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kcanvan wrote:

unquantumphysics wrote:Good idea--unfortunately, though, the sand timer needs to run for at least 15 seconds. The balloon would rise as soon as the lever is lifted

Right, the balloon would rise as soon as the lever is lifted, however, there's obviously going to be a counter-weight on the other side of the lever. The sand timer will run it's course, and the weight of the sand will lift the counter-weight. It would just be a matter of figuring out how much sand will lift some mass counter-weight.

Use the sand as your counterweight. Make your lever symmetrical--ie, put the same sand container on both sides at the same distance from the pivot point. To calibrate: run the sand timer for the exact number of seconds you need, then dump that sand into the other container. Your sand timer should now trigger at exactly the right number of seconds. (Replenish the sand in the original bottle to the exact same point, though, for accuracy.)

Just a tip: filter your sand smehow, maybe through a sieve or something, because if anything changes the rate of the sand flow at all, all accuracy is lost.