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

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.