Type K thermocouples find widest use at tem- peratures above 500°C due to their strong resis- tance to oxidation better than other base metal thermocouples.
Thermothal P/Thermothal N thermocouples cover all the standards required by international standards.
Each thermocouple element is manufactured so as to meet special tolerances when matched to platinum; this enables any coil of Thermothal P to be matched with any coil of Thermothal N to form the standard tolerance. Each coil is tested at 200°C/400°C/600°C/800°C/1000°C against Plati-num 67 and each one is provided with a tag which gives the EMF values against Pt 67.
The following table lists KP/KN EMF characteris- tics against Pt 67 (NIST Monograph 175).
Precautions in the use of type K
Indications are given below regarding the precau- tions to be taken when using type K couples. These are listed in ASTM STP 470A.
Type K thermocouples have a better resistance to oxidation than other combinations, except type N, and are recommended for oxidizing or inert atmospheres.
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.