How are hourglasses so accurate/How do they work?

Question

I know that it’s possible to buy hourglasses that run out after a set amount of time, like 30 mins/an hour/2 hours etc., and even water-drippers that measure every 30 minutes. How do they do that?

In: 2

Answer 1

One way is to calibrate every hourglass individually during their production. Create the hourglass structure, but leave one reservoir open. Put in enough sand/water, then let them dip into the closed reservoir for exactly 30minutes. Remove the excess sand/water from the open reservoir, then close it.