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Over-pressure valve actuation pressure?

Discussion in 'Buoyancy Compensators (BC's) and Weight Systems' started by danstrider, Jan 29, 2019.

  1. iamrushman

    iamrushman Loggerhead Turtle

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    very interesting concept...thanks for sharing the details.
     
  2. danstrider

    danstrider Angel Fish

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    Thanks for all your replies, and thanks Luis for posting the articles on the CVS and for posting several descriptions of common issues. Sounds like the CVS widget worked, but was just finicky or unreliable or too bulky for its value. Luis, what other "several flaws" did it have in operation with regard to the physics?

    Got it. This is a great summary.

    A normal bladder BC won't vent until it is fully inflated, so I can't figure the over-pressure feature is commonly used. Certainly, the thick plastic shell should tolerate more pressure than a bladder.

    I bought a DGX over-pressure / dump valve to measure how much pressure it takes to actuate. These are fairly stiff springs. Matching herman's comment about adjusting them for a constant bleed to control ascent with a drysuit, these valves can be unscrewed some to change the spring preload, so decreasing the activation pressure.

    I also bought a DGX power inflator hose with pull-to-dump. This mushroom valve looks to vent at a lower differential pressure, but they also have a non-dump solid elbow.

    I get that the dedicated regulator would equalize the pressure inside the shell. And I get that this is similar to the physics of a second stage. It's probably doable to convert a cheap second stage for this purpose, if equalizing this way is neccesary.

    At risk of a stupid question, what if another over-pressure valve was mounted backwards to the shell instead of using the equalizing air regulator? The shell's pressure would still equalize occasionally, but this would change the air/water ratio and "reset" the neutral trim depth. If this reset occurred when outside of say a ~10ft band, then you'd simply swim down and the simple valve would match your new desired trim depth, no? I don't yet understand what value there is in using air to equalize the internal shell pressure.

    The 1977 article mentions about 60lb positive on the surface, giving roughly 7.5gal of displacement. Another article mentions most divers should aim for 2/3 to 3/4 full of water for neutral trim ... this means to me only roughly 2gal of air is needed, meaning the CVS is multiple times larger than it needs to be. If it were roughly the same size as a typical tank, and strapped into a dual tank configuration, would that help with the bulk? Also, how long does it take to drain the ~40lb of water out of the shell?

    I transcribed the 1977 Skin Diver article, so at least you can read the text more easily!

    The Dacor CVS patent is also very helpful.

    Pretty much committed myself to building something at this point, so really want to understand what pieces I'm missing...

    danstrider
     

    Attached Files:

  3. JackD342

    JackD342 Dive Shop

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    Ha! I of course had to click on the patent doc to at least take a glance, and one of the inventor names sounded a bit familiar. Sure enough, Jack Bohmrich was at SeaSoft for a while as VP of Sales. LinkedIn says he is still there, but a few years ago I was told he had left. Maybe he is back again, but it has been a while since I have needed to order anything there so I wouldn't know. So I suppose if you really want to speak with the designer with any questions, you have a route for tracking him down.
     
  4. Luis H

    Luis H Instructor, Scuba

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    @ danstrider, Thanks for the attachments. Really appreciate the text and a copy of the patent.

    The over pressure valve on a flexible BC was added decades ago to avoid the bladed from rupturing if it accidentally got over inflated during an ascent. But if you have that much air to fully inflate any BC, you are surfacing out of control. You have big problems.

    An over pressure valve in a lifting bag (or an SMB) could serve a big purpose, by keeping the bag from rupturing if you send it to the surface while the air is expanding.


    The purpose of the CVS (Constant Volume System) is to maintain a constant volume ratio (of air and water) at all depth. Since the water is incompressible, adding water when going down would defeat the purpose. The shell has a lot of flat surfaces that would not tolerate much differential pressure. The pressure inside needs to be close to the pressure outside. Therefore, since the air is what expands and contracts as function of pressure change, air is what needs to be added or vented to maintain a constant volume.



    A while back I thought about making a constant volume BC system using two Schedule 80 PVC pipes. The reason for the heavy wall PVC pipe is to use them as ballast cylinders, but not necessarily compensate for the changing pressure. The schedule 80 PVC pipe can take a reasonable amount of internal pressure, and I can calculate how much external pressure it can tolerate before it buckles (due to elastic instability) and implodes.

    Some day, I may get back on that project. It could be a really fun project if I had the time.
     
  5. danstrider

    danstrider Angel Fish

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    The DGX over-pressure valve took 2.71lb to activate during a hang-test. I had the valve suspended upside down in a fixture, with stagnant water in the valve body. When enough water was poured in the hanging bucket for the stagnant water to drain out through the valve seal opening, that was the point to stop. The valve hole is 0.75in in diameter, giving 1.767in2 area. This yields a pressure differential of 2.711lb/1.767in2 = 1.53psi. 1.53psi is approximately 3.5ft of water.

    Theoretically, the CVS works by flooding the vessel until at some depth, say 30ft, then add air until equalized, then close off the valves for the remainder of the dive. If I descend, the pressure vessel should maintain its air/water ratio to maintain neutral trim. It shouldn't keep burping water in, or burp out air during ascents, because that changes the fixed volume displaced. The caveat is that a closed pressure vessel is subject to implosion or explosion based on its structure, so over/under-pressure valves are a safety requirement. How far away from the (neutral) depth set-point those should burp is an open design question.

