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Intermediate Pressure

Discussion in 'Regulators' started by brittlestar, Sep 13, 2019.

  1. Luis H

    Luis H Instructor, Scuba

    # of Dives: I'm a Fish!
    Location: Maine
    Well, the idea that the single stage double hose regulator predates the two stage regulator is a common error. I guess some people think that it is a logical progression and they continue to spread that misconception, but it is wrong and well documented.

    There are many pictures and descriptions of the early Cousteau double hose regulators (like the CG45) and then the ones sold in the US. The first Aqua Lung sold in the US was the Spaco around 1949 (basically the same design as the CG45). That was followed by the US Divers Aqua Lung from 1950 to 1954. All these regulators had two stages. The second stage was just a demand valve with no venturi flow assistance.

    The history of the single hose regulators is also kind of interesting and some of the dates are not as well documented.

    One of the early US made single hose regulator has mostly being forgotten. It was the Rose Aviation and it came out in 1956. It is a side breather with a smaller diaphragm (bellows style) than the Poseidon Cyklon. I don’t know how well they worked. I actually have two of them, but I have never had time to look into them.

    I have read that Emile Gagnan actually had one of the earliest designs for a single hose regulator, but Spirotechnique/ US Divers and the Cousteau group liked the looks of the double hose and therefore the first US Divers single hose was not in the market until relatively late.

    The Scott Hydro-Pak was another single hose regulator competing against the Aqua Lung. The same company that makes the Scott Air-Pak SCBA still in use today for fire fighting and other industrial contaminated air applications.

    Scott Aviation Corporation of Lancaster, New York, was founded by Earle M. Scott in 1932. The civilian version of the Hydro-Pak was around from 1953 to about 1962, also predating the Cyklon and maybe even the Porpoise (but that is another debate).

    The Scott Hydro-Pak included a full-face mask and the tank was worn with the valve on the bottom for easy access (like most SCBA). They were a bit expensive and lost market to the Aqua Lung. It would have interesting if they would have been more popular, the full face mask might have become much more popular and we all may be using cylinders with the valves on the bottom.

    Here is information on the Scott Hydro-Pak
    Double Hose Regulators - Scott
    АлександрД, couv and Pao like this.
  2. JackD342

    JackD342 Dive Shop

    # of Dives: 500 - 999
    Location: Highland Park, IL
    Just found a nice little "historical highlights" for Aqualung on their website. This might fill in a few gaps.
    Our Story
    Luis H, АлександрД and couv like this.
  3. Fibonacci

    Fibonacci Manta Ray

    # of Dives: 500 - 999
    Location: Melbourne, Australia
    Yes Ted Eldred's Porpoise CA model was in production from 1952-1954 marketed via the Breathing Appliance Company based in North Melbourne, Australia.

    The CA was an unbalanced first stage diaphragm regulator with the two body halves screwed together.
    It's heritage borrowed heavily from a 1940's model CIG Comweld ox-acetelyene welding regulator... IP was set at 40psi for the CA and CA1.

    The upgraded CA1 model was introduced in 1954 and continued until the much-improved 'Universal' model was launched in 1955 after some research in conjunction with the Royal Navy and Siebe Gorman in England.

    The Universal could produce 290L/min which was confirmed in tests with the US Silverman Respiratory Laboratory with IP at the new limit of 105psi +/- 5psi.

    The Porpoise Universal was a unique design which was never patented... even though some of Ted's original layout drawings are marked as such.
    The Universal was a high performance balanced diaphragm regulator, which had some features that were not in widespread use for about another 10 years.
    It also featured an integral reserve, which was activated via the black plastic knob at the end (the Porpoise single or twin cylinders being normally positioned inverted for easy reach).

    It has been copied, in one way or another, by most modern SCUBA regulator manufacturers ever since.

