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IPO and back mounted counter lungs

Discussion in 'Rebreather Diving' started by Gareth J, Apr 23, 2019.

  1. Gareth J

    Gareth J Manta Ray

    # of Dives: 1,000 - 2,499
    Location: UK
    Hi All

    A question.
    Does anyone know if the incidents of IPO are higher when using back mounted counter lungs, compared to the over the shoulder counter lungs.

    I've always hated the clutter of front mounted counter lungs. I had the electronics upgraded 18 months ago, and a BOV fitted. So, I'm considering upgrading to back mounted counter lungs. Finally getting back to my OC days of an uncluttered front.

  2. Brad_Horn

    Brad_Horn Nassau Grouper


    Based on only being able to find one reference attributed to IPO the answer would have to be yes. But then as the Aqualung CRABE doesn't as I understand it meet CE standards for: WOB or hydrostatic or a whole host of other requirements that is likely not unexpected either.
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    Noting BMCL can offer the best WOB and hydrostatics of any configuration. Might it be simpler to not just verify what the WOB is for your unit as fitted with BOV and BMCL or FMCL?

    Then this data from the manufacturers testing will also allow you to them compare the different hydrostatic performance of each configuration. Allowing you some scientific basis to support either configuration.

    Examples of this comparison and available data from manufacturers
    A Comparison of CE Test Data for two Closed Circuit Rebreathers – Joseph's Diving Log
    happy-diver likes this.
  3. wedivebc

    wedivebc CCR Instructor ScubaBoard Supporter

    I don't know of any studies exploring this theory but I do know this about BMCLs:
    There are two main types of WOB issues. Mechanical restrictions like hose diameter, turbulence, laminar flow etc. and hydrostatic loading. The design of a back mounted counterlung must be such that it positions the gas bubble within the CL to as near as possible to the center of the divers lungs to reduce the hyrostatic pressure. It does not take much difference in water pressure between your lungs and the forces acting on it to make for a big difference in WOB. This requires the counterlungs to be mechanically restrained as the tendency is for the gas contained within the CLs to float up and away from the diver, thereby increasing the WOB. Many BMCL divers find their position of trim and the position of the rebreather on their body play a big role in reducing WOB. This distance from the diver's body can be exacerbated by gas volume in a drysuit and the luft of the drysuit underwear.
    With front mounted CLs the majority of the gas bubble is restrained by the diver's body most commonly the chest area where the gas bubble is lifted by buoyancy to be as close as possible to the diver's lungs which reduces static lung loading and WOB.
    I do believe high WOB does contribute to IPO so based on that knowledge I would suspect back mounted counterlungs could easily cause a higher incidence of IPO.
    Hoyden, rjack321 and happy-diver like this.
  4. silent running

    silent running Manta Ray

    # of Dives: 1,000 - 2,499
    Location: Brooklyn, N.Y. U.S.A.

    Hi Brad, how can BMCLs have the best WOB except possibly in their one optimal position? Don’t even small changes in position change the hydrostatic head and WOB in BMCLs? That was certainly my experience diving the KISS and a CCR Draeger. As soon as I moved out of the slightly heads up position, they breathed much worse, either inhaling or exhaling. Isn’t the point of OTS CLs that they are less affected by changes in diver orientation because they are closer to the actual lungs to begin with?
  5. Brad_Horn

    Brad_Horn Nassau Grouper

    The best documented WOB of any rebreather is 1.44J/L at 40m on Air at 75lpm
    This rebreather is a BMCL unit. It also has its 3D CLs as close to the divers back as possible as the units frame is the backplate with the harness built in. It doesn't have an optimised position because it was designed to work in the full range of positions.
    Testing at http://www.deeplife.co.uk/or_files/DV_OR_WOB_Respiratory_C1_101111.pdf

    That said there are also BMCL units inclusive of the KISS and Drager designs with extremely high WOB but this is also reflected in their inability to be CE'd. Balanced by there being a lot of FMCL designs that barely scrape through the CE minimums of 2.75J/L when 40 year old MK16's offer much much lower WOB.

    If the set be it BMCL or FMCL is fitted correctly on the diver, then the WOB ought not to change from that tested. The manikin used to test WOB is not that much different from the average diver. Which Dave is I think where your statement has a fallacy, in that you have assumed a FMCL may offer better WOB over BMCL! Certainly possible but by no means a certainty.

    Obese divers might have issues in experiencing increased WOB, due to these CL's being further displaced from the lung centroid but I don't think anyone has had sufficient interest to investigate that as a factor. This would equally affect FM as well as BM CLs.

    It would certainly be interesting being in the OP's position and being able to directly compare the WOB of his set based on the manufacturers testing of it with optional BOV fitted with both BMCL and FMCL options. Pending knowledge of what the actual WOB is for both configurations any discussion is purely theoretical anyway.

