[PSUBS-MAILIST] Hatch interlock

Thu Mar 3 01:22:11 EST 2016

Hi Sean,
I really like the idea of vacuum monitoring for hatch leaking - nice one.
Also Alan a good idea for a single central transmitter monitoring the whole
thing.

A couple of thoughts from me:
- as well as the vacuum breaker valves on the actual hatches, you'd need
individual manual isolation valves in the 1atm chamber so you determine
which was leaking, and in case one leaks.
- You could protect the transducer from over pressure with a relief valve
in the 1atm chamber (installed on the 1atm side of isolation valves)
- I'm not sure how likely it really is, but if you got salt water into the
vacuum space the water would quickly evaporate, leading the salt residue to
accumulate over time
- oil lubricated vacuum pumps, or at least the ones I've used, at only
moderate vacuum can create smoke/oil vapour which would be uncomfortable in
a sub. Using a diaphragm pump or similar you'd probably achieve a much
poorer vacuum but no atmospheric contaminants
- a vacuum switch would probably be cheaper and more likely to survive the
over pressure, at the cost of less detailed info, ie. This one is good to
200m
http://www.suco-tech.com/product.php?p=44&c=12
- actually on reflection, I sell digital gauges for SCUBA gas mixing
(shameless plug:
http://m.ebay.com/itm/Electronic-Digital-Pressure-Gauge-for-SCUBA-Blending-Nitrox-Trimix-Oxygen-/262307605334?nav=SEARCH
) and they are -1 to 400bar(g) in 0.1 bar increments, which would be ok for
both the vacuum and over pressure, so it can't be too hard, and requires an
ADC resolution of only 12 bits - if 10 levels of vacuum measurement would
be ok?
- I have some very reasonably priced transducer samples coming from the
same supplier - if they work out ok I could send you one or at least the
specs.
- For a quick and easy way of turning high res analogue signals into useful
digital data I can highly recommend this Arduino shield:
http://rascalmicro.com/docs-precision-voltage-shield/
Brandon has made a couple of 18-bit versions for me, which are good enough
for 0.1% readings from oxygen cells and the like. I've been meaning to
order some more.

Well that turned into a bit of a ramble, hopefully some is useful.

Cheers,
Steve
On 3 Mar 2016 3:52 pm, "Sean T. Stevenson via Personal_Submersibles" <
personal_submersibles at psubs.org> wrote:

> That's a smart idea. I would also tie that valve actuation mechanically
> into the actuation mechanism for the hatch dogs, so it remains intuitive
> and doesn't require a second action.
>
> Sean
>
>
> On March 2, 2016 9:05:54 PM MST, Alan James via Personal_Submersibles <
> personal_submersibles at psubs.org> wrote:
>>
>> Yes, didn't think of the pressure you get when you open the door.
>> You would need to have a valve to release the vacuum before being able
>> to open the door, perhaps this valve could be a 3 way valve, letting air
>> in between the o-rings
>> & closing the flow to the transducer simultaneously. This would stop a
>> low pressure transducer
>> being destroyed & stop the vacuu! m pump cutting in as it would maintain
>> a vacuum in the line.
>> Cheers Alan
>>
>> ------------------------------
>> *From:* Brian Cox via Personal_Submersibles <
>> personal_submersibles at psubs.org>
>> *To:* Personal Submersibles General Discussion <
>> personal_submersibles at psubs.org>
>> *Sent:* Thursday, March 3, 2016 4:32 PM
>> *Subject:* Re: [PSUBS-MAILIST] Hatch interlock
>>
>> Sean,  If you are only vacuuming that 1" or so circle in-between the O
>> rings will the vacuum be enough to hold it?   Seems like you'd have around
>> 280# pounds with a 10 Hg vacuum.   or you could  have close to a total
>> vacuum?   25 Hg  ?  then you could get upwards to 700# +  ,  If the O rings
>> squeezed all the way down you might get metal to metal, then would y! ou
>> still reap the benefit of that area acting as a force?   Seems like there
>> would be very little volume of vacuum.
>>
>> Brian
>>
>> --- personal_submersibles at psubs.org wrote:
>>
>> From: "Sean T. Stevenson via Personal_Submersibles" <
>> personal_submersibles at psubs.org>
>> To: Personal Submersibles General Discussion <
>> personal_submersibles at psubs.org>
>> Subject: Re: [PSUBS-MAILIST] Hatch interlock
>> Date: Wed, 02 Mar 2016 14:53:19 -0700
>>
>> I will have one transducer per hatch, so that I can track the interlock
>> status and seal integrity per hatch. Each transducer is therefore exposed
>> to whatever pressure exists at the flange between orings, which in the case
>> of the lockout hatches must necessarily include the pressure at full
>> lockout depth, because those flanges are exposed to full pressur! e when
>> the lockout is operated.  This means that in order for this conceptual
>> design to work, I must accept a larger range, lesser resolution measurement
>> for those hatches, but it occurs to me now that in every case, I would need
>> to accommodate the maximum anticipated pressure on either side of each
>> hatch, if I expect to be able to track progressive seal leakage without
>> damaging a transducer. Ergo, only the 1 atm spaces could make use of 0 - 15
>> psia transducers. The rest would have to be 0 - 250 psia or whatever, and I
>> may require higher resolution signal conditioning (24 bit?) to eff!
>> ectively measure the range below 1 ata.
>> Sean
>>
>>
>> On March 2, 2016 1:19:54 PM MST, Alan James via Personal_Submersibles <
>> personal_submersibles at psubs.org> wrote:
>>
>> Sean,
>> why not leave the transducer in the 1 atm compartment & just attach it
>> to the pipe from the compressor that would run through the wall to the
>> various
>> compartments & sealing flanges. There are of course other complications
>> with
>> releasing the vacuum pressure on the individual hatches.
>> Alan
>>
>>
>>
>> ------------------------------
>> *From:* Sean T. Stevenson via Personal_Submersibles <
>> personal_submersibles at psubs.org>
>> *To:* Personal Submersibles General Discussion <
>> personal_submersibles at psubs.org>
>> *Sent:* Thursday, March 3, 2016 8:35 AM
>> *Subject:* Re: [PSUBS-MAILIST] Hatch interlock
>>
>> It is not the personnel transfer hatch that presents the problem, but
>> rather the egress hatches in the lockout chamber, as those flanges will!
>> see the lockout pressure in normal operation.  I can get away with a larger
>> range pressure transducer for those hatches, but then I lose measurement
>> resolution.
>> Sean
>>
>>
>> On March 2, 2016 9:32:49 AM MST, Alan James via Personal_Submersibles <
>> personal_submersibles at psubs.org> wrote:
>>
>> Sean,
>> that sounds a good idea. Nuytco set the o-ring on the deep worker hatch
>> externally with a vacuum
>> pump, but with your twin seal idea you could do this from within the sub
>> without decreasing the hull pressure.
>>   If the transducers are just monitoring the vacuum between the o-! rings
>> can't the transducer be
>> m! ounted on the 1atm side of the diver lockout hatch & not be exposed to
>> diver lockout pressure?
>> Alan
>>
>>
>>
>> ------------------------------
>> *From:* Sean T. Stevenson via Personal_Submersibles <
>> personal_submersibles at psubs.org>
>> *To:* "personal_submersibles at psubs.org" <personal_submersibles at psubs.org>
>>
>> *Sent:* Thursday, March 3, 2016 1:41 AM
>> *Subject:* [PSUBS-MAILIST] Hatch interlock
>>
>> Doing some further design on my lockout submersible project, I came up
>> with a novel way to implement hatch interlocks, which doubles as a seal
>> condition monitor, ! and a means of establishing a preliminary seal in the
>> absence of a pressure differential without relying on the hatch dogs to p!
>> rovide the initial o-ring squeeze.
>> My design entails two o-rings per hatch (vessel has six hatches: cabin
>> loading / escape, outer lockout loading / escape, inner lockout loading /
>> escape, inner lockout egress , outer lockout egress, and transfer). These
>> o-rings are concentric face seals, each residing within a half dovetail
>> groove for positive retention of each o-ring when the hatch is opened or
>> manipulated. The grooves are oriented such that the flat face of each half
>> dovetail faces the intermediate space between the two rings. This
>> intermediate volume is not isolated, but rather connected (on the sealing
>> flange side) to a vacuum transducer, and piped through appropriate valving
>> to a vacuum pump. When the hatch is closed, this intermediate space is
>> pulled to vacuum (as strongly as the pump allows), then locked off, and the
>> strength of this vacuum is measured by the transducer and continuously
>> monitored. The interlock is clear as long as the va! cuum holds, a! nd
>> activates the moment the seal is rele! ased, instead of relying on some
>> arbitrary movement of the hatch to indicate that it is open.
>> Apart from the obvious expense, I see a potential problem with exposing
>> those vacuum transducers in the lockout hatches to high pressure,
>> necessitating either a less sensitive transducer that will withstand the
>> pressure, or some means of isolating the transducer when the pressure
>> approaches the limit of its range - I'm still working this out in my head,
>> but I thought I would share anyway.
>> Sean
>>
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