When choosing a thermocouple, you must consider the following:
• What are you measuring the temperature of (a surface or immersed in solid,liquid or gas)?
• The temperature range you need to measure (As a general rule your operating temperature should not be in the higher end of the thermocouple temperature range as it will depreciate a lot faster than it should, e.g your operating temperature varies from 650° to 690°C, rather than selecting a type J Thermocouple (rated +20 to +700°C) you would select the next highest which is type E (rated 0 to +800°C)
• If it is essential to have a fast response time or not, to learn the various factors of choosing a response time read more on our Thermocouple technical page.
• The specific sizes required to fit your application e.g probe diameter, probe length, compression fittings, type of connector required etc.
• Do you require a specific sheath material?
• Do you need your sensor calibrated?
• Will the sensor need to be resistant to chemicals/abrasion/vibration or any other environmental considerations?
• Is there high emf (electromagnetic interference) potential from power switching, rectification, or radio waves?
• Any other installation considerations? (e.g does the sensor need to be bent in to shape before installation)
Reference : sterlingsensors