[PSUBS-MAILIST] Acceptable cabin pressure swing

Sun Feb 28 13:03:35 EST 2016

The requirements only state that some means be provided of equalizing pressure on both sides of a hatch prior to opening. To me, this reads like something as simple as a small diameter needle or ball valve connecting the two sides, as there is no particular requirement for speed of equalization, and in fact the smaller flow area is safer in the event of something leaking.  The rules indicate that no possible failure by design should be capable of raising the interior pressure 1 atm, so really, the only reason to need a high volume OPV is if some lower pressure than that caused by some sort of failure could potentially unseat a dome, hatch or other fixture with limited capability to withstand internal pressure, in a way that is both potentially dangerous and can't otherwise be addressed in design.  The other possibility is if you want to accommodate a salvage blowdown at depth as a flooding control measure, and then be able to vent that excess pressure on the way up, in which case a
high flow valve is helpful, but I would still pay particular attention to how one might accommodate a failure of such a valve to reseat.

Sean

On February 28, 2016 8:57:54 AM MST, hank pronk via Personal_Submersibles <personal_submersibles at psubs.org> wrote:
>Brian,Where to put the relief valve, in my opinion it should stay on
>the store shelf lol.   I have a vent valve in the hatch, it is a 1\4
>inch valve.  If you have a 1\2 psi relief valve and you want it to open
>at the surface, what is the point in having it?  You want a relief
>valve to prevent your hatch blowing off or ports popping off.  Well if
>your at the surface it is to late, the relief valve needs to open on
>the way to the surface.  That means you need to surface super slow and
>stop at just the right depth and wait for the valve to vent off
>pressure before you continue to the surface.  So if your going to put a
>relief valve in you best go big, like Emile and Alec have  or it will
>be like waiting for a giant inner tube to deflate that has one pound of
>air pressure in it.   Your sub does not need a relief valve, can't hurt
>but no need in my mind, because you have small bolted ports and a good
>hatch securing system.  It would be different if you had a big dome
>that was not strapped  on like Gamma has. We talked a while back about
>this and I remember Alec talking about rubber straps holding the hatch
>down.  Now that is how I think it should be done.  As long as there is
>catch so the hatch can only open an inch or two.  Hank 
>
>On Saturday, February 27, 2016 11:20 PM, Brian Cox via
>Personal_Submersibles <personal_submersibles at psubs.org> wrote:
> 
>
>I guess if the relief valve is more than a couple of feet below the
>water line, that would be a 1/2 psi per foot, which would render the
>valve superfluous. Brian
>
>--- personal_submersibles at psubs.org wrote:
>
>From: Hugh Fulton via Personal_Submersibles
><personal_submersibles at psubs.org>
>To: "'Personal Submersibles General Discussion'"
><personal_submersibles at psubs.org>
>Subject: Re: [PSUBS-MAILIST] Acceptable cabin pressure swing
>Date: Sun, 28 Feb 2016 17:36:06 +1300
>
>I know Emile has had it working but is it  possible for the O’ring to
>dislodge with flow.  I think the the O’ring should be more captive like
>in a dovetail groove in Parker book.  It is nice and simple but it
>should be tested at different pressures and flows.  Comments?Hugh From:
>Personal_Submersibles [mailto:personal_submersibles-bounces at psubs.org]
>On Behalf Of Alan James via Personal_Submersibles
>Sent: Sunday, 28 February 2016 7:59 a.m.
>To: Personal Submersibles General Discussion
>Subject: Re: [PSUBS-MAILIST] Acceptable cabin pressure swing I sent
>this attachment & a couple of pictures but I haven't seen it yet,so
>here is  just the attachment only.This could be adapted in to some sort
>of snorkel for air on the surface.Alan From: Alec Smyth via
>Personal_Submersibles <personal_submersibles at psubs.org>
>To: Personal Submersibles General Discussion
><personal_submersibles at psubs.org> 
>Sent: Sunday, February 28, 2016 7:41 AM
>Subject: Re: [PSUBS-MAILIST] Acceptable cabin pressure swing The
>problem with that valve is that it's just got a 1/4" diameter, and you
>really want it to handle large volumes. Emile and I as mentioned built
>our own and they're really easy to make. Interestingly we made them
>independently but what we came up with is identical. I know Emile
>posted his design here a while back, so it must be somewhere in the
>archives. I never made a drawing of mine, just made the actual valve
>directly since it's so straightforward. It's machined from a SS round
>and consists of just two parts. The main body is a cylinder with a
>bunch of holes drilled through it, and one central hole for the stem.
>This cylinder is welded into the hull. The other part is the cap, a
>shorter length cut from the same solid round but with an O ring groove
>around the edge and no holes. The stem goes through the body and screws
>into the cap. You put a spring on the stem and a wing nut to adjust the
>cracking pressure. That's it. If interior pressure is stronger that the
>spring, the cap lifts and in so doing opens all the holes.  Best,
>Alec On Sat, Feb 27, 2016 at 10:07 AM, Cliff Redus via
>Personal_Submersibles <personal_submersibles at psubs.org> wrote:
>
>Brian, see McMaster Carr part number 4620K41.  This has a 1 psi
>cracking pressure, not 0.5 psi that I mentioned earlier. Cliff
>
>On Thursday, February 25, 2016, Brian Cox via Personal_Submersibles
><personal_submersibles at psubs.org> wrote:
>
>Cliff,          Where did you find that 1/2 psi relief valve? Brian Cox
>
>--- personal_submersibles at psubs.org wrote:
>
>From: Cliff Redus via Personal_Submersibles
><personal_submersibles at psubs.org>
>To: Personal Submersibles General Discussion
><personal_submersibles at psubs.org>
>Subject: Re: [PSUBS-MAILIST] Acceptable cabin pressure swing
>Date: Thu, 25 Feb 2016 19:05:00 -0600Jon for my boat, I have a pressure
>relief valve with a 1/2 psi cracking pressure to vent pressure while on
>the surface.  I have a high cabin pressure  alarm that triggers at 20
>psia (5.3 psig) and a cabin lower pressure alarm set at 12 psia (-2.7
>psig).   In the PLC I also record and display the cabin pressure at the
>time the hatch is closed and log all alarm states every second.   I
>have never had either a high or low cabin pressure alarm while diving. 
>I have had elevated cabin pressure caused by accidently having the
>backup manual O2 value open and a leak in an air fitting on anther
>occasion. Cliff On Wed, Feb 24, 2016 at 1:12 PM, Jon Wallace via
>Personal_Submersibles <personal_submersibles at psubs.org> wrote:
>
>
>I think this is somewhat subjective but what is general consensus on
>maximum cabin pressure swing (higher/lower) relative to hatch closing
>at the start of a dive before setting off alarms?  My thought is that
>an alarm should sound well before any point of emergency.  I'm
>considering sounding a warning at 2psi +/- and an alarm at 4psi +/- 
>but I'm not sure if this is too strict.
>
>Jon
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