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Hi Alan,
No matter how you go about it, the pressure outside the sub in
air is not going to be any higher than 14.7 psi; and if you removed every
molecule of air or water from inside the sub, 0 psi is as low as you can
go. So the difference is still going to be no greater than 14.7 psi.
Despite how we perceive it, a vacuum can do nothing
whatsoever. When you suck on a straw, you don't really
pull the liquid into your mouth. You really just decrease
the pressure in the straw so that the ambient air can push the
liquid into you mouth.
There are so many factors that go into engineering and
fabricating a fiberglass pressure vessel. The considerations for factoring
the effect of dive cycles are also much different than steel. At least
with steel we get our specs together, then have a commercial producer
manufacture the tube for us.
Do you plan to buy a fiberglass tube (or whatever
shape) for the hull or fabricate it yourself?
Cheers,
Jim T
In a message dated 1/18/2011 10:26:08 P.M. Central Standard Time,
alanjames@xtra.co.nz writes:
Hi Jim,
I was more concerned with structural integrity than air leaks with my
posting.
I'm building a small sub out of fiberglass / epoxy & will be needing
to thoroughly
test it.
From what I gather you can't pull more than an absolute vacuum because
there's
no air for your pump to push out.
I'm wondering if you filled your hull up with water whether a high
pressure water pump
could create a negative pressure in the hull. Or am I barking up the same
tree.
Alan
----- Original Message -----
Sent: Wednesday, January 19, 2011 4:04
PM
Subject: Re: [PSUBS-MAILIST] Pressure
testing with a vacuum pump
Alan,
Still you can take it down to about 30 feet without ever
leaving your shop. If it doesn't leak air at that pressure
differential that's good. If it does leak, you've only got air
(or a leak detector) instead of dirty water or salt water into the
defect.
So this begs two questions:
1. What's the best product and/or procedure
for locating a small leak flowing from the outside of a vessel to
the inside?
2. Would it be better to try to seal the hatch and
pressurize the vessel on the inside to detect air leaking to the
outside? That's putting the higher pressure on the "wrong" side of the
hatch/canopy.
Jim T
In a message dated 1/18/2011 8:20:05 P.M. Central Standard Time,
alanjames@xtra.co.nz writes:
Thanks everyone for clearing that up,
I was thinking that if
you had a pump that could produce 250 psi & it's sucking
from
somewhere, that you could attach it's intake to a hull & produce a
negative
250 psi pressure. Have done a bit of "Googling" with your
direction & have sought
of got my head round it.
Darn! it
seemed a lot easier option than dropping my hull down a deep lake on a
long rope.
Regards Alan
----- Original Message -----
From: "Jens Laland" <laland@artematrix.org>
To:
<personal_submersibles@psubs.org>
Sent: Wednesday, January 19,
2011 1:19 PM
Subject: Re: [PSUBS-MAILIST] Pressure testing with a
vacuum pump
> This one I find quite useful with regards to
measuring vacuum, ref
>
>
http://traktoria.org/files/instrumentation/vacuum/Pressure_Conversion_Table1.pdf
>
> Best regards,
> Jens Laland
>
>
>
>
>
>
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