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The general barometric equation to determine pressure at altitude is:

Palt = 33 *  exp (-0.038 * (alt/1000))

So for a 2000 foot change (sea level to 2000) the pressure change would
be:

Palt  = Psea * exp (-0.038 * 2) =  Psea * 0.9268

This would be the same as 2.4 feet of water (this is from sea level to
2000)

As a general rule of thumb a 1 inch change of mercury (Hg) in barometric
pressure is about a 1000 foot change in elevation.  A 2 inch change in
mercury (or 2000 feet of elevation) would be the same as a 27 inch (2.25
feet) change in water.  The specific gravity of water is 1.0 the
specific gravity of Hg is 13.5.  So a small change in mercury is the
equivalent of a large change in water.

I commonly dive an alpine lake with a surface water elevation of 6550
MSL.  To get there we have to travel over a 8,800 foot pass.  The time
from the base of the pass at 7,200 MSL to the top usually is 15-20 min.
I have done this within 2 hours of repetitive dives and have not noticed
any physical symptoms.  The difference in an elevation change of 7,200
to 8,800 feet is the equivalent of 1.6 feet of water, from the lake to
the top of the pass it is equivalent to 2.2 feet of water.  The change
in barometric pressure is not linear and you should know what elevation
you are starting from and where you will be ending at.  Then determine
the delta P accordingly.

bgg


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