Thermowell needed with bimetallic thermometer?

When installing a bimetallic thermometer (http://www.teltru.com/images/temp/bimetal.gif) in a copper water supply line, is a brass thermowell (http://www.teltru.com/images/temp/thermowells.gif) needed? All the bimetallic thermometer manufacturer spec sheets say a thermowell is needed for any "pressure application". However, looking at the spec sheets for thermowells, their idea of a pressure application is 5000 psi, not the 50psi of a residential water system.

If a thermowell is not needed, is there a dissimilar metals problem with having a stainless thermometer screwed into a copper fitting?

Looking at their brochure, it says that Thermowell would be recommended for corrosive, hi pressure, or hi velocity applications....since the stem of the thermometer is not constructed to withstand these extremes. Additionally, the use of a thermowell allows removal of the thermometer for calibration or replacement without shutting down the system. So in your application, it would not be required but would be a good idea.

There is not a problem with stainless steel and copper.