[PSUBS-MAILIST] Leak, pressure and life support test

[hank pronk]

Hi Jim,
I am no expert on the subject, but I do a test with as much vacuum as possible.  When you dive the hatch is secured tight so there is always pressure on the seal.  Before I converted my k350 hatch to open from both sides, I did the vacuum test without securing the hatch.  I does not take much vacuum to suck the hatch down tight.  Honestly I am not sure about a low vacuum test, you could be right.   I just assumed it should be more than less.   Before I tested my last sub in the lake, I did a vacuum test and I did loose vacuum overnight.  It was very minor, so I went anyways.  Sure enough I had a leak, I took on a couple of litres of water at 200 feet for an hr.  I think my connection point from the acrylic cylinder to the hatch land was the problem.  There is no seal, it is cast in place.   
Hank

From: "JimToddPsub at aol.com" <JimToddPsub at aol.com>
To: personal_submersibles at psubs.org 
Sent: Monday, September 2, 2013 8:57:18 AM
Subject: Re: [PSUBS-MAILIST] Leak, pressure and life support test



Hi Hank,
 
See if you agree with my line of thinking:  At a hatch depth of 20 feet you'll have about 9 psi differential on your hatch and your other penetrations as well.   At a hatch depth of 1 foot you have about .445 psi differential.
 
For many things, the greater the pressure differential, the greater the chance of a leak or seepage.  However on the hatch in particular (and possibly on some other things), the greater the pressure forcing the hatch against the landing, the more tightly it will seal.  So the hatch might seal well at depth, but leak when barely submerged
 
It seems it would be good to do both a high vacuum and a low vacuum test.  If the hatch seals well at high vacuum, but leaks down at low vacuum, more work is needed on the latches or the mating surfaces.
 
Best regards,
Jim

In a message dated 9/2/2013 9:21:58 A.M. Central Daylight Time, hanker_20032000 at yahoo.ca writes:
Hi James,
>I always do a vacuum test before my test dives.  I use a compressor as a vacuum pump and draw out as much air as possible.  Then I have a valve on the hull and a vacuum gauge.  I let it sit for 24 hr.  If it holds the vacuum your set.  
>Hank
>
>
>From: James Frankland <jamesf at guernseysubmarine.com>
>To: personal_submersibles at psubs.org 
>Sent: Monday, September 2, 2013 3:45:27 AM
>Subject: [PSUBS-MAILIST] Leak, pressure and life support test
>
>
>
>Hi All,
>
>I did some tests at the weekend.  Very small pics attached as i havent updated my site yet.
>
>First i tested the battery pods and seals with old inner tubes.  Managed to pump them to about 5psi.  Not much but i think it was ok to indicate any leaks.  I couldnt see or hear anything and pressure remained steady.
>
>Then i did the very sophisticated "saucepan test" over the hatch viewport.  I was just a bit suspicious of this one as it has no gasket, the lense seats directly to the face.  No leak, but agreed, not much test pressure although the saucepan was full despite the spillage out the sides.
>
>Next i did a life support test.  I filled and ran the scrubber and then sat inside for an hour with the hatch sealed.  CO2 seemed to level out at about 5500ppm and i topped up O2 when it got to 19%.      
>
>i also dropped half a psi of internal pressure and that seemed to hold, so i am hoping the boat is tight.
>
>I was going to do a leak test in a local fresh water resovoir, but as im a bit more confident its not going to leak now and its such a logistical fuss, im going to go straight in the sea.  Next week.
>Kind Regards
>James
>
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