Re: [PSUBS-MAILIST] MBT Vents

[Jon Wallace <jonw@psubs.org>]

Frank,

I've never considered calculating PSI that way, but yes it gets you very 
close to the right answer (just slightly over).  Hugh's recommendation 
of splitting the depth in half is always the quickest way to figure PSI 
unless you are being very particular about the exact pressure.  Dividing 
by half results in a number that is slightly more conservative than your 
method, but still acceptable in most cases.

Fresh water = .433 psi / foot
Salt water = .445 psi / foot

These are the most common values I've come across in regards to the 
weight of water.  For scientific work, you'd have to do an analysis on 
the particular water you were interested in since Distilled water is 
going to weight less than your average lake water (which has minerals in 
it); and the weight of salt water is going to depend upon the amount of 
salt absorbed by that water, which varies around the globe.

250 foot depth x .433 = 108.25psi
250 foot depth x .445 = 111.25psi
250 foot depth by Franks method = 111.36psi
250 foot depth / 2 = 125psi

Simply dividing depth by half introduces a slight error (about 10% in 
this case), however its a conservative error which provides a bit of 
safety margin.  But I like your method since the 33 foot rule was 
pounded into our brains during dive training and is easier to remember 
(in my case) than decimals, when you want a more exact number.

Jon



ShellyDalg@aol.com wrote:
> I just wanted to make sure my limited math skills aren't way off.
>  To determine pressure at depth you take the depth ( 250 ft. ), divide 
> by 33 to get atmospheres, then multiply by 14.7 to get pressure ( 112 
> psi. ).
>  Is this right ?
> So if your max depth, not operating depth, is 500 feet, the max 
> pressure is 223 psi.
>  
> Now let's get crazy......the pressure pushing down on a 24 inch 
> diameter hatch at 500 feet is.......
> radius squared ( 12 X 12 ) times pi for the square inches ( 452 ), 
> times the psi ( 223 )............
> 100,831 pounds. Yikes !
> Frank D.
>  




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