Hi David

My goal was to get a fairly accurate reading
With a Hot tub the pipework remains all fairly close within the integrated unit, and could be arkward to find a place to cut a tee into, unless you have a separate Plant room. If you attempt to install an ABS TEE then you might need to make sure you have enough room for a slip coupling, careful if you are going to use a solvent cement, you only get a couple of seconds alignment. Much better to fix a "strap boss" and reduce down to your pocket. Consider mounting the entry to the pipe line from below horizontal thus reducing the requirement for venting(the pocket could fill up with air). Choose a place (any place, you can calibrate to any temperature) that has the most consistent water temperature ie before any heat exchanger or inline heater. So you are really looking at the area at the return drain. Do not worry about setting the pocket "into" the flow as turbluence will take care of uniformity, plus you will find that there will be less wear on the pocket itself, believe it or not but you actually have a water based sand blaster going on. If you have a cartridge or sand filter then you can place the pocket after them.

Then of course the fun starts by integrating the PIC to optimise the heating circuit to reduce overshoot and hysterisis in the heating circuit.


HTH

Duncan