Duncan
Thanks for the input. Where the water comes into the filter reservoir there is a cover that I can mount the Thermowell to and drop my DS1820 right in. So I really don’t need to tap into the water lines. I think this is what you suggest, keep the sensor it in the return part of the process.

This afternoon I opened up the doors where the controller is and found a thermowell for the existing temperature sensor. Now it looks like I might be able to fit the DS1820 in the same well, but that probably is not the best place. My overall goal of this project is to only monitor the water temperature remotely. We have frequent power bumps in our area and my GFI trips occasionally, so I want to catch the trip by alarming the water temperature. The other reason is that my wife wants to make sure the tub is up to temperature before she runs outside. We have turned into real pansies.

Did a search on EBAY for a “thermowell.” Surprised me, found lots of them.

As far as replacing the pump/heater/light controller it might not be too bad. My tub is vintage so all I have to worry about is time-of-day, minimum temperature, run temperature, pump high/low speed and the light. I would probably use 24 VAC controlled contactors for the 30 amps of 240 VAC current, then use the PIC to control everything else. Not ready to go there yet, but it sounds like a fun concept to design.
David