Industrial temperature measurement is often characterized by challenging environments. Choosing the right thermocouple is crucial for process efficiency as well as safety and/or product quality. Although every application is unique, in this article we will consider some factors to take into consideration when choosing a thermocouple for each of the most common operating atmospheres:
- Vacuum
- Reducing
- Oxidizing
- Inert
In the following four sections, the most compatible sheath materials and appropriate thermocouple types will be specified for each environment along with some examples of industrial applications. We hope this will help you make an informed decision.
This article assumes a basic knowledge of what must be taken into consideration when choosing a thermocouple and designing an assembly. Get an overview on more general factors such as temperature range, response time, placement and accuracy, please refer to this article.
Measuring temperature in a vacuum
A vacuum environment is one that is devoid of matter and, in practical terms, is defined as a near absence of gas pressure (much less than atmospheric pressure). Factors such as pressure, density, lack of heat transfer by conduction or convection and low thermal conductivity are all relevant to the accurate functioning of thermocouples in a vacuum.
Industrial applications involving vacuum environments
- Aerospace
- Vacuum furnaces
- Semiconductor applications
- PVD applications (Physical Vapor Deposition)
- Crystal growing
- Nuclear power
- Advanced ceramics
- Solar cells
Considerations for thermocouples in a vacuum
The following table indicates the properties of the thermocouple types most suited to a vacuum atmosphere.
| Type | Junction materials | Temperature range | Sensitivity | Notes |
|---|---|---|---|---|
| J | Iron-Constantan | 0°C to 760°C | 55uV/°C | Low cost Requires a sheath in a vacuum. |
| K | Chromel (Nickel&Chromium) – Alumel (Nickel&Aluminium) | -184°C to 1260°C | 39uV/°C | Low cost Requires a sheath in a vacuum. |
| T | Copper-Constantan | -184°C to 400°C | 45uV/°C | Reliable up to 370°C and stable at low temperatures. |
| N | Nicrosil-Nisil | 0°C to 1100°C | 10.4uV/°C | Lower cost than B, R and S types. Very accurate and reliable at high temperatures. Requires a sheath for use in a vacuum. |
| C | Tungsten (5%)/ Rhenium – Tungsten (26%)/ Rhenium | 0°C to 2300°C | 16uV/°C | Excellent for high temperature vacuums Unprotected thermocouple |
| B | Platinum (6%)/Rhodium – Platinum (30%)/Rhodium | 38°C to 1800°C | 10.4uV/°C | Intended for very high temperature applications. Only functions reliably for short periods of time in a vacuum and requires the protection of a sheath. |
| R | Platinum (13%)/Rhodium – Platinum | 0°C to 1593°C | 6uV/°C | Suited for very high temperatures and short periods of time in a vacuum. Easily contaminated and should be used with a sheath in a vacuum. |
| S | Platinum (10%)/Rhodium – Rhodium/Platinum | 0°C to 1538°C | 10.4uV/°C | Usually used with a sheath. Only intended for short periods of time in a vacuum. High cost so only for specialized applications |
Sheath materials
Sheath materials suited for temperature measurement in a vacuum include tantalum, molybdenum, INC600 and niobium 1% zinc. Visit our overview of sheath materials and their different properties.
High temperature measurements
At temperatures higher than around 1200°C regular thermocouple alloys come too close to their melting points and cannot be used for accurate measurements. For this reason, Kamet offers a range of specialized high temperature thermocouples which can be relevant for certain vacuum environments, such as furnaces.
Vacuum heating challenges
Kamet is able to provide solutions not only for temperature measurement in a vacuum, but also for the unique challenges of heating in a vacuum.
As experts in the heating field with decades of innovation and experience, we are able to advise you on any vacuum heating issues you may encounter. You can read more about our (custom-made) heaters or get in touch with us to discuss the various solutions and options with one of our experts.
Measuring temperature in a reducing environment
A reducing environment is one in which oxidation is prevented because the thermocouple is surrounded by reducing agents (gasses that remove oxygen, usually hydrogen or nitrogen).
Examples of industrial processes with reducing environments
- Metallurgy
- Heat treatment (annealing furnaces)
- Welding
Considerations for thermocouples in a reducing atmosphere
In selecting the best thermocouple for a reducing environment, consideration needs to be given to the low levels of oxygen and the effect of the reducing agents (such as hydrogen, nitrogen and ammonia). Reducing agents can, for example, cause alloys to oxidize and thereby reduce the EMF output causing the thermocouple to read low.
The following table indicates the properties for the thermocouple types most suited to a reducing atmosphere.
| Type | Junction materials | Temperature range | Sensitivity | Notes |
|---|---|---|---|---|
| Type J | Iron-Constantan | 0°C to 760°C | 55uV/°C | Use for dry applications only Rapid oxidation of wire above 540°C. |
| Type T | Copper-Constantan | -184°C to 400°C | 45uV/°C | Resistant to moisture and condensation corrosion. Reliable up to 370°C and stable at low temperatures. |
| Type N | Nicrosil-Nisil | 0°C to 1100°C | 10.4uV/°C | Only suited for dry applications. |
| Type C | Tungsten (5%)/ Rhenium – Tungsten (26%)/ Rhenium | 0°C to 2300°C | 16uV/°C | Unprotected thermocouple Suitable for high-purity hydrogen atmospheres. Cannot be exposed to oxygen. |
It is theoretically possible to use all the other types of thermocouples as long as they are appropriately protected. We have not included type K in the summary above because it is susceptible to ‘green rot’ in reducing environments.
