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?
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maths.
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you create a opening between the two halves of the hourglass this is going to let a known about of sand though a minute (worked out by experiments with previous hourglasses). you can then just add however much sand is needed to get to the wanted time (say 1 hour).
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its not PERFECTLY accurate, but a lot of the time, you dont need extreme accuracy. your food cooking for a hour is not going to be burnt if it stays in 61 minutes or be raw after only 59 mintues, for example
They’re not actually that accurate, hour glasses vary quite a bit across different hour glasses of the same “time” and even within the same hour glass on different uses.
But if you’re using an hour glass you don’t need that good of accuracy. Does it really matter if your 1 hour hour glass runs for 60 minutes and 15 seconds vs 59 minutes and 48 seconds? Not really.
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.
You make a prototype, you put some sand in it, and you measure how long it takes to run down. If it’s too fast, add more sand, if it’s too slow, take some out. Repeat until it’s as accurate as you want, and measure how much sand you have. Then make duplicates, fill them with the same amount of sand, and presto: a production line of sand times. If overall accuracy is important to you, (a) don’t use hourglasses, and (b) test each one before you ship to the customer.