[PSUBS-MAILIST] Hatch interlock

Thu Mar 3 08:32:22 EST 2016

Hi Sean,Since the goal here is to monitor o-ring condition.  How about a central pump with a manifold with a vent valve and gauge piped to your dash.  In the name of keeping it simple, there is no need for anything electronic. Steve has the gauge that can handle the pressure boundaries. Your set up does not need to indicate witch o-ring is in trouble because your going to abort regardless of witch one it is.  A visual inspection at the surface will locate the problem.  Hank 

    On Thursday, March 3, 2016 5:32 AM, Sean T. Stevenson via Personal_Submersibles <personal_submersibles at psubs.org> wrote:

 Actually, I'll be using a PAC - a slightly more powerful version of a PLC, in the form of a National Instruments CompactRIO controller, running an embedded HMI on touchscreen hardware very similar to Cliff's implementation. I want as much as possible for the only manual controls and indicators to be those required by the rules as manual overrides, and of course the isolation valves at the through hulls. Everything else should run automatically or "fly-by-wire" during normal operation. Fortuitously, one of my projects at work in 2014 was the development of a smooth, high accuracy pressure ramp and control system for automatic pressurization and depressurization to 60,000 psi, both hardware and software development, so I know how to do this.Sean

On March 2, 2016 11:56:31 PM MST, Alan James via Personal_Submersibles <personal_submersibles at psubs.org> wrote:
Wow some good products Steve,I had a look at your Ebay product inventory http://www.ebay.com/sch/tfmengineeringaust/m.html?_nkw=&_armrs=1&_ipg=&_from=I can see you will be a great source of knowledge for tracking down those tricky bits & pieces.    If Sean puts an LED between the pressure sensor & the solenoid valve or between the solenoid valve & it's power source, he will be able to see which set of o-rings is not sealing. And if he was Cliff he wouldbe feeding the information to his PLC & displaying it on his HMI.Cheers Alan

      From: Stephen Fordyce via Personal_Submersibles <personal_submersibles at psubs.org>
 To: Personal Submersibles General Discussion <personal_submersibles at psubs.org> 
 Sent: Thursday, March 3, 2016 7:22 PM
 Subject: Re: [PSUBS-MAILIST] Hatch interlock

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,
SteveOn 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 ableto 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 pumpcutting 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 stillreap 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: W! ed, 02Mar 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 whenthe 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! ectivelymeasure 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 itto the pipe from the compressor that would run through the wall to the variouscompartments & sealing flanges. There are of course other complications withreleasing the vacuum pressure on the individual hatches.Alan

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