That float sounds pretty good.
> [contamination]
Perhaps some further research will turn up satisfactory sealing methods. There are huge advantages to an instrument that has one moving part, that doesn't use electricity, that is (potentially) easy to repair.
Before dismissing, this advantages, it might be worth defining exactly what contamination and how much would realistically infiltrate the tank and threaten the water quality. People lived high and wide on water from the "wild" for thousands of years. There is no "pure" water, just water with different levels and types of chemical, organic and bacterial loads. Cut a water supply pipe in your house some time and examine the interior. Likely you'll be SHOCKED at what you find. The pipes you're been living with all your life all have biofilm (aka slime) lining them and often various corrosion products. It's' not whether there is something in your water, just how much "something".
Pretty much every
water heater in the world incubates Legionnaire's Disease. But unless the system allows the bacteria to grow to large numbers, there is no problem for anybody. Thing are rarely either/or, just/so.
Regards,
Rufus