I plan on using dual shur flo pressure demand pumps, one for each fresh water tank, mounted at the lowest water level possible. (Down in the bay)
Its about 10 feet of head pressure, which is easily accommodated by one of those pumps for a shower head.
So, yeah gravity primed. I’ll set up a cross flow connector to equalize the tanks before the pumps. After the pumps, check valves, then tee them together to a common auxilary pressure bladder. That then feeds the rest of the vehicle.
Normal operation would be both pumps powered.
I have these neat little pwm flow sensors (cheap!), for the fluid status system.
The fluid status is an arduino that takes analog capacitance values of the tanks to get their level. Its basically some self stick metal foil on the side of the tank and a capacitor, with some fancy programming.
That gives me tank levels, and the same controller can cut power to either pump and trigger alarms if they are running with no flow after a certain amount of time.
All this nerd box crap costs some time and effort, but the savings are realized when I can use a cheaper instant hot water heater (Gerard?) They are pretty sensitive to flow rate changes, and I should have the components to intelligently compensate for flow rate changes.
The same system will monitor temperature in the bay. The heaters for the tanks will be very simply wired (no computer) but will alert the control system if its available. Bay will be foam insulated as well.
Link to flow meter
Capacitive level idea: (there are lots of variations)
Some other considerations:
Using the “odometer” on the pumps can give interesting information, like a failing pump, blockage, and water usage stats.
Its just a matter of knowing initial volume and usage over time to calculate a time vector to empty. (Just like a car computer miles to empty)