Re: Interesting pressure question

Conshelf <Consh...@xxxxxxxxx> wrote:
"Lee Bell" <pleeb...@xxxxxxxxxxxxx> wrote:
Thermal expansion of water? You planning on diving in boiling seas?

Admittedly, it would be a very minor difference, but water does vary in
density even over normal temperature ranges.

Agreed. And there's also minor variations (as already pointed out) in
local gravity, as well as variations from that variation caused by
relative position of the moon and sun (as is illustrated by day-to-day
differences in tide heights). Fortunately, most of these are 2nd and
3rd order variables that we can pragmatically ignore.


The record low barometric pressure (during a hurricane, of course)
[versus] Standard sea level atmospheric pressure...would result
in a change of actual depth to pressure depth of approximately 4.5 ft.

Nice analysis.


To show an extreme of this, let's assume that it was possible
to dive on the moon. If you were physically 120 ft below the
surface of the water, your earth calculated pressure gauge would
read around 20 ft instead since the gravity of the moon is 1/6th of
earth.

Since we're having the fun of being pendantic, this isn't quite
right.

The gage wouldn't read 20ft because we don't have a 15psia atmosphere
on the moon like we have on Earth.

The Earth gage is calibrated with the assumption of the approximately
15 psia atmosphere present being equal to 0ft depth. Thus, 15psia =
0fsw, 30psia = 33fsw, 45psia = 66fsw, etc.

As such (and using the round-off of 33ft = 1 ATM), the depth gage
that's reporting 120fsw is doing so based on a measured absolute
pressure of:

15+ (120/33*15) = 69.5 psia.

On the moon, the atmosphere is zero psia (ie, none), and assuming
1/6th the gravity, then each 6*33fsw = 198fsw of water = 15psi, so a
depth of 120fsw on the moon would be calculated as:

0 + (120/198*15) = 9 psia.

Since 9psia is less than 15psia (Earth Sea Level), this would be
interpreted by said Earth gage as less than a zero depth (in water
units, approximately negative 13fsw). If it was a fancy dive
computer, it might translate this pressure into a perceived altitude,
which would be equivalent to being at roughly 13,000ft elevation on
Earth...can you say "Dive Lake Titicaca"? :-)


-hh

.

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