Fuel drained for sampling should not be put back in the tank
Why not? If no trace of water, then surely it's greener to pour it back in the tanks. It was flying in USA that first introduced me to this concept as a UK pilot I'd always chucked it onto the grass.
The other posts remind me of a mate who put a few dashes of Castrol racing oil in his lawnmower. Smelt like a Grand Prix when he cut the grass ;-)