Harry,
The relationship of adding one "atmosphere" of pressure every 10 m of
water depth is due to the relative densities of air and water and due
to the amount of air above us.
One "atmosphere" of pressure is 101.325 kPa. Imagine a column of water
10 m tall over a 1 m2 area. The volume of that would be 10 m3, with a
mass of 10 Mg. The weight of 10 Mg is 98 kN, so that column exerts a
pressure of 98 kPa at its base. Now account for sea water having about
3 % greater density than fresh water and the pressure is ~101 kPa.
Gee, you know I didn't even have to reach for a calculator or even
pencil and paper to do that. Isn't the SI neat and easy? In my
submarine days we were required to work that out in feet and pounds to
show that every 100 ft depth increase raised pressure 44 psi. We used
64 lb/ft3 as the density of sea water (62 lb/ft3 for fresh). I don't
care to do that in my head just now since I haven't had my second cup
of coffee.
Jim
On Sunday 29 April 2001 0412, Harry Wyeth wrote:
....
> It is interesting that for scuba, divers must know that pressure
> increases one atmosphere every 33 feet (as taught in the US), which
> is, of course, 10 m. The usual safe diving limit is around 100 feet,
> which is rounded off to around 30 m. Not being a physicist, I wonder
> what relation there is between air/water pressure doubling at exactly
> 10 m depth, and adding another atmosphere at 20 m. Anyone know? Why
> 10 m and not 9 or 11?
.....
--
James R. Frysinger University/College of Charleston
10 Captiva Row Dept. of Physics and Astronomy
Charleston, SC 29407 66 George Street
843.225.0805 Charleston, SC 29424
http://www.cofc.edu/~frysingj [EMAIL PROTECTED]
Cert. Adv. Metrication Specialist 843.953.7644
