Effecient automatic Hot tub

[Stewfish]

I have a solution that works for me.

For starters, My tub is covered with a semi-clear plastic when I want more heat and I cover that with an opaque cover when I am getting too much heat.

I also have a tiny pump controlled by 2 temperature control relay boards ($4 each). The water is pumped through 100feet of 3/4" black pipe, in a coil similar to yours. This pipe also has a semi-transparent plastic cover to help trap the heat. A sensor in the tub detects when heat is needed and turns on the relay. A sensor near the outlet end of the heat pipe detects when the pipe water is hot enough to bother turning on the pump. When both relays are on, the pump runs. When the hot water in the pipe is replaced by cooler water from the tub, the pump turns off and waits for the pipe water to get hot again. This keeps the pump from running when it will not help. The pump runs off a 12v battery with a 10watt panel. The temp can easily go up from 30c in the morning to 36c in the late afternoon, even with the opaque cover on...on a hot sunny day.

I saw this post by DThames and thought it would be good to get more info, hopefully he will chime in.

Solar heating hottub (assisting heating)

Hello, I have a 900L inflatable HotTub which costs a small fortune to heat, I have recently lifted it off the floor onto some pallets and rubber matting on top of the pallets to reduce the heat lost through the floor. I then built a small solar thermal box with around 30 metres of 1/2in pipe...

diysolarforum.com

I could use a way to automatically regulate my black pipe/water solar system. I had good success with the system manually last summer, but it was a pain. It seemed to loose the most when I turned it on and left it running too late, clouds, or I forgot to turn it on. I had a giant spool of home depot 1/2" irrigation pipe I just left in the yard. I need to make a box w plexiglass, automate it, and a shade/cloud sensor would be good too.

Can anyone point me in the right direction of parts etc or other suggestions. I know how to wire a 12v automotive relay, but I'm not sure what sensor and relay are needed for this.
A keezer temp regulator first come to mind and simple but I'm not sure its the right way to go. Maybe If I dipped the sensor in epoxy up above the water line to waterproof the sensor.

Thanks for pointer guys.

Similar just wondering is there a cloudy day sensor that I could hook things up to like the keezer or a timer but cloud cover would prevent usage of a device? Light sensors used for motion sensor lights don't work for that.

https://www.amazon.com/dp/B015E2UFGM/ref=cm_sw_r_apan_i_57AX36BS5EQ54N4N8JMA