Green rot refers to the oxidation of chromium, a process that typically occurs at high temperatures, ranging between 800°C and 1260°C , particularly in low-oxygen environments. Under normal circumstances, an oxide layer on the surface of the NiCr leg shields it from oxidation. However, in environments containing hydrogen or other reducing agents, this protection is compromised, leading to accelerated chromium oxidation. This results in a scaly green layer of corrosion forming on the positive leg.
Sheath materials
Sheath materials that could be considered for a reducing environment are Hastelloy X and Molybdenum. For more information on the properties of these materials go to this page.
Measuring temperature in an oxidizing environment
An oxidizing environment is one in which the thermocouple will be in contact with gasses that cause combustion (mainly oxygen).
Examples of industrial processes with oxidizing environments
- Combustion engines
- Waste Incineration
- Glass industry
Considerations for thermocouples in oxidizing atmospheres
As the name implies, there is a very high oxidizing potential in these atmospheres which needs to be taken into consideration when choosing the best thermocouple. The following table indicates the properties for the thermocouple types most suited to an oxidizing atmosphere:
| Type | Junstion materials | Temperature range | Sensitivity | Notes |
|---|---|---|---|---|
| Type E | Chromel (Nickel&Chromium) – Constantan | 0°C to 982°C | 76uV/°C | Only use up to 900°C in oxidizing environment. Protect from sulfur attack. |
| Type J | Iron-Constantan | 0°C to 760°C | 55uV/°C | Dry applications only. Rapid oxidation of wire above 540°C. Vulnerable to sulfur above 540°C. |
| Type K | Chromel (Nickel&Chromium) – Alumel (Nickel&Aluminium) | -184°C to 1260°C | 39uV/°C | Good for clean oxidizing environments. Low cost. Needs protection from sulfurous atmospheres. |
| Type T | Copper-Constantan | -184°C to 400°C | 45uV/°C | Reliable up to 370°C and stable at low temperatures. Resistant to moisture and condensation. |
| Type N | Nicrosil-Nisil | 0°C to 1100°C | 10.4uV/°C | Lower cost than B, R and S types. Resistance to oxidation at high temperatures. Very accurate and reliable at high temperatures. Vulnerable to sulfur attacks. |
| Type B | Platinum (6%)/Rhodium – Platinum (30%)/Rhodium | 700°C to 1800°C | 10.4uV/°C | Intended for very high temperature applications. Use with protection in reducing atmospheres. Can be used unprotected in oxidizing environments. |
| Type R | Platinum (13%)/Rhodium – Platinum | 0°C to 1593°C | 6uV/°C | Suited for very high temperatures. Easily contaminated and usually used with a sheath. Can be used without protection in oxidizing environments. |
| Type S | Platinum (10%)/Rhodium – Rhodium/Platinum | 0°C to 1538°C | 10.4uV/°C | Usually used with a sheath. Requires protection from reducing atmospheres and contamination. Can be used without protection in oxidizing environments. High cost so most suited for specialized applications. |
Sheath materials
Sheath materials that are suitable for use in oxidizing environments include: Pt10%Rh, Pt20%Rh, INC600 and stainless steels, such as AISI 316, AISI 321 and AISI 310.
Measuring temperature in an inert environment
An inert environment is one that contains no oxygen or only very low oxygen levels. It is primarily (a mixture of) non-reactive gasses such as nitrogen, argon, helium and carbon dioxide.
Examples of industrial processes with inert environments
- Semiconductor, such as thin film deposition
- Sintering of powder metallurgy
- Heat treatment of metals
- Crystal growth
- Aerospace materials testing – for example to test a hall effect thruster
Considerations for thermocouples in inert atmospheres
The presence of the inert gasses and the very low levels of oxidation are factors to take into consideration when choosing a thermocouple to suit industrial processes in inert environments.
The following table indicates the properties of the most suitable thermocouple types. All of them can be used either protected or unprotected in an inert environment.
| Type | Junction materials | Temperature range | Sensitivity | Notes |
|---|---|---|---|---|
| Type C | Tungsten (5%)/ Rhenium – Tungsten (26%)/ Rhenium | 0°C to 2300°C | 16uV/°C | Suitable for high-purity inert environments. |
| Type E | Chromel (Nickel&Chromium) – Constantan | 0°C to 982°C | 76uV/°C | Only use up to 900°C in inert environments. |
| Type J | Iron-Constantan | 0°C to 760°C | 55uV/°C | Life span reduces at high temperatures. |
| Type K | Chromel (Nickel&Chromium) – Alumel (Nickel&Aluminium) | -184°C to 1260°C | 39uV/°C | Low cost. Should be used if environment is completely inert. |
| Type T | Copper-Constantan | -184°C to 400°C | 45uV/°C | Reliable up to 370°C and stable at low temperatures. |
| Type N | Nicrosil-Nisil | 0°C to 1100°C | 10.4uV/°C | Lower cost than B, R and S types. Very accurate and reliable at high temperatures. |
| Type B | Platinum (6%)/Rhodium – Platinum (30%)/Rhodium | 38°C to 1800°C | 10.4uV/°C | Intended for very high. temperature applications. |
| Type R | Platinum (13%)/Rhodium – Platinum | 0°C to 1593°C | 6uV/°C | Suited for very high temperatures. Easily contaminated and usually used with a sheath. |
| Type S | Platinum (10%)/Rhodium – Rhodium/Platinum | 0°C to 1538°C | 10.4uV/°C | Usually used with a sheath. High cost so more appropriate for specialized applications. |
Would you like to know more?
There are also situations where thermocouples have to withstand a combination of these environments. Kamet excels at providing specific advice for your unique process and environment. Custom-made solutions are also one of our specialities. So, if you haven’t been able to find all the information you need on our website or knowledge base, then please contact us and we will be happy to help.