Thermowells

Thermowells provide protection to temperature probes against unfavorable operating condition such as corrosive media, physical impact and higher-pressure gas or liquid. Their use also permits quick and easy probe interchanging without the need to "open-up" the process.

Thermowell take many different forms and utilizes a variety of material (usually stainless steel); there is a wide variety of thread or flange fitting depending on the requirement of the installation. They can either be drilled from solid material for the highest pressure integrity or they can take the form of a thermo pocket fabricated from tubing and hexagonal bushes or flanges; the later construction allows longer immersion length.

Thermowells transfer heat to the installed sensor and due to that thermal inertia is introduced.

Any temperature change in the process took longer time to affect the sensor if thermowells are present; thus the response time is increased. This factor must be considered when specifying a thermowell; except when thermal equilibrium exists, a temperature measurement will probably be inaccurate to some extent.

Optimum bore is an important parameter since physical contact between the inner wall of the thermowell and the probe is essential for thermal coupling. In the case of a thermocouple, which is tip sensing, it is important to ensure that the probe is fully sheathed (in contact with the tip of the thermowell). For Pt 100 sensor, which is stem sensing, the difference between the probe outside diameter and bore must be kept to an absolute