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.

[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]

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

[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 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.

[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 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

[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...

[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?

[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..........

........

[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

[Jocke]

Isn´t the big difference water is wet and air is dry?



Jocke

[Nucengineer]

Hi Art,

Let me see if I can help with your question. Basically, pressure at sea level is 14.7 psi or 29.9 in Hg. As you increase elevation, you decrease atmospheric pressure. So if you climb Mt. Everest, you're at approx. half the pressure of sea level maybe even a little less, say 7.35 psi or 14.5 in Hg. If we continue into outer space we appraoch a near perfect vacuum of about 0.5-1.0 in Hg.

Now for water, the pressure increases 0.4333 psi/ft for fresh water and 0.444 psi/ft for sea water. So at 1,000 ft in depth your pressure is 444 psi.

Now for your watch question: if we calibrate and seal it at an increased elevation equal to half the pressure at sea level, let's say 7.5 psi, your watch would have at sea level approx. 7.5 psi differential pressure between the inside and outside of the watch case. This is no problem because most divers are rated to 1,000 ft and that's differential pressure (444 psi to 14.7 psi from outside to inside the watch case, respectively). So say you continue on down from sea level and dive to 1,000 ft, you now have 0.444 psi/ft x 1,000 ft + 7.5 psi = 451.5 psi differential pressure across the watch case. The 444 psi differential pressure that the watch is rated for vs. the slightly increased differential pressure of 451.5 psi should not make a difference on a watch rated for 1,000 ft, because in all likelyhood, there is some safety factor engineered into the crush pressure of the watch case. Engineers don't design pressure vessles to an exact rating without some safety factor designed in.

Also, remember air is a compressible fluid and for all intents and purposes, water is considered incompressile. That is why watches like the oil filled Sinn divers can obtain such depths. It is because, with the oil filled cavity inside of the watch, no differential pressure is created accross the watch case the deeper you submerge. This is because the oil equalizes the pressure that the sea water exerts on the case.

Hope this helps to answer your question. The bottom line is the most differential pressure you would create would be about a half of an atmosphere (approx.7.5 psi) and this is of no consequence considering how modern watches are designed.

[vjb.knife]

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

[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.

[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:

[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

[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

[Art161]

Wow, this stuff gets complicated. In any event, it seems definitive that water pressure changes with changes in depth more than air pressure changes with changes in elevation.

I think that if you inflate your car tires to 32 psi at sea level and drive to 7000 feet, the pressure will increase to approximately 39 psi (1 psi increase for each 1000 feet of increased elevation). This assumes the same ambient temperature and same tire temperature at both elevations.

When we talk about tire pressure being 32 psi, we are talking about the differential between the pressure in the tire compared to the pressure of the ambient air. A flat tire still has approximately 14.7 psi in it at sea level (ambient pressure). It has only equalized its internal pressure to the ambient air pressure; it has not created a vacuum.

If we have 7.35 psi ambient air pressure at the top of Mt. Everest, we haven't lost 1 psi per thousand feet. If we lost 1 psi for every thousand feet, we would have negative pressure at the top of Mt. Everest. Obviously this is not the case.

Now I am thinking that perhaps the decrease in ambient air pressure is not linear with an increase in altitude. It might change more per foot at the lower elevations than at the higher elevations.

In any event, it appears that the differences in elevation on planet Earth do not affect the watch adversely unless it is immersed in liquid beyond its rated depth.

If we leave planet Earth, even virtually 0 psi ambient air pressure will likely cause no ill effects to the watch.

[Art161]

Originally Posted by vjb.knife

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

Originally Posted by vjb.knife

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

If it will make you feel better, I will tell you that the outfit you are wearing in your avatar does not make you look fat.

[vjb.knife]

Originally Posted by Art161

If it will make you feel better, I will tell you that the outfit you are wearing in your avatar does not make you look fat.

Thanks, because I definitely was not fat at that time at least. Carrying that 200 lbs. of gear around on the surface in between dives does tend to burn the calories.

[Art161]

I think I just had a moment of clarity.

I don't think we are dealing with depth in water directly, only indirectly. What we are dealing with is pressure. The rating is given as depth in water because that is what makes sense. If you said the watch could withstand xxx pounds psi, people would say WTF? A rating of 330 feet or whatever is something more meaningful.

Presumably the watch crystal and/or gaskets would fail at some point if the watch were enclosed in a dry chamber and subjected to high pressure.

Perhaps Rolex could add the psi and Hg ratings to the dial. That would provide a good reason for a 45 or 46 mm case.

[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]

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.