I am trying to use some DS18B20 temperature probes to monitor the temperatures of (1) the water, (2) the inside of the tub enclosure, and (3) the ambient air temperature. I wanted to use these with another Sonoff device which is a THR320D - which is a switch which also has an RJ9 input for the temperature probes. But I am having issues getting the third party temperature probes to work and I might have to use a Raspberry Pi instead for that.
Once I have the temperature probes I may then just write my own code to use as a "thermostat" and keep the tub temperature setting at the max. This way I can have more control over heating both in terms of when to use the heater and whether I also want to let the temp go down a bit on weekdays as I almost never use the tub during weekdays, usually just on weekends.
And having a probe inside the enclosure lets me monitor the temperature inside the enclosure to assess the risk of freezing. If the enclosure is always well above freezing, even on the most frigid days, then I need to worry less about having the heater kick on to keep the temperature up. But it seems like a 1200 litre tub of water, even well insulated, and a heater kicking on a couple of times a day keeps the temperature warm-ish inside the enclosure. And this is despite some gaps at the bottom of the enclosure which are, presumably, air vents.