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Regulator - maybe a silly question

Discussion in 'Regulators' started by Dr. Eaver, Aug 13, 2008.

  1. Charlie99

    Charlie99 Solo Diver

    # of Dives: 500 - 999
    Location: Silicon Valley, CA / New Bedford, MA / Kihei, Maui
    Two important characteristics of a regulator are the cracking pressure and how much venturi assist it has. Cracking pressure is simply how much pressure needs to be applied to the diaphram such that it opens up the poppet valve a bit and air starts to flow. In normal use you suck on the reg, this pulls in the diaphram and airflow starts. So that we don't feel like we are sucking air through a straw, this cracking pressure is set very low --- something like 0.03psi. This can also be expressed as 1/2 to 1" of water (This means it is the same pressure it takes to lift water 1"). If you have a regulator with air inside of it and the diaphram (the face of the regulator) downward, the diaphram will see 1/2" or so of pressure difference and air starts to flow. If the regulator is full of water, no pressure is applied across the diaphram and air won't start to flow. This is why some posters above recommended that you let water fill the regulator.

    Normally, an octopus is adjusted such that the cracking pressure is high.

    The second important characteristic of 2nd stages is the venturi assist. Often this is done by inserting a little vane inside the regulator such when airflow starts it hits against the vane and further opens the poppet valve and increases airflow more. Most regulators have an adjustment (often labeled + and - or something like Surface and Dive) that allows you to turn the venturi assist on and off.

    The reason that shoving your thumb over the mouthpiece often stops a freeflow is that this increases the pressure inside the mouthpiece of the regulator, increasing pressure on the inside of the diaphram, which then closes the valve.

    Charlie Allen

    p.s. A crude but simple check of cracking pressure to to face the mouthpiece up and then slowly dip the 2nd stage into water, noting how deep into the water you can go before the regulator starts delivering air.

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