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  1. #11

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    Quote Originally Posted by v101 View Post
    Question for ones who fill tanks from a cascade system.

    A friend of mine I fill for has a 4 bottle cascade system.
    The bottles are 434 cu ft @4500 psi

    Typically I fill the bottles to 4300-4400
    So here is question. If we want to get the most fills on these bottles when we do remote dive trips. We typically start with full side mount tanks.
    This trip he started with 2 bottles for low pressure, stopping at 22-2500
    Then next bottle to 3000, final bottle to 3500.

    However not many fills were achieved this way, before bottles got drained down more quickly than we anticipated.

    We are filling HP 119s and also HP 100s

    Any advise or suggestions on filling strategies?
    This is the first time I've ever seen anyone adopt an approach of filling off the low bottles first BUT STOPPING AT A DESIGNATED PRESSURE.

    It sounds fine in theory but in practice there are two issues:

    1) The tanks naturally sort them selves out into a ascending order of pressure, and as other have noted, getting all you can from the "low" bottle first maximizes the pressure of the gas that is preserved in the higher pressure bottles further down the cascade.

    If you save pressure in a bottle in the lower end of the cascade, that same pressure is going to have to come out of one or more of the higher pressure bottles, and you just equalize all your bottles to the same useless pressure a lot sooner. Even before that point is reached, the pressure in the low bottle may be higher, but it will still be useless because you won't be able to get a usable fill in the tank once the last bottle in the cascade drops below the desired fill pressure for the tank you are filling.

    In other words I'd much rather have 1200 psi in the low bottle than 2500 psi in the low bottle, as at 1200 psi it means that other 1300 psi is still present in the last bottle in the cascade.

    2) If the cascade bottles are not connected on a common system (you don't specify if they are plumbed together or you are just filling off 4 separate bottles) you end up with more loss in transfers since you are making switches from "low" to "medium" to "high" pressure cascade bottles (and perhaps a switch between the high pressure bottles) for each and every tank you are filling, when you may need no switches or just one switch for the initial fills.

    Obviously, plumbing the tanks together in a portable system with flexible hoses or pigtails is more efficient, but it's more expensive and requires the tanks be more or less solidly and permanently mounted on a trailer.

    ----

    Someone mentioned octopus whips to fill more than one tank at once. I do use a 2 DIN connector whip when I trans fill and then boost O2 to my 2L O2 rebreather bottles. I do that as it keeps the fill rate lower and is a time saver when filling all bottles. But it does result in greater transfer loss unless you have a shut off valve and purge at the end of each whip on the octopus.

    NACD Cave DPV Cert # 666: Cave DPV Anti-christ

  2. #12
    Member
    Join Date
    Jun 2005
    Location
    Chelsea, VT
    Age
    60
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    3,659

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    Quote Originally Posted by Rustykfd View Post
    Open bottle 1 until you reach fill pressure or equalize, repeat with 2,3,4. Have filled thousands of bottles from cascades in the fire service and some dive tanks from my home cascade as well.

    I do the same. Learned in the fire service as well.


    Mike


  3. #13

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    Quote Originally Posted by DA Aquamaster View Post

    Someone mentioned octopus whips to fill more than one tank at once. I do use a 2 DIN connector whip when I trans fill and then boost O2 to my 2L O2 rebreather bottles. I do that as it keeps the fill rate lower and is a time saver when filling all bottles. But it does result in greater transfer loss unless you have a shut off valve and purge at the end of each whip on the octopus.
    also important to note that when you cascade to multiple bottles at the same time it tends to lower your cascade efficiency by a little bit. We use a 4:1 octopus when filling, but the cascade is also being fed by a 25cfm compressor when we are filling. For us, the efficiency is less important than the speed and filling 4 pairs of doubles at the same time helps keep everything a lot cooler.

    We do have shutoffs and bleeds on the end of each whip though. A lot easier for us to work with, especially since they are often fed off of a 100ft hose lead from inside the building when we fill in the truck


  4. #14

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    Clearly it's best to max out each bottle, and the vast number of bank configurations and fill configuration as well as values like temperature changes and lost gas from bleeding whips make it hard to build a calculator, but I threw together something simple to show effects of filling bottles on a bank. If you write down the ending values after filling the first tank and update the starting values you can simulate multiple fills.

    It's rough and maybe has mistakes, but it was interesting to think through the math behind what we all used to watch happen bottle by bottle at Wayne's. And God damn the people who didn't wait for bottles to equalize and robbed us of higher pressure fills by rushing through the banks!

    Www.divebhb.com/gas-planning

    Quote Originally Posted by JJ1987
    "But nothing gets accomplished in sidemount!"


 

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