    Let's say I had the pressure vessel with the correct air/water ratio to be neutrally buoyant at 30ft of water. This 1.53psi DGX valve means if I swim up by just 3.5ft of water, the over-pressure valve would burp. Not sure if that's the desired depth range or if it defeats the purpose of "constant volume" at too narrow of a depth excursion before the safety feature activates.

    It should be possible to add an insert to the DGX valve opening to decrease the area, thereby increasing the activation pressure differential. That could be easier than finding another spring, and would put less pressure on the plastic housing.

    The spring is 0.955in diameter with relaxed length of 1.20in and wire diameter of 0.043in, and is easily replaceable. The spring rate, based on a quick Hooke's Law experiment, deflected 8mm for 422g, giving approximately a 2.95 lb/in spring rate. I found a replacement from McMaster (PN 9657K524) of the same OD and length, but with 14 lb/in rate. That should yield a 4.75x factor in activation pressure, giving +-16.6ft of depth range. That sound better. If the plastic housing can take it. And if the pressure vessel can take it too.

    @Luis H, do you have a feel on the depth range you have/had before hearing the pressure equalize or over-pressure valve burp?

    @JackD342 thanks for the lead on the CVS designer. I may try to look him up at some point.

    danstrider
     
  6. Luis H

    Luis H Instructor, Scuba

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    The CVS is not a pressure vessel. The pressure differential between the inside and the outside of the case is always very small.

    It vents air as you go up and the regulator adds air as you go down with relatively small motions, maybe a foot or two of pressure differential. It might have been a bit more, but not a lot more. I never paid close attention, but it vent and adds air with relatively small pressure changes (depth changes). Somewhere in the specification I should be able to find this information (allowable pressure differential)

    In order to maintain a constant air volume (inside the case) without creating much pressure differential the CVS has to vent air and add air as you move up and down, respectively. The actual volume of air inside the case stays constant. It only vents the expanding excess air (going up) and add air as needed to compensate for the compression of the air.



    Normally the diver adjust the water to air volume ratio at the beginning of the dive near the surface (but it can be done at any depth). It was common to re-adjust once at depth to compensate for wetsuit compression. The vent system and regulator combination, then maintains the constant volume by adding and venting air to compensate for pressure changes. It is supposed to be an automatic continues process. The volume of air inside the case stays constant (but the air density is obviously changing with depth). It does work.

    A couple of issues is that it doesn't take into account for the buoyancy change due to wetsuit compression or the buoyancy change due to the fact that the scuba tank is getting lighter during the dive as the air is consumed.
     
  7. Luis H

    Luis H Instructor, Scuba

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    The patent papers says that the pressure differential is maintained to within 2 psi (or less than) from the ambient pressure. That is almost 4.5 feet of depth change before activating the vent (going up) or the supply regulator (going down). That seems a lot more differential than I recall (I haven't used one in years).

    The patent papers does say that pressure differential is kept under 2 psi. I think it was a lot less than that.


    I need to fix the one I have and get it back in the water.


    I should add that the 2 psi only applies to the positive pressure (the internal pressure above ambient that would cause the relieve valves to vent).

    I have not seen any information on how much negative pressure the system can tolerate or is designed for, but it would be less that the positive pressure.

    Also the air supply regulator looks and feels just like a low end octopus regulator. Just by feeling the force required to press the equivalent of a purge button, I would say that this regulator operates in the same cracking effort range as a low end regulator, or a few inches of water pressure differential (or something in the range of 0.1 psi or about 3 inWC).
     
  8. rongoodman

    rongoodman ScubaBoard Supporter ScubaBoard Supporter

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    What problems were they trying to solve with those complicated systems that we're obviously handling without much issue these days with a simple wing or BC? Is it that diving styles have changed, or was there something else going on?
     
  9. Luis H

    Luis H Instructor, Scuba

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    It was supposed to be a complete hands free system. Once you set it (in theory) you didn’t touch an inflator or air dump again during the dive and it maintained perfect buoyancy at all time.

    Also you have to keep in mind that when BC were starting to show up, there was a large group of divers who thought they were un-necessary and even potentially very dangerous. The idea was that they could cause an un-controlled ascent. Many believed that BC should only be used by very experienced divers that could handle the potential of a run-away ascent. This was actually even before the use of the LP inflator device.

    Most early BC had CO2 cartridges and a few like my Fenzy had its own small air cylinder under the BC. Take a look at my avatar.

    If you read the patent description, you can see that even in 1977, the fear of a run-away ascent was still a strong consideration.

    Early BC’s also didn’t have dump valves on the bottom or even multiple dump valves.




    I will admit that the systems we have (the modern BC's, BP/wings, etc) do the job well, but they far from perfect.

    The freedom of diving without any BC (in warm water with minimal or no wet suit) is IMHO the ideal diving setup.

    In some ways, the opposite is actually carrying this (CVS) small refrigerator size box on your back
     
  10. Coztick

    Coztick Barracuda

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    What's the name for those devices that are ridiculously complex that perform the simplest of tasks....
    ...that...

    Take an appropriately sized bag.
    Blow in it when needed. Maybe twice?
    Vent on accent.
    Brainchild of a Ford engineer, my guess.
     

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