My sump blisters contain an added accessory, a liquid level sensor designed for fuel tank installation. It's made of polysulfone, which is both fuel and ethanol resistant, and has a 1/4" NPT tap so it's easily installed and completely accessible from outside the tank in case it ever needs to be serviced or replaced.

I actually installed a series of these sensors at even spacing up the tank wall (which was a bit of doing, actually - John saved my wrists by doing all the tapping in the aluminum plates installed in the tank walls, and I owe him - it was a lot of work doing that tapping!

The idea here is that people have had a lot of trouble with tank level sensors, especially when ethanol is in the fuel (as it will be in mine). But a mechanical sensor has fewer failure modes if it's properly installed. These units add an additional safety factor in that they use magnetic switches, so there is no risk of spark inside the tank. They're not cheap, at about $11 per, but they work very well and I'm looking forward to using them. I'll hook them up to a simple LED bar graph indicator, with an additional annunciator warning on the lowest tank sensor, and the sensor in the sump.