[apalrd]

I would be a fan of the pressure transducer, because the Nason one is so heavy for it's function (well, all of the brass fittings are...), if you wanted pressure.

Any reasonable input diagnostics would prevent simple shorting to turn on the compressor, with a positive voltage-pressure curve. If the sensors limits do not include the extremes (0v-0.5v, 4.5v-5v) then the input diagnostics should prevent operation in any case.

If the slope of the pressure->voltage curve is positive, then shorting to 5v would cause the pump to turn off anyway. Shorting to ground should produce a fault which would shut off the pump, so you would need a middle value to force it to come on.

If the slope of the pressure->voltage curve is negative, then 0v would be full and would naturally turn the pump off. 5v would be naturally low pressure, and turn the pump on. This would not be failsafe in a 5v short, but would be better than the other option. The current solution is not failsafe to a ground short, and it's probably easier to short to ground than 5v, so I would consider it acceptable for FRC.