Following points must be considered while selecting a thermowell for a particular application:
Thermowell Immersion Length: The immersion (U) length of a thermowell plays a crucial role in deciding the accuracy of temperature measurements. In general, “U” length of a thermowell is limited to five times the thermowell’s external diameter for getting accurate results. This is the immersion length maintained in case of processes involving liquids whereas in case of gas or air associated processes, the required minimum U length is set to ten times the external diameter of the Thermowell.
Resistance to vibration: Sometimes, the performance of thermowells gets adversely affected by turbulent vibrations. These vibrations are created by gas or liquid flowing in the process area which is subjected to temperature measurement. The intensity of these vibrations depends upon the thermowell’s diameter and the liquid (or gas) flow. To prevent or reduce the occurrence of these vibrations, thermowells must be designed with excellent stiffness. Most preferred type of thermowell design which can stand against vibrations is the tapered thermowell design since it offers superb stiffness without even compromising the temperature sensitivity of a thermowell.
Thermowell material: Appropriate material must be chosen for construction of an effective thermowell. In case the thermowell is made up of unsuitable material, there will always be risk of thermowell failure. Hence, thermowell must always be chosen keeping in view the process temperature along with the corrosion and erosive conditions which may possibly be allied with the process.