometry. These include ensuring that thermocouple conductors are physically and chemically homogenous if they are in a temperature gradient. Equally, the junctions themselves must be in isothermal areas. If either of these conditions is not satisfied, additional, unwanted EMFs will result.
Incidentally, any number of conductors can be added into a thermoelectric circuit without affecting the output, so long as both ends are at the same temperature and that homogeneity is ensured. This leads to the concept of extension leads and compensating cables, enabling probe conductor lengths to be increased. See Part 2, Section 3.
Returning to figure 2.2, in fact the output, ET, is the same for any temperature gradient distribution over the temperature difference T1 and T2, provided that the conductors again exhibit uniform thermoelectric characteristics throughout their lengths. Since the junctions M, R1 and R2 represent the limits of the EMF-generating conductors, and since the remaining conductors linking the measuring device are uniform copper wire, the output of the thermocouple is effectively a function only of the two main junctions’ temperatures. In essence this is the basis of practical thermocouple thermometry.
The relevant junctions are the so-called measuring junction (M) and the junction of the dissimilar wires to the copper output connections (usually, a pair of junctions), called the reference junction (R), as in Figure 2.2. So long as the reference junction (R) is maintained at a constant, known temperature, the temperature of the measuring junction (M) can be deduced from the thermocouple output voltage. Thermocouples can thus be considered as differential temperature measuring devices - not absolute temperature sensors.
There are important points to note at this stage. Firstly, thermocouples only generate an output in the regions where the temperature gradients exist, not beyond. Secondly, accuracy and stability can only be assured if the thermoelectric characteristics of the thermocouple conductors are uniform throughout. Finally, only a circuit comprising dissimilar materials in a temperature gradient generates an output.
Cold Junction Compensation
There are calibration tables (emf tables) for each thermocouple type, ralting output voltage to the temperature of the measuring junction. Throughout thermocouple thermometry it is clearly necessary to refer sensor voltage output to these in some way to ascertain true temperature
Most importantly, different net voltage outputs are produced for a given temperature difference between the measuring and reference junctions if the reference junction temperature itself is allowed to vary. So, the calibration tables mentioned above always expressly assume that the reference junction is held at 0°C.
This can be achieved by inserting the copper junction(s) into melting ice, via insulating glass tubes, or into a temperature controlled chamber, like an isothermal block with suitable temperature sensors. For industrial measurement today, this kind of function is normally performed by temperature correcting electronics - while linearising electronics (usually digital), harnessing curve fitting techniques, look after the inherent non-linearities as per the calibration tables (more in Part 1, Section 5).
Essentially, reference temperature variations are sensed by a device such as a thermistor as close as possible to the reference junction. An EMF is then induced which varies with temperature so as to compensate for temperature movements at the reference terminals.
Cold Junction Compensation Methods
As explained earlier, thermocouples provide an output which is related to the temperatures of the two junctions. For them to function as absolute temperature measuring devices, rather than differential, the reference junctions must be maintained at a known temperature (see Figure 5.1).
An established, simple method of maintaining reference temperature stability, still used in laboratories today, is to immerse the reference junctions in a slush of melting ice. Given that you have pure water ice, the temperature plateau during the melting process is established at a constant 0°C within ±0.001°C. In practice, all that is required is an ice-filled Dewar flask and the set-up is then potentially quite accurate. However, it does require regular attention and replenishment for anything other than short term use and is clearly inappropriate for industrial requirements. Sources of error include the 4°C reference offset which will occur if enough ice melts so that the reference junctions are actually immersed only in water - with the ice floating above! Also if the ice used has been stored in a freezer, it will be a lot colder than 0°C.
Figures 5.1: Dewar flask with Reference Junctions
Not surprisingly, there are more practical alternatives for industrial use, also designed to provide a reference temperature of 0°C. One involves an automatic temperature-controlled enclosure, into which the reference junctions are inserted. This holds the junctions continuously at the ice point, using semiconductor thermoelectric cooling (Peltier) devices. Here temperature errors are typically less than 0.1°C. The use of an ice point reference, or its equivalent (however generated), is still preferable to the alternatives, not only on the grounds of accuracy and stability, but also because the reference tables for thermocouples are based on a 0°C reference temperature.
