+ Reply to Thread
Results 1 to 4 of 4

Thread: Pictures not working

  1. #1
    RBW Member Velkarr is an unknown quantity at this point Velkarr's Avatar
    Join Date
    Nov 2008
    Location
    Germany
    Posts
    2

    Pictures not working

    Hi !

    I´m new to rebreathers and curious about that "CMF" thing and how it let a scr work. But in http://www.rebreatherworld.com/showthread.php?t=12559 there i may find the answers the pictures doesn´t work and its hard for me to get through all the english test.
    Is there another source for this Pictures ? Would be much easier for me to understand.

  2. #2
    Mark Hilley firemandiver is just really nice firemandiver is just really nice firemandiver is just really nice firemandiver is just really nice firemandiver is just really nice firemandiver is just really nice firemandiver is just really nice firemandiver is just really nice firemandiver is just really nice firemandiver is just really nice firemandiver's Avatar
    Join Date
    Sep 2007
    Location
    gulfport, ms
    Posts
    829
    Kiss Sport

    Dolphin, Ray, Homebuild

    Re: Pictures not working

    Can't help you with the pics...but CMF in SCR is pretty simple. Constant Mass Flow: A gas, Nitrox in the case of most SCR, is constantly added to the loop or counterlungs at all times. It is usually done through an orifice, either pre metered or adjustable, to deliver gas at a give rate. As long as the cylinder valve is turned on, the gas is being dumped into the system. This gas is typically make up gas to replace the O2 used by the diver.

    I don't have the link handy, but if you'll do a little digging for SCR/Dolphin threads there are several precise technical explanations as well as flow charts and diagrams.

    Hope this helps.

    FD

  3. #3
    RBW Member mgkarl is on a distinguished road mgkarl is on a distinguished road mgkarl's Avatar
    Join Date
    Jun 2009
    Location
    Massachusetts
    Posts
    57
    Explorer Sport, Inspo Classic

    Dolphin UBS40 Ray

    Re: Pictures not working

    Quote Originally Posted by Velkarr  View Original Post
    Hi !

    I´m new to rebreathers and curious about that "CMF" thing and how it let a scr work. But in http://www.rebreatherworld.com/showthread.php?t=12559 there i may find the answers the pictures doesn´t work and its hard for me to get through all the english test.
    Is there another source for this Pictures ? Would be much easier for me to understand.
    The simplest way to think about the "CMF Thing" is to remember that for a given amount of work, your body will consume the same number of oxygen molecules no matter how deep you are. So if your body needs 1,000,000 molecules of oxygen to swim for a minute against a current, it will use 1,000,000 molecules whether you are at the surface or 100 feet/ 30 meters.

    But at 30 meters the air is much more dense, and each breath takes in four times as many molecules as at the surface. An SCR flows Nitrox constantly into the breathing loop to make up for the oxygen you consume. If it flowed a constant volume of gas into the loop, say 10 liters per minute, at 30 meters it would be flowing four times as much gas as at the surface, giving you 4,000,000 oxygen molecules. You don't need the extra oxygen molecules at depth, so this flow rate is four times higher than needed, and the extra gas is dumped out of the loop through the exhaust valve. Operating this way the SCR is no more efficient than open circuit scuba.

    By using a sonic orifice as a flow controller, the gas injection becomes constant mass flow instead of constant volume flow. So if 10 liters per minute of volume corresponded to that 1,000,000 molecules of oxygen at the surface, at 30 meters because the gas is four times as dense, those 1,000,000 oxygen molecules fit into 2.5 liters. The sonic orifice makes sure the injection flow is 1,000,000 molecules per minute no matter how deep you are, and this is what is meant by constant mass flow. The mass flow remains the same, but the volume flow decreases with depth. This is why the SCR is more efficient than open circuit scuba. (The reason why the sonic orifice works this way is because sonic flow is the maximum possible flow through the orifice. Once flow becomes sonic the mass flowing through the orifice becomes constant no matter what the downstream pressure becomes.)

    You can see this work in real life. Diving an SCR with 40% nitrox the exhaust valve bubbles like crazy near the surface, but as you dive deeper the bubbling slows down a lot. In a way, the SCR is becoming more gas efficient (volume efficient) with increasing depth.

    A couple of links you could look at are:

    Untitled 1
    Untitled 1
    Untitled 1

    You could also google for "sonic orifice" or "choked flow" or "adiabatic expansion of gas through an orifice" but you may get a lot more math than you are looking for.

    Mark

  4. #4
    RBW Member Velkarr is an unknown quantity at this point Velkarr's Avatar
    Join Date
    Nov 2008
    Location
    Germany
    Posts
    2

    Re: Pictures not working

    Thank You!

+ Reply to Thread

Posting Permissions

  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts