You might not need to get the sensor wet, the outside of the pipe may suffice. If the pipe is plastic, you might insert a copper section. The pic shows the return pipe under the dial thermo with two thermowells, a homebrew dry one with DS18B20 above a commercial wet one. The feed pipe simply has a DS18B20 for Arduino and another sensor for the digital thermo underneath a bit of insulation in the short horizontal section. It seems to be just as effective as the thermowells. Arduino at the top right matches the red heat meter. At the bottom of the feed and return pipes, you can see PT500 sensors, laboriously installed, never used.
I would suggest a DS18B20 if your fluid is in range -25>125C. It is digital, accurate, easy to use, cheap, and well-supported on this forum.
