Re: [PSUBS-MAILIST] question about general design?

[David Buchner <buchner@wcta.net>]

[sorry you're seeing this twice, Ken]



>Is my reasoning flawed?  If you have a off the shelf compressor bottle that is rated to 125psi that is outside your pressure hull, it would limit your diving to depths greater than 125psi (I don't know how many feet that is)   Otherwise, you aren't gonna blow ballast at 150psi with 125psi air.  So, if the tank always has pressure inside of it greater than the pressure outside, it ought to be happy as a clam.  They way I figure it, if the outside presure is 50psi and the tank is holding 75 psi, that's like having a tank at 25 psi with no pressure on the outside.  Being a newbie, I may be all wet, if so someone please  straighten me out.   Ken Harris

Think about the volume of air. That's what I'm thinking about, anyway. One of those portable air tanks can inflate maybe a couple of regular-sized car tires to around 30 psi.


I don't know how big of ballast tanks you're thinking about, but most of them look to be in the 20's-30's-more gallon range. I'm going to embarass myself here with my public school not-remembering-basic-science-from-the-1600's, but say your ballast tank was the same size as the portable air tank. At 120 psi, it's holding like 8 times as much air as outside (15psi). (here's where the embarassment comes in. for all I know, the pressure goes up as a square or cube or something, making it way way less...) But if you're underwater, and the pressure's already higher, you've got a lot less total air to play with. Once you spread your 120psi air out into the ballast tank, "a couple of tires worth" might not be enough for one complete blow, let alone diving and surfacing a second time without going to SuperAmerica and waiting for their dinky coin-operated compressor to fill your tank again. At most, a 12 gallon, 120psi tank can fill 12 gallons to 120 psi. No wait, that's not right -- as soon as the pressure it equal between the two tanks nothing will happen, so you've got 12 gallons at 60 psi and 60 psi left in your tank. Okay I'm in over my head and I've had way too much coffee and it finally cooled off here so I should shut up before I embarass myself further, get outside and sort my scavenged lumber and put more epoxy on my Mountain Dew drum...


(By the way, since somebody mentioned it, mine was around $30 and holds 12 gallons. I bought it for inflating tires, not submarines.)
-- 
David
buchner@wcta.net - MN, USA