    Universal 1 Web.jpg
    Universal 2 Web.jpg
    More here:
    Porpoise CA-1 regulator
    Luis H, АлександрД and couv like this.
  4. АлександрД

    АлександрД Manta Ray

    # of Dives: 100 - 199
    Location: Moscow, Russia
    small offtopics:

    All of you forget about Benoît Rouquayrol :)
    Their (with Auguste Denayrouze) AUTONOMOUS (*) breathin apparatus was invited first. It was in 1860-1863.
    Scaphandre autonome was first realisation of Single hose 2nd stage (*) SCUBA.

    Those times had one problem: poor industry level, and access to ruggedized matherials. Due to that it was impossible to produce small tanks for high pressure.
    First Rouquayrol-Denayrouze Scaphandre autonome had surface supply air hose, but already had intermediate tank between "low pressure" supply and ambient pressure for breathing. And had Demand valve!

    (*) Late was as idea, Draggable tripple tanks assembly :)

    with maual adjusted regulator

    combine them two - and you got real Scaphandre Autonome :)

    Late was Ogushi system but without demand valve (with teeth control :) )

    And some remarks and main question answers...
    For most regulators it is true, But! not for all.
    Isolated regulators noted above, but exists very famous and popular regulator, that Increase IP (IP always accepted by default as relative pressure) with depth.
    It is Aqualung Legend regulator. (late most Apeks regulators had Overbalansing)
    This allow to compensate air density gows with depth.

    and main reasons for IP range around 135 psi.
    I think, and it looks real, that it is just a balance. Balance between: 2nd stage size, hoses hardness, hoses diameter, air density on the depth, and maximal diving depthes.
    For example:
    first Soviet SCUBA - AVM-1 (two stages, double hose, upstream demand valve) had 5-7 atm IP (73-103 psi).
    another Soviet SCUBA Ukraine-2 (or same model SCBA - ASV) (two stages, single hose, UPstream valve) has IP 4.5-5 atm (59-73 psi).
    it was enough, because this SCUBAs was designed for depth limited diving, it use upstream valves, which works very well even with pressure till 2-3 atm (30 psi).
    E.g. normal working IP for AVM-3 even 3 atm! (44 psi). It has big seat orifice, and VERY low breathing force. And AVM-1 and AVM-3 use very big membranes - like for Mistrals.
    With appearance of downstream 2nd stages was required more stronger IP to prevent long idle run of lever and decreasing of breath force for breathing in. First 2nd stages had big membrane and works well with high IP, and allow small breathing force even with high IP.

    For me - I preffer to use IP around 6-8 atm (88-117 psi) and upstream 2nd stages. It allow to get best balance between icing, hose hardness, breathing force, hoses lifecycle.
    Fibonacci, Luis H, couv and 1 other person like this.
  5. brittlestar

    brittlestar Angel Fish

    So it seems as if the common IP range was determined for most manufacturers regulators to perform at their optimal level in relation to 2nd stage diaphram size, orifice diameter, etc. It could be 20psi or 250psi dependent on regulator design. Right?

    Do I have this correct...
    The IP pressure remains constant, say 145psi above ambient pressure. As the diver takes a breath IP is compensated for whatever the ambient pressure is? So at 99' (4ata) gas is being delivered at about 60psi? Yes/No?
  6. АлександрД

    АлександрД Manta Ray

    # of Dives: 100 - 199
    Location: Moscow, Russia
    COULD be. Yes. But it also depends on most accessible hoses on the market, and balance was found near 145 psi.
    for example, Ukraine-2 (and similar ASV) had lower IP, and also use more thiner hoses:
    looks like garden hose, right? :) but it is not selfmade, it is standard hose, it keep 100 psi easy.

    yes for first part.

    No for second part:
    145 psi is IP. This IP adjusted at surface, with ambient pressure 14.7 psi approx. And 145 - it is RELATIVE pressure from ambient. (absolute (vacuum related) will be 159.7 aprox, but nobody use it for adjust)
    Delta (difference) between 0' and 99' is around 44 psi. DELTA! not absolute pressure.
    So, IP at 99' will be 145 + 44 = 189 psi. Relatively of the surface pressure!
    But relatively to ambient conditions at 99' - IP will be the same - 145 psi :)
    Is it clean? (sorry, English is not my native, also as imperial system)
    Bob DBF, JackD342 and Luis H like this.
  7. Luis H

    Luis H Instructor, Scuba

    # of Dives: I'm a Fish!
    Location: Maine
    Keep in mind that in the early days (in the 60’s) we had several second stage demand valves that use the up-stream needle valve design. That type of demand valve performs much better with lower IP. See the bottom of the post from @АлександрД.

