Difference between air and water pressure

[Art161]

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?

[Lisa]

Erm... can't we just talk about how do it know when to turn the lume on?

[wanasub]

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.

[swatty]

I will think you will find only a small air pressure difference between sealevel and 7000ft , however the pressure 7000ft underwater would crush you

[Art161]

Originally Posted by swatty

I will think you will find only a small air pressure difference between sealevel and 7000ft , however the pressure 7000ft underwater would crush you

So is it primarily a difference in density between water and air?

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.

[Art161]

Originally Posted by Lisa

Erm... can't we just talk about how do it know when to turn the lume on?

The problem is that often it doesn't know.

[jdc]

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

[swatty]

Originally Posted by jdc

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

Got to love Wikipedia

Dr Lowe Hung

[Art161]

Originally Posted by jdc

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

Is that the same as:

Pie=good
Pie=high calories
Good=high calories
High calories=good

Not sure this computes.

I think I hijacked my own thread.

[jdc]

Originally Posted by Art161

Is that the same as:

Pie=good
Pie=high calories
Good=high calories
High calories=good

Not sure this computes.

I think I hijacked my own thread.

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

[chicagowatchman]

Originally Posted by jdc

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

:r ofl:

[GBJIV]

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...

[Tools]

Originally Posted by GBJIV

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...

If you go to Costco or any other good department store you can get the "Foodsaver" brand vacuum seal freezer baggers..

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..........

........

[Mrdi]

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.

[BigHat]

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?

[Art161]

Originally Posted by Mrdi

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.

Originally Posted by BigHat

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?

It makes sense that it's the density difference between water and air that accounts for depth ratings in water, but it doesn't matter if you take your watch from sea level to the top of Mt. Everest. So if you had an ocean of Classic Coca-Cola rather than water, you could not take your watch to as great a depth in Classic Coca-Cola as you could in water because the Classic Coca-Cola is more dense than water.

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!

[wuzzzer]

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.

[BigHat]

Originally Posted by wuzzzer

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.

Actually, there is no "legally" about it. Many posts about it. Do a search.

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

[vjb.knife]

Originally Posted by Art161

It makes sense that it's the density difference between water and air that accounts for depth ratings in water, but it doesn't matter if you take your watch from sea level to the top of Mt. Everest. So if you had an ocean of Classic Coca-Cola rather than water, you could not take your watch to as great a depth in Classic Coca-Cola as you could in water because the Classic Coca-Cola is more dense than water.

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!

Please tell me that this question is also just being made as a humorous statement.

[Andad]

Originally Posted by vjb.knife

Please tell me that this question is also just being made as a humorous statement.

Which weighs most.

An atom of feathers or an atom of chicken.

[WaltherPPK]

Originally Posted by Art161

...

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!

No WAY! Water is an ATOM? Air is HELIUM? Things just got interesting!

[toph]

Originally Posted by Mrdi

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.

exactly- it took 12 posts for this. what is TRF coming too

[Art161]

Originally Posted by toph

exactly- it took 12 posts for this. what is TRF coming too

Chris, it's people such as you that make TRF what it is today, whatever that is.

[JohnEaton]

Originally Posted by Mrdi

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.

cubic foot = 12 x 12 x12 = 1728 cubic inches

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

[vjb.knife]

Please tell me that this question is just being made as a humorous statement.

[JohnEaton]

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.

[vjb.knife]

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.

[Lisa]

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.

[vjb.knife]

Originally Posted by Lisa

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. Erm... can't we just talk about how do it know when to turn the lume on?

Sorry, that one is over my head.

[Art161]

Thanks for the further explanation, Vince.