[PSUBS-MAILIST] Buoyancy dynamics

[Jon Wallace via Personal_Submersibles]

 Sean, for the K-sub ballast tank configuration at 600 feet that would equate to about 32 pounds positive buoyancy, or .5 cu ft displacement.  You method sounds like a good way to calculate for K-subs.  Not sure if it works universally but I wouldn't be surprised if it did.
Jon

    On Sunday, August 16, 2020, 05:40:44 PM EDT, Jon Wallace via Personal_Submersibles <personal_submersibles at psubs.org> wrote:  
 
  Great interpretation, and yes.  As long as that initial amount is enough to overcome any forces on the way up (thermocline, etc).  Worthy of some calculations, thanks for the tip Sean.
Jon


    On Sunday, August 16, 2020, 04:57:54 PM EDT, Sean T. Stevenson via Personal_Submersibles <personal_submersibles at psubs.org> wrote:  
 
 For the most efficient use of air, if you didn't care about the speed of ascent, wouldn't the correct volume be that which would expand just enough to completely empty the tank by the time you reached the surface?

Sean




-------- Original Message --------
On Aug. 16, 2020, 12:42, Jon Wallace via Personal_Submersibles < personal_submersibles at psubs.org> wrote:


 Vance, I agree from a practical perspective.  I'm sure I'm looking beyond a practical application but I'm imagining entering my target dive depth at the surface and then having the software predict if there is enough HP air volume to blow the tanks at that depth to initiate surfacing.  Two variables are, 1) enough remaining pressure in the air tank to exceed pressure at depth for a given volume of air, 2) enough volume of air at a specific depth to displace enough water in the MBT to initiate surfacing.  I'm guessing it's not all that much, but I'm not sure what "enough" is and trying to quantify that.  Half cubic foot (30 lbs or so?)
Jon


    On Sunday, August 16, 2020, 12:31:33 PM EDT, via Personal_Submersibles <personal_submersibles at psubs.org> wrote:  
 
 Complicated. Just watch the bubbles. 60 feet per minute, give or take.Vance

-----Original Message-----
From: River Dolfi via Personal_Submersibles <personal_submersibles at psubs.org>
To: via Personal_Submersibles <personal_submersibles at psubs.org>
Sent: Sun, Aug 16, 2020 12:10 pm
Subject: Re: [PSUBS-MAILIST] Buoyancy dynamics

Jon, are you asking about how to correlate buoyancy condition with vertical speed through the water? IE "If I drain 50lb from the VBT and make myself 50lb positively buoyant, how long will it take me to reach the surface 600ft up?"
You would have to know the drag coefficient and a reference area of the sub in question in the up and down direction. Usually that kind of number is determined with fancy CFD or wind tunnel testing. I've got a couple data points about buoyancy vs vertical speed that I could crunch to get an approximate drag coefficient appropriate for small submersibles, then you could calculate the speed for any sub of a given cross sectional area.
Thanks,
-River J. Dolfi
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