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joe
the tanks they are refering to
are the soft ballast tanks.
which does not effect submerged displacement, they
effect surface displacement. soft ballast is normally free flooding
the only time they effect submerged displacement is
durring emergency blow. but due to the large quantity of air require to do this
and the chance of an uncontrolled accent this is not thee prefered method of
operation.
lets look at what is happening
in the soft ballast tanks if you want to use them for a controlled accent , for
ease of the numbers we will use an accent for 66 ft or 3 atm
absolute. fixed factors accent raate is basesd upon drag and the
positive displacement. we will use an accent rate of 60 ft /min and a
positive displacement of 128 or 2 ft^3.
durring the first thirty three foot rise you
will have to vent 1 ft^3 of air thru an oriface/ valve with a differential
pressure. in most tank designs this space would be approx 1 inch
allowing for a differential pressure of .03 psi ie a really shitty flow
rate.
for the next thirty three feet
of rise you will need to vent off 2 cf of air , while the air density has
decreased the flow is not linear based sloely on density
so you would have
to throttle the vent valves in order to made any kind of controlled accent.
this could be done using control valves and a pid controller incorporated
in a plc.but that waywould increase costs dramatically.
the cheapest way to accomplise
this is to use a small trim tank open at the bottom that has only the capacity
to give a small quanity of positive displacement. as the air in it expands it
will just blow out the bottom. this method will using more air the a sealed trim
tank if you are planing multiple accents and decents durring a dive but
eliminates the need of a high preesure tank and control system for the water
intake valve. although it would require a crew wiegh in to establish basic trim.
a standard scuba tank is 80 cf a
3000 psi/200 bar. assuming a full tank at 1000 ft or 500 psi abient
pressure. you will have a reserve lift capacity of
tanks size = .4 cf
volume air 80 cf air compressed to five hundred pai
==2.35 cf
an available air supply of 1.95 cf or 125 lbs of
lift.
the formula is p1v1=p2v2
size of tank
80*14.7=3000x
80*14.7/3000=x
.392= x
3000*.4=500*x
(3000*.4)/500=x
x=2.4 cf total airremember the air that will stay
in the scuba tank
2.4-.4 = 2 cf of available air.
rick miller
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