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21 September 2008, 03:40 PM | #1 |
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Difference between air and water pressure
Rolexes are rated to be waterproof to a certain depth. What is the difference between air pressure and water pressure?
Let's say you have your Rolex serviced at a high elevation. At that point I would assume that the air pressure outside the watch and inside the watch is the same. Now you take the watch to sea level. If you have come down in elevation 7000 feet, why doesn't the watch implode? It must be different than taking the watch from sea level to 7000 feet under the water. Are the air molecules smaller than the water molecules, and thus the pressure inside the watch and outside the watch equalizes in air but not in water because the air molecules can enter or leave the watch to equalize pressure? Or something else?
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21 September 2008, 04:06 PM | #2 |
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Erm... can't we just talk about how do it know when to turn the lume on?
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21 September 2008, 04:13 PM | #3 |
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I would guess that it doesn't cause any problems because you're dealing with a VERY small volume of air in a relatively strong enclosure.
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21 September 2008, 04:29 PM | #4 |
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I will think you will find only a small air pressure difference between sealevel and 7000ft , however the pressure 7000ft underwater would crush you
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21 September 2008, 05:03 PM | #5 | |
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Quote:
I think that normal air pressure at sea level is approximately 14.7 psi, and that it decreases by about 1 psi for each 1000 feet of increased elevation. So you have about a 50% change in ambient air pressure.
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21 September 2008, 04:54 PM | #6 |
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The problem is that often it doesn't know.
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21 September 2008, 04:28 PM | #7 |
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An often asked question and a relative one in todays society. I like Dr Lowe Hungs brilliant answer, although some think it is a bit simplistic in content
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21 September 2008, 04:30 PM | #8 |
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21 September 2008, 06:01 PM | #9 | |
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Pie=good Pie=high calories Good=high calories High calories=good Not sure this computes. I think I hijacked my own thread.
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21 September 2008, 07:06 PM | #10 |
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As that great cook Archimedes noted pies are a very important part of everyday life. I refer you to his experiment of comparing the relative values of a steak and kidney pie with a blue berry pie. Both were tasty and high in calories but the nutritional value of the Brit pie wins out. Hope this answers your question
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22 September 2008, 08:24 AM | #11 |
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:r ofl:
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21 September 2008, 11:42 PM | #12 |
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I was thinking about a related question.... When you here that a legal case is air tight. Is it better to be air tight or water tight? I guess it depends on the application. However, I recently bought a piece that I am thinking about keeping as BNIB for a long time - let's say 20 years. I am thinking maybe if I vaccum seal it in a bag i.e. remove air - maybe the watch, paperwork, Box etc... would not diminish as much. Should I ever want to sell it in 30 years.... It would be almost as if it was NOS.
Then I think it's too much work and not to worry about it either way...
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22 September 2008, 01:49 AM | #13 | |
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This is so easy, you can vacuum seal your watch every night if you want, or at least for the weekend if it's off your wrist.... You can seal up all that paperwork and those unsightly boxes, put them in cold storage and they will look like brand new in a half century when your heirs decide to sell all your stuff after they put you in a "Home".. I'll bet that if you just change the name on one of these to "Watchsaver", there's money to be made.......... ........
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22 September 2008, 12:21 AM | #14 |
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Quite simply:
Water is more dense than air 5 cubic feet of water weighs approx 40 lbs 5 cubic feet of air weighs a few ounces Therefore the pressure asserted at depth is much greater than the pressure exerted at the same relative altitude. |
22 September 2008, 01:35 AM | #15 |
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Is it fair to say a watch isn't "air" tight? I mean would the DS need a relief valve if that were the case? A molecule of H2O is much bigger than an atom of Helium isn't it?
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22 September 2008, 02:01 AM | #16 | ||
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Quote:
Quote:
Also, my guess is that the watch probably is not air tight and that the pressure inside the watch equalizes to the ambient air pressure. I would think that an atom of water is bigger than an atom of helium. Thanks!
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22 September 2008, 05:19 AM | #17 |
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Rolexes are tested to both positive and negative pressure. That's one of the reasons why they legally can call their watches waterproof, not simply water resistant. They many times use the term pressureproof as well.