    SR, I'd agree with you based on my own experience on certain units. Folk are certainly re-experiencing these issues with untested sidemount units My Journey Into Sidemount Rebreathers but where BMCL are designed for optimised performance, the evidence http://www.deeplife.co.uk/or_files/DV_DLOR_HydroImbal_101116.pdf doesn't agree with your experience to date.
    Compared to the expense of designing and testing BMCL to ensure they actually have low WOB, OTSCL are cheap!
  6. silent running

    silent running Manta Ray

    # of Dives: 1,000 - 2,499
    Location: Brooklyn, N.Y. U.S.A.
    Thanks for all the detailed info Brad. So the gist of your post is BMCLs can be just as easy to breathe as OTS with proper engineering? But how is this possible, given our lungs sit closer to the front of the chest with the spinal column, rear rib cage and shoulder musculature putting any BMCLs further away from the lung centroid to begin with?

    If OSEL had an OTS option I would certainly consider it as I tend to dive in places with high current and do find myself in high exertion situations. It is for this reason WOB through all positions is important to me. I think it ought to be important to every diver because the ocean and other environments people favor CCR, like caves, can be unpredictable in terms of currents and our orientation while negotiating any number of other environmental challenges...
  7. JohnnyC

    JohnnyC PADI Pro

    # of Dives: I'm a Fish!
    Location: United States
    Doesn’t matter either way, you can’t dive a pdf......
    rjack321 likes this.
  8. 1c3d1v3r

    1c3d1v3r Rebreather Pilot

    # of Dives: 500 - 999
    Location: FInland
    I think there is a flaw how the certification test measure the WOB of rebreathers.
    It calculates the area inside the "Lissajou pattern".
    It works well for second stages and passive gas masks because the pattern is on both sides of the horizontal zero axis.
    With a rebreather you have elastance and hydrostatic lung loading (hydrostatic imbalance). Elastance is the pressure difference between end of inhale and end of exhale. It is caused by the counterlung emptying or filling up and the lowest point of entrapped gas is changed in the water column.
    The tests ignore the pressure difference between the lung centroid and lowest point of entrapped air in the counterlungs (with a loose fabric counterlung the pressure inside the counterlung is the same as where the lowest point of air is in the water column. Bellow type counterlungs work differently). It does not matter for the lungs if the pressure difference is caused by air flow restrictions or hydrostatic imbalance. The work is the same.
    In the picture below there is a huge negative pressure inside the lungs all the time.
    The WOB is calculated by integrating the area of the Lissajou pattern. This would breathe badly even though the test says it is as good as the same pattern circulating the zero axis.

    A better way to calculate may be to integrate the exhale and inhale areas separately. For exhale you integrate the are on the positive side to zero axis (red lines). For inhale you integrate the area of the negative side and the zero axis (green lines).
    This takes into account both resistive wob and hydrostatic imbalance. A counterlung may also assist breathing. For exhale negative hydrostatic imbalance assist breathing. Positive hydrostatic imbalance assists inhaling.

    I'm sensitive for negative hydrostatic imbalance. It feels way more uncomfortable than the same amount of positive hydrostatic imbalance.

    Studies show slight positive pressure is better than negative. Diving long with backmounted counterlungs with negative loading increases the the risk of IPO.

    Link to a good document of WOB:
    rjack321 likes this.
  9. Brad_Horn

    Brad_Horn Nassau Grouper

    Sorry SR, I've only just seen your post. With proper engineering, BMCL will breathe better across the full spectrum of unmanned test requirements than OTS. It's not a guess. Given suitably indepth unmanned testing eventually being been done on OTS units and the results (with calibration) published, anyone can readily do this comparison against that offered on the Apoc. Deep Life Design Team: Selected Design Validation Reports for DL & Open Safety Equipment Ltd's Rebreathers
    OSEL even sell a breathing simulator to make this R&D easier for rebreather manufacturers, so they can offer improved products with better breathing performance. http://www.opensafety.co.uk/files/Datasheet_iBreatheMkIV_1906.pdf

    In response to your specific question, note the shape and material used in the Apoc IV & II counterlungs. Some of this is disclosed in the FMECA http://www.deeplife.co.uk/or_files/FMECA_OR_V4_140831.pdf
    You won't get the high breathing performance offered if you replaced the CLs with MSR waterbags or added a SS backplate and wing into the equation. This is why the design (noting the Apoc is a spin-off from the 350m rated commercial saturation diving primary life support system that DL was originally commissioned to design) started from a clean sheet of paper, with money not being a factor, and anything that didn't improve performance (or hindered user safety) was critically examined.

    I'm in full agreement with you on the importance of WOB. But can't see OSEL offering OTS unless a military client has sufficient mission justification for the need and accepts the decrease in breathing performance. In which case, like with the various Incursion models OSEL is shipping, this wouldn't be offered to recreational divers.

    You'll obviously improve the PRISMs WOB with fitment of the ALVBOV, which should even subjectively, be noticeable; however, I don't recall that the USN published WOB for the PRISM (don't think SM/PR tested it?) and without that we can't discuss just how much of a WOB improvement you'd see swapping to an Apoc. Still like to see one fitted with analogue secondary and modern primary HUD though.

    As I understand it, one can move the centre line in the lissajou easily by adding or withdrawing gas, so the method you've described would not work effectively.

    Under the current 14143 standard there is an elastance limit, a breathing resistance limit and a work of breathing limit. There are also hydrostatic limits. The elastance, and hydrostatic limits are related but independent of WOB.
  10. Superlyte27

    Superlyte27 Cave Instructor

    # of Dives: 5,000 - ∞
    Location: Florida
    I can't believe people still listen to that guy

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