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Thread: An An attempt to clear the misconception on CMF

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    RBW Member diveoceanos has a spectacular aura about diveoceanos has a spectacular aura about diveoceanos has a spectacular aura about diveoceanos has a spectacular aura about diveoceanos has a spectacular aura about diveoceanos's Avatar
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    An An attempt to clear the misconception on CMF

    Below there is a summary of what has been wrote about this issue.

    A greatly misunderstood and misapplied notion is that of “choked flow”, also referred to as “critical flow”. In gas flow through an orifice there is an occasion where the gas velocity reaches sonic conditions. This occurs for oxygen flow when the absolute pressure ratio is about 0,526 i.e. when the downstream absolute pressure (P2) is 52.6% of the upstream absolute pressure (P1). Beyond that point further increase of the upstream pressure does not cause any further increase to the velocity. The same does not apply to the mass flow.

    In Rebreather technology erroneously we use the term “constant mass flow” orifice although the correct term is constant velocity flow orifice. In case of KISS and Draeger Dolphin rebreathers and similar types the term Constant Mass Flow applies under certain conditions while this is not the case for rebreather that use a normal regulator to feed the Oxygen Orifice like Voyager etc. This is a result of a common misconception. Designers select an orifice diameter so that the flow achieves a critical sonic velocity at the range of operation of the unit. Once sonic velocity is achieved in an oxygen orifice (P2/P1 = 0,526), it is easy to "assume" (by error) that the mass flow rate is constant for all pressure ratios less than 0,526. For example, when P2 is 1 bar (surface) and P1 is 11 bar (10 bar above ambient), P2/P1 = 9,1%, sonic velocity occurs through the orifice. As P1 further increases there is no further increase in the velocity of the air flowing through the orifice and that is clear.

    Let us now consider all factors. The mass flow rate through an orifice is a function of three basic parameters.

    • Velocity
    • Density
    • Orifice Area

    Therefore it is wrongly concluded that the mass flow rate also does not increase as upstream pressure increases. When the upstream pressure (P1) increases, the density of the gas inside the nozzle also increases; and since the mass flow rate is also a function of density, the mass flow rate increases linearly with pressure (P1).

    CMF1.png

    What happens if a Rebreather uses a sealed first stage and what happens if it uses a non-sealed (non-modified) first stage?

    • In case of sealed first stage the Intermediate pressure remains constant and it is not affected by depth changes. Considering that the Pressure differential remains within the sonic conditions the critical mass flow will remain the same. If diving deeper than a critical depth then the flow velocity begins to slow down and eventually when the outlet pressure equals the inlet pressure ceases. The only way to maintain PPO2 beyond the depth where ambient pressure equals the supply IP is by operating a bypass valve attached to a depth compensating regulator (or non-sealed one).
    • In case of a normal (non-sealed) regulator as one goes deeper the critical mass will increase despite the fact that the velocity remains the same and at sonic conditions. The critical mass is directly proportional to the upstream pressure according to formula whilst downstream pressure does not cause any effect to the flow velocity. The mass flow rate is adjusted by the diver pre-dive. That means that special caution should be exercised so that the settings would be such as the oxygen supply is enough at shallow depths and does not exceed the metabolic consumption at the deeper part of the dive. Diving beyond the limits of sonic conditions is possible but again special caution is needed so that oxygen flow rate in the loop does not exceed metabolic consumption at the deep.
    Picture3.jpg

    IMPORTANT NOTE: There is no way to achieve a CMF by using a depth compensating regulator (non-sealed one).

    References:

    How Dolphins work
    http://www.okcc.com/PDF/Choked.pdf



  2. #2
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    Re: An An attempt to clear the misconception on CMF

    Below there is a comparison between the expected Mass Flow in a Sealed Regulator (Constant IP) Vs a non-sealed Regulator (Depth Compensating IP) in systems with continous gas bleed in the loop.

    Note: The below examples are using a round orifice of a given diameter. There are not actual measurements. The use of a needle valve instead is expected to give the same flow changes with depth as an orifice valve.

    Mass Flow Sealed reg.jpg

    In a Sealed (Constant IP) regulator an orifice is feeded with a constant supply pressure. Below a certain depth depending on the IP the Flow Velocity slows down and becomes Subsonic. At the depth where the IP equals the ambient pressure the flow ceases.

    Note: The Metabolic Consumption needs are not constant duting a dive. However this type of gas injection system allows for optimum calibration at the surface so the mass is always below the metabolic consumption, hence there is no risk of going Hyperoxic. At the same time the Oxygen injection is near to the metabolic consumption at the surface and only if a diver increases the workload or diving beyond the sonic area will need to increase the frequency of manual injections to the loop.