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22 September 2008, 05:23 AM | #18 | |
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Also I have never heard your assertion regarding positive and negative pressure. The testing process as well documented as pulling a vacuum and assessing crystal deflection. Please tell us more about what you mean. Edit: I pulled up one of the threads for you: http://www.rolexforums.com/showthrea...ght=Waterproof |
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22 September 2008, 08:23 AM | #19 | |
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You are kidding again right?
Quote:
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23 September 2008, 03:12 PM | #20 | |
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An atom of feathers or an atom of chicken.
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26 September 2008, 05:10 PM | #21 |
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No WAY! Water is an ATOM? Air is HELIUM? Things just got interesting!
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22 September 2008, 08:25 AM | #22 |
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exactly- it took 12 posts for this. what is TRF coming too
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22 September 2008, 11:48 AM | #23 |
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Chris, it's people such as you that make TRF what it is today, whatever that is.
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22 September 2008, 09:19 AM | #24 | |
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1728 / 231 = 7.48 gals / cubic foot 7.48 x 5 = 37.4 gallons 37.4 gallons x 8.4 lb/gal (salt water) = 314 pounds I don't believe your sailboat would float if water had the density you quote You can joke about anything but math
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22 September 2008, 08:18 AM | #25 |
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You are kidding right?
Please tell me that this question is just being made as a humorous statement.
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22 September 2008, 12:38 PM | #26 |
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PSI = Depth of salt water in feet X .445
12,800 ft X .445 = 5696 PSI However Rolex actually tests the DSSD to 15,000 ft or 6675 PSI An aluminum scuba tank is around 3000 PSI standard fill pressure and usually hydro-tested to 5000 PSI once every 5 years.
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23 September 2008, 01:42 AM | #27 |
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Sorry for the wise-ass comment earlier.
Sorry for the wise-ass comment earlier. I suppose there are many people that do not have technical training that would make all of this very simple and obvious. Pressure is pressure whether it is in water or in air. A water column or depth in water produces pressure at a constant rate depending on the density of the water which varies from fresh water to salt (and can be a bit different due to the level of salinity). Since water is basically non-compressible the rate of change in pressure as you descend in water is constant, as stated earlier, about .445 psi (pounds per square inch) per foot of seawater. In the Atmosphere at sea level the absolute pressure standard at 67 degrees F is approximately 14.7 psia (pounds per square inch absolute) or zero pisg (pounds per square inch gauge). One atmosphere of pressure is equal to this pressure absolute at standard temperature at sea level is 101.325 kPa or in other units that are equivalent: 760 mmHg (torr), 29.92 inHg, 14.696 PSI, 1013.25 millibars.
The difference to a watch between air pressure and water pressure is none. The seals will operate the same and the watch will perform and or fail the same whether it is under 1000 psi of water pressure (about 2247 feet under seawater) or if it is in a pressure vessel pumped up to 1000 psi of air pressure. The whole question about opening the watch case at high elevation is nearly nil since the amount of pressure change would be less than 1 atmosphere or 15 psi, which is miniscule to the pressure that a diving watch is rated. Change in elevation vs. change in absolute pressure is not a constant because air as a fluid medium is highly compressible, so the change in elevation can not be stated as a specific psia change per a certain change in feet in elevation and also because the relative density of air in the atmosphere is affected greatly by temperature, humidity, wind, etc. The one problem you might have if a watch was opened at a very high elevation is that there would be a greater pressure exerted on it once it is brought back to sea level that could make it hard to open there. But with a screw on case back , the relatively large hand wench used to remove it and the relative low differential pressure between the interior of the case acting on the approximately 1 square inch surface area of the case back, it would pose no problem. I apologize again for the initial comments. My response should have been - I feel that the issue that you have raised would have extremely little or zero effect on the watch. |
23 September 2008, 01:48 AM | #28 |
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Thank you, Vince. There are people like me whose minds work in very simple ways. I appreciate those of you who want to take the time to explain some of this. As for me, I'm still wondering about the lume thing.
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23 September 2008, 04:58 AM | #29 | |
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Sorry, that one is over my head. |
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23 September 2008, 11:08 AM | #30 |
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Thanks for the further explanation, Vince.
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