"U" is the insertion depth of the thermowell. This is how much of the thermowell will be inserted into the process fluid.The method of attachment (thread-o-let, pipe fitting…) will affect the insertion depth and needs to be considered when specifying this measurement.

"T" is the lag length of the thermowell. The purpose of lag is to provide extra length to clear any insulation, refractory… that may be between the process connection and the temperature sensor connection point.

"P" is the process connection pipe size of the thermowell. The standard length of the threads for all pipe thread sizes is 1”.

"Q" is the root diameter of the thermowell and is based on the “P” dimension.

"V" is the tip diameter of the thermowell and is based on thermowell design.V = Q for straight-shank wells.

"S" is the bored depth of the thermowell.This dimension accounts for the length of the wrench flats, lag length, thread make-up, and insertion depth, less ¼”. This length is the same “S” length specification for spring loaded assemblies, designed for use with thermowells. The overall length of the thermowell is S + ¼”, since the thermowell is bored within ¼” of the tip.

The hex wrench flats are machined to ¾” in length.

For example, if we want our thermowell to be immersed 4” into the process liquid, and have 2” of insulation on our pipe:

U = 4”, T= 2”, the wrench flats = ¾”, and the threads = 1”.So the overall length of the thermowell is 7-3/4” and the “S” length is 7-1/2”.

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