Thermocouple atmosphere compatibility
Filter the eight standardised types by oxidising, reducing, vacuum or moist service and see which alloys are recommended, which are to be avoided, and why.
| Type | Verified range | Recommended in | Avoid in | Verdict |
|---|---|---|---|---|
| Type J | −40 … +750 °C | reducing atmospheres | oxidising service above ~550 °C, vacuum, sulphur | - |
| Type K | −200 … +1370 °C | clean, oxidising atmospheres | reducing / vacuum service 800-1050 °C (green rot) | - |
| Type T | −200 … +350 °C | clean, slightly oxidising and moist atmospheres | sustained service above ~350 °C, strong oxidising | - |
| Type E | −200 … +900 °C | clean, moderately oxidising atmospheres | reducing service and heavy oxidation above ~900 °C | - |
| Type N | −270 … +1300 °C | oxidising, vacuum and low-oxygen atmospheres | sulphur-heavy and metal-vapour-laden zones, which contaminate any base-metal pair over time | - |
| Type R | 0 … +1768 °C (continuous ~1600) | clean, oxidising atmospheres only | metal vapour, sulphur and reducing gases, which poison platinum | - |
| Type S | 0 … +1768 °C (continuous ~1600) | clean, oxidising atmospheres only | metal vapour, sulphur and reducing gases, which poison platinum | - |
| Type B | 0 … +1820 °C (continuous ~1700) | clean, oxidising high-temperature service | reducing atmospheres and metal-vapour contamination | - |
Choose an atmosphere to mark each type. A blank verdict means the atmosphere is neither recommended nor called out as a hazard in our own datasheets, it is not a rating.
The one atmosphere failure worth memorising
Green rot is where a Type K reading quietly leaves specification in reducing or low-oxygen service between 800 °C and 1050 °C. The chromium in the Chromel leg oxidises selectively, the EMF falls, and the loop reads low, often by enough to matter, and without any visible damage from outside. If the process can reach that band without free oxygen, the alloy decision has to be made before the wire is ordered, not after the reading is questioned.
Frequently Asked Questions (FAQ)
Which thermocouple type suits a reducing atmosphere?
Type J and Type N are the usual answers, with Type N the better choice as temperature climbs. Type K is specifically the wrong choice in reducing or low-oxygen service between 800 °C and 1050 °C, where the Chromel leg oxidises selectively, the failure known as green rot.
Why is a vacuum furnace a Type N decision above 800 °C?
In vacuum there is no oxygen to support the protective oxide film on the Chromel leg, so Type K drifts. Type N was formulated for oxidising, vacuum and low-oxygen service, which is why it holds its reading where Type K does not.
Is the recommended type the only type that works?
No. A type outside its recommended atmosphere will still read a temperature, but its drift and life are no longer bounded by the specification. Where the loop has to stay inside an accuracy class, atmosphere has to be treated as a hard requirement rather than a preference.
What about moist service?
Type T is the alloy built for clean, slightly oxidising and moist atmospheres, and it is the most accurate base-metal type below zero. Its ceiling is the limit, sustained service above about 350 °C is not what the copper leg is for.
Unsure which alloy your furnace needs?
Tell us the atmosphere, the ceiling and the accuracy class, we will confirm the type and supply the matching spool.