Genghis said:-
4) Aparently the operator didn't provide a fuel dipstick, which I'd have thought was standard for a low wing light aircraft in this class
Dipsticks are only useful if the base of the tank is flat, so 1inch on the dipstick = X gallons, 2 inch = 2X gallons etc etc.
If the wing has dihedral and if the aircraft sits nose-high on the ground, then all the fuel will drain into the lowest corner, as is the case in my aircraft.
If the gauges in my aircraft read 1/4 full, we know that the tanks are actually 1/2 full (by checking the amount of fuel needed to fill them), but the sloping floor of the tank is bone dry. and a dipstick will not be the slightest bit damp.
In this case, it is very difficult to design an accurate fuel sender based on a float mechanism, as if the sender is near the lower end of the tank, the float will register full even when the tank is half full, but if the sender unit is at the top of the tank, the float will drop to the bottom of its travel when the tank is half empty.
Possibly the best method would be to isolate the tank from the airframe on rubber mounts and calculate the weight of fuel by the deflection of the mounts. Until you start to move, when vibration will cause the mounts to move and affect the reading of the gauges.