Thermocouple type K cannot be used directly or without protection in the following cases:
1.Alternatively oxidizing and reducing atmospheres.
2.Atmospheres with sulphur containing gases. The alloys are attacked causing intergranular corro- sion, embrittlement and cracking of the negative leg (Thermothal N).
3.Any protection sheaths should be cleaned to remove oils, fats, carbon containing substances, dirt.
4.In a vacuum, except for a short time, chromium evaporates mostly from the positive leg Thermothal P, causing the EMF of the couple to alter.
5.Reducing atmospheres such as hydrogen, dissoci- ated ammonia or carbon monoxide which has low oxygen content.