[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.