    Mass Flow Non Sealed.jpg

    Above there is an example of the expected flow in a system that uses a depth compensating regulator (A classic non-modified SCUBA regulator) which is preset at the surface and no modification is being made by the diver during the descent. It is obvious that the diver will need to manually inject Oxygen close to the surface and while going down. In this case the Flow mass will continue to increase even in the Sub-sonic velocity area. The deeper the diver goes the more difficult will be to maintain a constant PPO2 in the loop as he will need to add diluent to keep the O2 down to acceptable limits.

    This type of diving creates a lot of task loading and requires the diver attention to PPO2 on a full time basis. It is considered extremely difficult to have a stable PPO2 in the loop.

    Some rebreathers are using this method of gas injection. Furthermore the divers are instructed by the manufacturer and their instructors not to adjust the flow while descenting...
    Last edited by diveoceanos; 6th January 2010 at 16:55.

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    Re: An An attempt to clear the misconception on CMF

    Damn, Thanks that really cleared things up....

    Mike

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    Re: An An attempt to clear the misconception on CMF

    good write-up

    Mods: should consider moving over to an archived 'RBW article'

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    Re: An An attempt to clear the misconception on CMF

    Quote Originally Posted by diveoceanos  View Original Post
    Some rebreathers are using this method of gas injection. Furthermore the divers are instructed by the manufacturer and their instructors not to adjust the flow while descenting...
    Sotos well done.

    Happy Birthday as well.

    can you name some of those rebreather please? just curious mate...

    Spyros

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    Re: An An attempt to clear the misconception on CMF

    Quote Originally Posted by Outlaw  View Original Post
    can you name some of those rebreather please? just curious mate...
    It is not difficult to immagine, how many ccr with a non sealed first stage and with a "backyard agency" teaching the use of it do you know?

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    Re: An An attempt to clear the misconception on CMF

    Bravo Sotos !! many thanks..

    John

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    Re: An An attempt to clear the misconception on CMF

    Quote Originally Posted by Outlaw  View Original Post

    can you name some of those rebreather please? just curious mate...

    Spyros
    Hi Spyros,

    We have seen in other threats here in RBW, at least one manufacturer, who is officially claiming that by using a needle valve and a non-modified first stage can reach constant mass flow thoughout the dive without further adjustments on the fly. It is not a secret.

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    Re: An An attempt to clear the misconception on CMF

    Hi Sotos, i am not sure i follow you to 100% or i perhaps i do not agree with you.

    First example i agree totaly. But the second one i am having some questions...

    In the second you are having depth compensation, but your flow is increasing linear. This is where i am not agreeing... or are you using a constant mass flow (aka KISS valve) on a depth compensation reg? Because if you were using a needle valve you would either increase or decrease the flow and the curve (green line) would be a sin curve.

    /Johnny

    Quote Originally Posted by diveoceanos  View Original Post
    Below there is a comparison between the expected Mass Flow in a Sealed Regulator (Constant IP) Vs a non-sealed Regulator (Depth Compensating IP) in systems with continous gas bleed in the loop.

    Note: The below examples are using a round orifice of a given diameter. There are not actual measurements. The use of a needle valve instead is expected to give the same flow changes with depth as an orifice valve.

    Attachment 17572

    In a Sealed (Constant IP) regulator an orifice is feeded with a constant supply pressure. Below a certain depth depending on the IP the Flow Velocity slows down and becomes Subsonic. At the depth where the IP equals the ambient pressure the flow ceases.

    Note: The Metabolic Consumption needs are not constant duting a dive. However this type of gas injection system allows for optimum calibration at the surface so the mass is always below the metabolic consumption, hence there is no risk of going Hyperoxic. At the same time the Oxygen injection is near to the metabolic consumption at the surface and only if a diver increases the workload or diving beyond the sonic area will need to increase the frequency of manual injections to the loop.

    Attachment 17571

    Above there is an example of the expected flow in a system that uses a depth compensating regulator (A classic non-modified SCUBA regulator) which is preset at the surface and no modification is being made by the diver during the descent. It is obvious that the diver will need to manually inject Oxygen close to the surface and while going down. In this case the Flow mass will continue to increase even in the Sub-sonic velocity area. The deeper the diver goes the more difficult will be to maintain a constant PPO2 in the loop as he will need to add diluent to keep the O2 down to acceptable limits.

    This type of diving creates a lot of task loading and requires the diver attention to PPO2 on a full time basis. It is considered extremely difficult to have a stable PPO2 in the loop.

    Some rebreathers are using this method of gas injection. Furthermore the divers are instructed by the manufacturer and their instructors not to adjust the flow while descenting...

  10. #10
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    Re: An An attempt to clear the misconception on CMF

    Hi Jonny.

    The function it is not linear in the second example but appears to be one. I will try to upload an other graph where the IP is lower and the curving will be obvious.

    We are using a small diameter orifice in both cases.

    The curves are derived by an excel file where we have as input all the parameters and a widely known formula for sonic and subsonic flow is being used.

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