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Is it possible to make an hourglass that measures different amounts of time in one direction than the other? Say, 25 minutes right-side up, and 5 minutes upside down, for pomodoros. Moving parts are okay (flaps that close by gravity or something) but it should not take additional effort to flip.
One question I have about both your solution and mine is how easy it is to vary the time drastically by changing the size of the hole. My intuition says that too large holes behave much differently than smaller holes and if you want a drastic 5x difference in speed you might get into this "too large and the sand sort of just rushes through" behavior.