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Thread: CMF - Understanding Constant Mass Flow

  1. #11
    RBW Member Paul S is on a distinguished road Paul S is on a distinguished road Paul S's Avatar
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    Re: Understanding Constant Mass Flow

    Quote Originally Posted by paulraymaekers  View Original Post
    as for talking about volume at surface pressure: indeed it is easier for people to understand than mass: as everybody seems to understand that 1.35 liter at surface is to much, and 0.75 is +/- correct
    1.15 gr/min, or 1.55gr/min.... nobody seems to know what is the best
    Honestly, I wouldn't know which was correct in either units. I'd prefer to talk grams if we're talking about mass, and I find (for me) the extra step of converting to surface pressure adds confusion.

    But that's just me, and I also know I'm the weird one for being educated in the dark arts of science.

    Hey, at least nobody is talking about 0.19 hogsheads per hour of O2 flow, although that would still be about right

    One question though: Why do you need a first stage on the O2 for the orifice? Couldn't you just have the orifice connected directly to the HP side? Then there wouldn't be a drop off at depth.

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    Re: Understanding Constant Mass Flow

    Quote Originally Posted by Paul S  View Original Post
    One question though: Why do you need a first stage on the O2 for the orifice? Couldn't you just have the orifice connected directly to the HP side? Then there wouldn't be a drop off at depth.
    euh, seems my article was not that clear maybe

    (flow depends on inlet pressure, constant flow means constant inlet pressure.... is the HP pressure constant? )
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    Re: Understanding Constant Mass Flow

    Paul -
    This is very well written.

  4. #14
    RBW Member Vilho is an unknown quantity at this point Vilho's Avatar
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    Re: Understanding Constant Mass Flow

    Hi Paul,

    Thank you for a good article.

    Two questions:

    1) If I understand this correctly, the depth limitation could be overcome by increasing the intermediate pressure of the O2 regulator at the factory (and sizing the orifice to match). What is the reason for not doing this (e.g. higher ip regulators don't exist, want to use standard regulators, etc.)?

    2) When exceeding the 80m limit, is it ok to plug in off-board gas (with standard regulator) and leave the orifice and O2 reg in hCCR setup or should the rEvo always be modified into a full eCCR when doing these dives?
    (I'm not there yet, just curious at this point.)

    Best,

    Vilho

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    Re: Understanding Constant Mass Flow

    1) If I understand this correctly, the depth limitation could be overcome by increasing the intermediate pressure of the O2 regulator at the factory (and sizing the orifice to match). What is the reason for not doing this (e.g. higher ip regulators don't exist, want to use standard regulators, etc.)?

    80m is already a nice depth, and I don't want to go outside the recommended IP of the apeks first stage

    2) When exceeding the 80m limit, is it ok to plug in off-board gas (with standard regulator) and leave the orifice and O2 reg in hCCR setup or should the rEvo always be modified into a full eCCR when doing these dives?
    (I'm not there yet, just curious at this point.)

    for deeper you can do both: either converting to pure eCCR, blocking the orifice and taking of the APR cap, or, injecting offboard gas, either O2 or rich dil: for very deep dives I inject rich dil, as I'm not so fond of using pure O2 at great depths

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    .... radial scrubbers give longer dwell time than axials...
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  6. #16
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    Re: Understanding Constant Mass Flow

    Allow me to answer eventhough its addressed to Paul:

    Quote Originally Posted by Vilho  View Original Post
    1) If I understand this correctly, the depth limitation could be overcome by increasing the intermediate pressure of the O2 regulator at the factory (and sizing the orifice to match). What is the reason for not doing this (e.g. higher ip regulators don't exist, want to use standard regulators, etc.)?
    That would requirer a special regulator to get the IP high enough (Dolphin uses 16bar), and also smaller orifices. Smaller orifices can clog more easily. Along with reinforced tubing to match the higer pressures. All things would make the system more proprietary, more expensive, and harder to maintain.

    There are ways around this, namely a stronger spring for the DS4 and a special orifice. That would give you some 2-3bars higer IP, and therefor also 20-30 m. deeper rating concerning O2-flow ONLY.

    Several other methods too keep the loop functioning below the absolute IP is available. Each has its pro and cons:

    * Hiher IP, with smaller orifice.
    * Uncompensated Needle-valve.
    * Uncompensated Offboard MAV O2-addition for the dive-time below IP.
    * SCR with diluent for the dive-time below IP.
    * eCCR config with no MFO.

    2) When exceeding the 80m limit, is it ok to plug in off-board gas (with standard regulator) and leave the orifice and O2 reg in hCCR setup or should the rEvo always be modified into a full eCCR when doing these dives?
    (I'm not there yet, just curious at this point.)
    By interpret your question as if its OK to plug in offboard GAS to a MAV, and maintain PPO though this MAV. IF so you should set the setpoint for the controller lower than the PPO, to avoid it firering uneccessarily. Also to avoid possible backwards flow of moist gas through the open solenoid (I dont know if it has a check-valve infront like the MFO).

    Remember the rEvo is "divable" to the point where IP matches the pressure, thats not 80m, but somewhere deeper. An IP of 11bars nearly gets you to 100msw.

    Nicolai

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    Re: Understanding Constant Mass Flow

    Nice clear article Paul.

    the rEvo in hybrid configuration still requires a blocked first and thus still has a depth limitation, correct? to dive below where ambient exceeds IP, it would need to be converted to full eCCR mode right?

    have you put together a system that is eCCR with offboard MFO and MAV o2, with the controller set to low, requiring manual injection. I'm wondering if something like this would give the person the advantage of manual injection and being a part of the monitoring/maintenance loop while still being true eCCR with no depth limitation. maybe there is no use in this.

    Gill Envy

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    CMF - Understanding Constant Mass Flow


    CMF - Understanding Constant Mass Flow
    by Paul Raymaekers
    This editorial is a simplified explanation of the CMF principle, and is only written to assist with the understanding of CMF rebreathers. The author of this article does not accept any responsibility for the use of this information for building or modifying existing rebreathers.


    Click on above image to read editorial

  9. #19
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    Re: Understanding Constant Mass Flow

    Quote Originally Posted by Hanssing  View Original Post
    Allow me to answer eventhough its addressed to Paul:


    That would requirer a special regulator to get the IP high enough (Dolphin uses 16bar), and also smaller orifices. Smaller orifices can clog more easily. Along with reinforced tubing to match the higer pressures. All things would make the system more proprietary, more expensive, and harder to maintain.
    If I am not wrong, a higher IP would not require a smaller orifice since we would always be reaching the same sonic speed (Vmax).

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    Re: Understanding Constant Mass Flow

    Quote Originally Posted by Davichin  View Original Post
    If I am not wrong, a higher IP would not require a smaller orifice since we would always be reaching the same sonic speed (Vmax).
    Thinking about it, I guess I am wrong as speed is the same but density is higher so we would have too much oxigen coming in...

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