RTD and thermowell distance requirement


Thread Starter


I need to know what is the length required between a RTD sensor tip and inner surface wall of a thermowell? Is there a minimum or maximum length or should the RTD sensor touches the inner wall of the thermowell?

Please advise if you have any knowledge or any standard that I can refer to. Thanks for your help.
Hi PC,

The length btw the sensor tip and the wall should be as minimum as possible, and generally sensor tip touches the wall. That is to obtain good response by preventing the lag time due to temperature propagation.

From my understanding, for you it would more important to focus on how and where you are going to install it-straight pipe or elbow, operating conditions like flow direction(!) and pipe size, etc. As for your question, yes, distance is minimal, but no strict requirement, except if it is a skin thermocouple then it definitely must touch the wall.

Good luck,
The tip of an RTD or other temperature sensor should have good metal to metal contact with the bottom of the thermowell bore. If possible you should select a spring loaded sensor to ensure that this contact is maintained after installation. If you are not sure of the length of the well you can order a cuttable spring loaded RTD.

The difference between the ID of the well and the OD of the probe should be approximately .01" (ex: a 1/4" OD probe is matched with a .260" bore thermowell). This accomplishes the ASME PTC 19.3 goal of creating intimate thermal contact between the well and sensor. It also improves the response time of your sensor.

This website provides a handy frame of reference for basic information about thermowells.