: how do you measure time with sand ?

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How do people who make hourglasses know how much sand to put in for the amount of time they want ?

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the make the glass put some sand and text it, if it took too long they remove sand or add sand if it took too little.

but if you industrialize the process and can mass produce that parts reliably enough you only need to test the first one and then you know how much sand to put in all of them

This is often done through a process called testing.

A set volume of sand can be measured with how long it takes to fall. You can then use math to figure out the exact volume needed for a certain time.

Once this is known, assuming you have a consistent hole, you can mass produce hourglasses with this volume of sand.

You can test it or you can make engineering math pretending that sand is a liquid with some fairly high factor of viscosity. Someone here will know how to explain that better than I ever could, but some solids can behave similarly to liquids if you use the proper comparison methods.

Ultimately, it’s all about how much of the particular sand goes through the hole (through the narrow part) in a certain time. Very few people make sandglasses these days to measure hour-long increments very precisely. Most sandglasses these days are used for much shorter periods — a minute or a few minutes. Even when people made bigger sandglasses, such as to time how long someone should stand watch on a ship, longer times often involved having to repeatedly turn the glass back over.

They make reasonably consistent hourglasses. They’re not precise, because that’d cost way too much, just close enough. They put too much sand in. An hour later they have an hour’s worth of sand in the bottom. They dump the remaining sand out of the top, seal it up and ship it.