Sand dials

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KevinLA
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27 Aug 2010, 11:42 am

When does time start when turning over a sand dial? Is it when the first grain of sand hits the bottom or is when the first grain leaves the top cylinder?

Last edited by KevinLA on 27 Aug 2010, 5:45 pm, edited 1 time in total.

TallyMan
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27 Aug 2010, 12:38 pm

I would image that the error inherent in such timing devices is substantially larger than the time it takes for the first grain of sand to fall, thus the question becomes irrelevant in practice.

I wonder what the variation in say a "5 minute" timer is on successive turns? I'd guess in the region of a few seconds. They are also likely temperature dependent as the hole is likely to expand slightly with warmer temperatures.

Would there be any deviation in the time taken for the sand to fall one way through the hole than the other due to any imperfections in the shape of the hole, or would this cancel out?

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ruveyn
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27 Aug 2010, 3:05 pm

KevinLA wrote:
When does time start when turning over a san dial? Is it when the first grain of sand hits the bottom or is when the first grain leaves the top cylinder?

It is a matter of convention. There is no Right Answer.

ruveyn

Mdyar
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27 Aug 2010, 7:33 pm

KevinLA wrote:
When does time start when turning over a sand dial? Is it when the first grain of sand hits the bottom or is when the first grain leaves the top cylinder?

Not being posed in the philosophy forum, I'd say:

I suppose that the synchronizing of it with a standard timepiece would begin immediately as it was turned , and stopped as the last grain fell to a stop.
This is how it would be used in actual practice.

Technically as it was turned the first grain would be in a free fall , so I vote- leaves the top cylinder.

Surya
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27 Aug 2010, 8:11 pm

Mdyar wrote:

Technically as it was turned the first grain would be in a free fall , so I vote- leaves the top cylinder.

Makes sense to me. When the first one leaves, there is as you said a 'free fall' as others fall, the distance gets less, which would take less time.

This is an amazing hourglass.

Quote:
Unveiled on May 1st 2004 to commemorate Hungary’s entrance in the European Union, The Time Wheel is made out of red granite, steel and bullet-proof glass and it combines one of humanity’s most primitive time measuring devices with a very precise computer. It lies in Budapest near the entrance to City Park. The sand in the hourglass flows from one side of the device to the other for an entire year and the last grains are programmed to flow exactly at midnight on New Year’s Eve. The flow is then turned manually so that it can start measuring time for another year. It takes 45 minutes for 4 people to turn it 180 degrees using metal cables.

The Time Wheel hourglass was designed by Istvan Janaki.

http://www.odditycentral.com/pics/the-time-wheel.html

Titangeek
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31 Aug 2010, 9:43 pm

TallyMan wrote:
I would image that the error inherent in such timing devices is substantially larger than the time it takes for the first grain of sand to fall, thus the question becomes irrelevant in practice.

I wonder what the variation in say a "5 minute" timer is on successive turns? I'd guess in the region of a few seconds. They are also likely temperature dependent as the hole is likely to expand slightly with warmer temperatures.

Would there be any deviation in the time taken for the sand to fall one way through the hole than the other due to any imperfections in the shape of the hole, or would this cancel out?

Would it not also depend on the size of the sand in question?

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danandlouie
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31 Aug 2010, 10:59 pm

would not the state of the device with regards to vacuum have an effect.

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StuartN
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01 Sep 2010, 5:08 am

KevinLA wrote:
When does time start when turning over a sand dial? Is it when the first grain of sand hits the bottom or is when the first grain leaves the top cylinder?

In movies, time begins when the base of the time-piece touches the table and ends when the last grain hits the bottom, which means the start and end point are defined by entirely different mechanisms.