Pool Temperature Monitoring

Put the hole in the line just *ahead* of the pump. There is negative pressure there. The pipe is relatively thin. You can make a stronger insertion point by adding a sleave at that point and tapping a hole through the sleave.

FWIW, I have thermal sensors inserted at that location, plus the supply and return pipes leading to the roof solar collectors. None of them have ever leaked.

There may be other methods but the simplest is to drill the pipe, wrap the device once with Teflon tape and insert it. There are threaded fittings available from swimming pool automation manufacturers if you want. My sensors are simple, press-fit devices. As mentioned, they don't leak.