Frank,
I did not consider this "internal reduction" in pressure and the resulting effect on the water level. But, this rise in level must surely be small, and less still with increasing pressures.... that 5 cf gets compressed even more. Interesting excercise...thanks!
Joe
From: ShellyDalg@aol.com
Reply-To: personal_submersibles@psubs.org
To: personal_submersibles@psubs.org
Subject: Re: [PSUBS-MAILIST] VBT's
Date: Fri, 25 May 2007 17:40:14 EDT
Thanks Joe. You're right about the divers mass. What I meant by having to count the volume displaced by the diver is this.....Let's say the diver weighs 200 pounds, and displaces 5 cubic feet of air within the sub's interior.When he exits the sub, the sub is lighter by 200 pounds and there is 5 cubic feet of space which was previously occupied by his body. The internal pressure in the chamber will drop by that percentage of the total volume of air space he took with him, so the water level in the lockout chamber will rise ( only slightly ) when the hatch is open and the diver jumps out.So to maintain neutral buoyancy with the hatch open, I'd need to add enough air to the lock-out chamber to blow out the water to the pre-exit/hatch open level, and release 200 pounds of buoyancy from the trim tank.If I was at 66 feet of depth, I'd need to put 10 cubic feet of air into the chamber ( 5 cu. ft. X 2 atmospheres )so that I wasn't gaining water weight inside the chamber.Now if the sub was stationary, with the hatch open and waiting there for the diver to return, you wouldn't need to dump the buoyancy to replace the diver's weight, because enough water will enter the chamber to roughly equal the divers body ( but not equipment ) because our bodies are not that much heavier than the water volume we displace.But if you want to blow the chamber dry, close the hatch, and return the chamber to one atmosphere, with-out the diver back on-board , and still stay neutrally buoyant ) then you must compensate for the extra buoyancy ( 200 pounds ) by adding water to the trim tank. ( but don't forget to go back for the diver! )I like that big chain idea, seems very simple, and as long as you can drop it if it gets entangled, it's like an anchor only variable.I've toyed with the idea of putting a hatch on the bottom of my sub, so I could reach out and grab something, but it seems that it would require too much additional equipment, to bring the sub to ambient pressure, and then back to 1 atmosphere, to be of any real use.You wouldn't want to open a hatch deeper than 100 ft deep, and it would take so long to pressurize the sub, open the hatch, catch whatever,pump out the water, close the hatch, dump the extra pressure, and monitor all the systems and pressures, that it seems like an easier way would be to have a basket and manipulator arm. That way, you never let any water into the sub, and you're never exposed to the ambient pressure.If you want to get out and swim around, why not just dive off the boat. Your bottom time is the same and with all the valve switching and door opening and closing you'd be doing, you'd probably have more time scuba diving than trying to exit a sub and then get back in.Another thing to consider is, a lock-out chamber takes up a lot of room. If you sub is 80 feet long, then OK.But what if your sub is only 15 feet long. Not much room for a sub within a sub.Don't mind me, I'm starting to ramble.I better get back to the shop and weld some more, or that damn thing will never see the water.You guys have a great weekend......Frank D.
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