    Even in the US some of the early single hose regulators use the needle up-stream demand valve design.

    All of those have gone away. The only up-stream demand valves I am aware of are servo/ pilot operated demand second stages (like the Poseidon).

    The typical downstream (non-pneumatically balanced) demand valve tend to operate better with the assistance of the higher IP. The Intermediate Pressure is actually trying to push the valve seat open and the spring force is what closes the demand valve. Ideally the pneumatic force and the spring force will balance against each other and just a small force from the lever will open the valve.

    Again, going back to the early days (the 40’s, 50’s, and 60’s) the first stages were either repurposed industrial oxygen regulators or they just took that basic design and packaged it into the early double hose regulators.

    Rounding 1 atmosphere from 14.7 psi to approximately 15 psi:

    The IP of 145 psi is gauge pressure (or 160 psi) absolute.

    At 33ft deep it is 160 psig

    At 66 ft deep it is 175 psig

    At 99 ft deep it is 190 psig

    To get the absolute pressure you just have to add one atmosphere to all the psig.

    Notice I use psig. That represents gauge pressure in psi. Gauge pressure by definition is always measured above one atmosphere. In other words when referring to gauge pressure, the surface pressure (1 atm) is always used as the reference.

    If you use an intermediate pressure gauge that is vented to the surrounding ambient and go diving with it (like I have done). You will read a constant 145 psi, no matter how deep you dive. This was using a Conshelf diaphragm type of first stage that is well balanced (not affected by tank pressure) and has it is just depth compensated.

    I used the setup shown in the pictures below to measure the intermediate pressure underwater, down to about 100 ft deep. The gauge attached to the Y fitting that looks cloudy was the one that I used underwater. It looks cloudy because I drilled big holes on the back to allow good water flow (to rinse after the dive). The cloudiness is from salt water corrosion.

    Since the gauge was flooded the pressure measurement was always the differential pressure above the surrounding ambient pressure. As I mentioned, the pressure was always constant for the entire dive. I have done this several times with similar regulators and the results are always the same.

    The “Y” fitting on the bottom is what I used to share one hose for the inflator and the IP gauge. I normally use this fitting to check the accuracy of IP gauges against my digital gauge. The digital gauge was verified against a calibrated precision gauge.


    The gauge on the left has been on several dives and it’s a bit cloudy when compared to the one on the right.


    Here are a couple of pictures showing the holes I drilled to help rinsing the inside of the gauge. You can see a bit of corrosion, but not bad.


    АлександрД likes this.
  8. Luis H

    Luis H Instructor, Scuba

    # of Dives: I'm a Fish!
    Location: Maine
    Your English is a heck of a lot better than my Russian. :)

    АлександрД likes this.
  9. Luis H

    Luis H Instructor, Scuba

    # of Dives: I'm a Fish!
    Location: Maine
    Someday I would like to test an Apeks or an Aqua Lung Legend underwater with my submersible IP gauge set-up. Those regulators claim to be "over-balanced" which is a miss-use of a technical term. They are actually over-depth compensated (or something like that).

    In other words, they have a dry ambient chamber with diaphragms (inside and out) with different "effective area" and a solid a solid pressure transmitting rod between them. The difference in the diaphragm "effective areas" created slight IP increase as a function of depth.

    This has been discussed a lot in the past, but I haven't had a chance to actually measure one.
    АлександрД likes this.
  10. brittlestar

    brittlestar Angel Fish

    Sorry for my confusion if this answer is stated and I am missing it. I know that IP is added to ambient pressure but I thought that the 2nd stage brings the IP down to ambient pressure. So is gas being delivered to the diver's mouth at ambient pressure or IP plus ambient?

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