Figures 5.2: Temperature controlled enclosure
Another very common device today is based on a temperature sensitive electrical network (there are several options) which tracks the reference junction temperature and develops an equivalent voltage. Such so-called cold junction compensation is incorporated into each thermocouple circuit, or the measuring instrument itself at the point of connection (see below). These devices are available as discrete modules, mains or battery powered and provide for accuracy within a few °C.
Many of the instruments designed to operate with thermocouples provide terminals for direct connection to the thermocouple or extension cable conductors without any need for a separate reference junction as such. Such devices as electronic thermometers, temperature controllers, data loggers, etc, frequently incorporate their own equivalent ice point reference voltage generators (as described above).
The temperature at the connection point might be determined by an integral resistance thermometer (see Part 2, Section 6), thermistor or transistor, and thus a suitable reference voltage developed. Incidentally, it is worth taking care over the physical siting of the reference generator, since accuracy and stability of the thermocouple reading are dependent on the network involved actually being in the same temperature environment as the connections themselves.
In any event, the reference voltage can be added to the thermocouple output either by inclusion in the electrical circuit, or, particularly in the case of controllers, data loggers and other digital systems, by data manipulation in the temperature calculations. In fact, many modern controllers, loggers, etc can accommodate the latter approach.
For large schemes involving many thermocouples, racking systems and cabinets are also available having, say, 100 equivalent reference junctions already fitted into a uniform temperature enclosure. The enclosure might be an ice point unit as already described, or it might equally be a thermally stable metal block which maintains a reasonably steady temperature close to that of its surroundings. In the latter case, the temperature of the block is continuously monitored by an electrical compensator, and again, the equivalent ice point voltage is then available to be added to each thermocouple signal output - electrically, or numerically.
Beyond these, there are also reference units designed for enclosures operating at elevated temperatures. These can be useful in areas with particularly high ambient temperatures, but the thermocouple outputs will have to be adjusted to the equivalent 0°C values. In essence, as long as the reference temperature is known, the temperature of the measuring junction can be derived by adding in a correction factor from standard tables covering the thermocouple concerned.
Most conducting materials can produce a thermoelectric output. However, when considerations like width of the temperature range, actual useful signal output, linearity and repeatability (the unambiguous relationship of output to temperature), are taken into account, there is a somewhat restricted sensible choice. Material selections have been the subject of considerable work over several decades, on the part of suppliers, the main calibration and qualifying laboratories and academia. So, the range of temperatures covered by usable metals and alloys, in both wire and complete sensor form, now extends from -270°C to 2,600°C.
Naturally, the full range cannot be covered by just one thermocouple junction combination. There are internationally recognised type designations, each claiming some special virtue. The British standard BS EN 60584-1 (formerly BS 4937) and the International standard IEC 60584 refer to the standardised thermocouples (these are described by letter designation - the system originally proposed by the Instrument Society of America (see Part 1, Section 3).
In general, these are divided into two main categories: rare metal types (typically, platinum vs platinum rhodium) and base metal types (such as nickel chromium vs nickel aluminium and iron vs copper nickel (Constantan)). Platinum-based thermocouples tend to be the most stable, but they’re also the most expensive. They have a useful temperature range from ambient to around 2,000°C, and short term, much greater (-270°C to 3,000°C). The range for the base metal types is more restricted, typically from 0 to 1,200°C, although again wider for non-continuous exposure. However, signal outputs for rare metal types are small compared with those from base metal types.
Another issue here is the inherent thermoelectric instability of the base metal thermocouple, Type K, with both time and temperature (although Types E, J and T have also come in for some criticism see Part 1, Section 3). Hence the interest in Type N thermocouples (Nicrosil vs Nisil), with the best of the rare metal characteristics at base metal prices, with base metal signal levels and a slightly extended base metal temperature range.
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 wires 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.
In practice, thermocouples can’t always be made to conform exactly to the published tables. So thermocouple output tolerances for both noble and base metal thermocouples are published as IEC 60584-1, manufacturers providing the sensors to these agreed limits (Table 3.3).
The tolerance values are for thermocouples manufactured from wires normally in the diameter range 0.1 to 3mm, and do not allow for calibration drift during use. Thermocouples other than those listed in these standards are usually supplied with manufacturers’ batch tables.
Table 3.3: Thermocouple Tolerances according to IEC 60584-1 (reference junction at 0ºC)