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Understanding the Intermediate Pressure Gauge

Discussion in 'Regulators' started by slackercruster, Oct 11, 2009.

  1. slackercruster

    slackercruster Solo Diver

    # of Dives: 50 - 99
    Location: NE US
    My intermediate pressure gauge I just bought yields a reading of about 140 on the Sherwood Magnum and 120 on my Sherwood Brut. Should they both be in the same range of 140?

    I've noticed the Brut is a little harder to breath than the Magnum setup. Is this because the pressure is 120 vs 140 or is it all in the 2nd stage?

    Also there is no creep, they are rock steady. They just drop about 5 pounds per breath and then rebound to full pressure.

    How should I interpret these readings? Are they about normal?
    Last edited: Oct 11, 2009
  2. knotical

    knotical perpetual student Staff Member

    # of Dives: 1,000 - 2,499
    Location: Ka'u
    The Brut and Magnum are both spec'd for an IP of 135 – 150 psi. 120 is rather low. Whether it is causing the Brut to breathe harder might not become known, since after you add a few shims to increase the IP you'll then need to readjust the second stage. If you really want to know you could adjust the second stage both before and after adding shims.
  3. meesier42

    meesier42 Divemaster

    # of Dives: 500 - 999
    Location: Kennewick, WA
    IP is different for each regulator. It will be set per the manufacture recommendations and design. Nearly all IP's are in the 110-150 psig, most balanced diaphragms are on the high end of that range and piston ones lower. after the rebuild, both the second stage is tuned while connected to the first stage, so the second stage should breathe nearly as well as it can. Depending on the design of the first/second stage there are benefits to being on one end or the other of the spec range, but these are things that a good reg tech will know, its more detailed than I feel like going into right now. I would recommend getting a copy of Regulator Savvy and finding a good dealer to take their regulator repair course (this book should be their textbook they teach from)
  4. tfsails

    tfsails Divemaster

    # of Dives: 200 - 499
    Location: Richmond VA
    All Sherwood first stages except the new SR1 are unbalanced piston designs. The Brut has a fixed HP orifice. The others have a movable orifice with five Bellville springs on it. These springs are compressed at high tank pressures and gradually rebound as tank pressure decreases. This makes the stage act like a balanced piston. To me, that's a pretty ingenious design. Because of this design, Bruts will allow IP to decrease as input pressure decreases; the others keep pretty much the same IP. I don't have the books in front of me, but Knotical is right in the ballpark concerning specs for these regs. You have to add shims to the piston to increase IP. I'd get that Brut looked at if it were mine.

    Probably all the Brut second stage needs is some adjustment AFTER the IP has been adjusted. Just keep in mind that the Brut is Sherwood's entry-level reg and is not designed to be the super-sweet breather their more expensive models are.

    Hope this helps some.
  5. slackercruster

    slackercruster Solo Diver

    # of Dives: 50 - 99
    Location: NE US
    Thanks for the replies.

    One other thing. Does it matter if a reg is set up in another LP port other than the R port? Or are they about the same?
  6. cerich

    cerich ScubaBoard Supporter ScubaBoard Supporter

    # of Dives: 5,000 - ∞
    Location: Georgia
    the r port give the best possible performance, mostly to do with IP drop during the inhalation cycle, but all the LP ports will work. Inlikely you would notice a difference.
  7. halocline

    halocline Solo Diver

    As tfsails posted, these are both unbalanced piston designs, but some sherwoods use spring loaded washers to move the orifice and stabilize the IP over the full tank range. If the magnum is one of those, and the brut is not (I'm not that familiar with the specific sherwood models), and you tested these regs with a near-empty tank, then the two readings would be about right. The unbalanced piston reg without the spring washers will drop about 15-20 PSI of IP over the range of the tank.

    So, an easy way to check this is to check the IP of the brut with a full tank; if it reads 140 or so, everything's fine, if it's 120, you should add a couple of shims (or replace the spring if it's wearing out) to raise the IP.

    On my MK2, which is a very similar design, I set the IP at 145 or so with a full tank. This allows the IP to still be in spec with a near-empty tank.

    IP rarely goes down as the reg is used over time; it's a little more like blood pressure in that almost all problems are associated with it rising. The first stage's job is to tank the very high tank pressure and drop it to IP, which is a measure of the pressure it takes to close the 1st stage valve. As that valve leaks or wears, inevitably it takes more and more pressure to close it, and that results in higher IP.

    I once listened to a "tech" at the LDS tell me that IP should be low on the MK2 because "that little reg can't handle the higher pressure." That was a classic....and one of the experiences that led me to start working on my own regs.
  8. Luis H

    Luis H Instructor, Scuba

    # of Dives: I'm a Fish!
    Location: Maine
    The Brut is an unbalanced piston design.

    The Sherwood Magnum (and I believe all the other Sherwood) are balanced piston by definition and as specified by Sherwood on their website.

    The definition of a balanced first stage is one that provides a constant IP independent of tank pressure. The mechanical and pneumatic forces inside the first stage (that open and close that first stage) are balanced and do not get out of balance by the changing tank pressure.

    I have seen IP test on Sherwood balanced piston regulators with changing tank pressure and I can guaranty you that the changing tank pressure is not unbalancing the system. The IP is as constant as the any regulator I have seen.

    In a Sherwood the so called “floating volcano orifice" is actually mounted on a secondary pneumatic piston. The air pressure behind the floating orifice is what balances the system. The system as a whole is balanced.

    Don’t think as the primary piston just by itself since the piston is just part of a balanced system. It may look like the piston of a Scubapro MK-2, but as a system it works different.

    It doesn’t have to be a flow through piston to be a “balanced system”. And BTW not all flow through pistons are balanced either. I have a US Divers Deepstar II which has an unbalanced flow through piston.
  9. awap

    awap Solo Diver

    # of Dives: 1,000 - 2,499
    Location: Central TX
    That's interesting. How did they do that?
  10. Luis H

    Luis H Instructor, Scuba

    # of Dives: I'm a Fish!
    Location: Maine
    Very simple… Think of a Mk-2 with a larger piston or just larger air flow path in the piston. Then just attach your LP hoses at the end of the cap instead of near the tank connection. The LP air will flow through the piston to the LP hoses, making it a flow through unbalanced piston.

    In addition to the Deepstar II, I believe there are some vintage European regulators of similar design.

    The Deepstar II was a very unique unit. The entire piston was made out of Delrin (or some similar plastic). It did have a replaceable seat on the piston, but the seat was in a female cavity in the piston. The volcano orifice in the main body was in a male structure that formed the HP chamber.

    Because the HP chamber was surrounded by the piston, there was no easy way to put an HP port on this regulator. In the late 60’s early 70’s when this regulator came out, not having an HP port for an SPG was the kiss of death. My understanding is that people and US Divers actually lost their jobs by allowing this design to go out into production.

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