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Thread: Overpressure valve

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    RBW Member jorgey is on a distinguished road jorgey is on a distinguished road jorgey's Avatar
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    Overpressure valve

    Hi, I have an Azimuth, and I have read a lot about the rig. There are some complains about the WOB of the rig. I found an article on a german forum where the author says that he has the same problem and by changing the setting of the relief valve from the recommended 14mBar to 8-9mBarhe solved it. I had read already an opinion about this here, though, the author at the german forum is a respected rb diver, with a experience I don't have. Therefore, I tend to believe what he found out empirically. My question is, do that make sense for someone here? What's the effect of the relief valve pressure setting on the rig performance? How is being calculated for a specific design? I would apreciate the comments about this topic. jorgeY

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    Normal people worry me jaap is a name known to all jaap is a name known to all jaap is a name known to all jaap is a name known to all jaap is a name known to all jaap is a name known to all jaap is a name known to all jaap is a name known to all jaap is a name known to all jaap is a name known to all jaap is a name known to all jaap's Avatar
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    Re: Overpressure valve

    Quote Originally Posted by jorgey  View Original Post
    Hi, I have an Azimuth, and I have read a lot about the rig. There are some complains about the WOB of the rig. I found an article on a german forum where the author says that he has the same problem and by changing the setting of the relief valve from the recommended 14mBar to 8-9mBarhe solved it. I had read already an opinion about this here, though, the author at the german forum is a respected Rebreather diver, with a experience I don't have. Therefore, I tend to believe what he found out empirically. My question is, do that make sense for someone here? What's the effect of the relief valve pressure setting on the rig performance? How is being calculated for a specific design? I would apreciate the comments about this topic. jorgeY

    My spontaneous thinking would be that its the opposite. On a BMCL-CMF-SCR (nice abbreviations:), anyway like an Azi) a slightly high OPV setting should generally give a better WOB than a slightly low setting.

    This is since the inhalation is generally the harder part, a higher pressure in the bag makes it easier to inhale. A higher pressure also makes the bag more inflated moving the average gas volume closer to your back reducing the hydrostatic imbalance.

    The above is of course assuming the diver is swimming belly down.

    But how it works out in real life might be a very different thing.
    I did not use my Azi in SCR-mode since a few years and its sold now so I don't recall what I tuned the OPV to.

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    Re: Overpressure valve

    Yeh, diving SCR I found a slightly higher setting to be more comfortable. If it is too low the ADV will hit too often, thus wasting gas.

    Dale

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    Re: Overpressure valve

    Thank you jaap and bletso,
    What you say sounds logical to me. On the other hand, the relief valve is on the exhalation bag, could that have an influence on the exhalation and less benefit on the inhalation because of the scrubber resistance? Just thinking loud.

    Next time I use the rig I'm going to do some tests. I had forgotten that I have a pressure gauge to measure WOB somewhere in cellar... I just need to make the connections :)

    jorge

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    Re: Overpressure valve

    Quote Originally Posted by jorgey  View Original Post
    Thank you jaap and bletso,
    What you say sounds logical to me. On the other hand, the relief valve is on the exhalation bag, could that have an influence on the exhalation and less benefit on the inhalation because of the scrubber resistance? Just thinking loud.

    Next time I use the rig I'm going to do some tests. I had forgotten that I have a pressure gauge to measure WOB somewhere in cellar... I just need to make the connections :)

    jorge
    Unless the scrubber is really, really hard to breathe through the pressure in the loop will aid inhalation and impede exhalation with rear mounted lungs. If the lungs are 50mm above your lungs then you have a pressure differential of 5mb to overcome, if they sit further away the pressure differential is greater-and remember that unless the lungs are fully inflated the gas will be trying to pull the lungs upwards and away from your body.

    If you assemble the loop outside the case [don't bother filling the scrubber or turning on the gas] and breathe through it for 2-3 breaths you will see the inhale CL expand as you start exhaling.

    With back CL's you can do a barrel roll as you dive. In the 'normal' position you will find it easier to exhale, on your side the resistance is neutral, and on your back you will get hamster cheeks as the gas tries to rush into you from the CLs.

    Most people find it easier to exhale than inhale. With a higher pressure in the loop and rear CL's you will have less of a pressure ratio between you and the CL's that you have to overcome on inhaling, but a slightly harder breathe out.

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    Re: Overpressure valve

    thank you all for the opinions, I think I understand now the function of the relief valve and the effect of its setting. On the other hand I re-read the article and the author complains about resistance by exhaling, peculiar for a rear mounted lungs, I think. If I understand right, he says it has nothing to do with the newer Azimuths.

    I will be testing my unit in the upper range of the recommended setting (14mbar +- 2mbar -> 12-16mbar) Now I had it in the lower limit, 12mbar. The range for the dolphin is 18-25mbar if I'm right, could that explain why people find it easier to breath as the azi?. Anyway, in an almost horizontal diving position a compensation of 16cm should be enough, I think. I'll see :)

    jorge

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    Re: Overpressure valve

    I'm not as familiar with the Azi as I am the Dolphin, it could be a design issue that causes problems.



    The Dolph [on the left] can be seen with it's inhale and exhale lungs. They sit pretty much one above the other so there is no variation on the pressure in the lungs. The bottom of the inhale lung is below the scrubber, which allows a smaller pressure differential between the inhale CL pressure and the diver's lung perssure. The smaller exhale CL allows the gas to transfer into the scrubber and inhale CL more quickly.

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