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What is nominal about nominal IP, most nominally?

Discussion in 'Repairing your own Gear' started by Bigbella, Jan 8, 2020.

  1. Bigbella

    Bigbella ScubaBoard Supporter ScubaBoard Supporter

    # of Dives: I'm a Fish!
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    "Nominal Inter-stage pressure."

    Gospel; or is there wiggle room? Is it described in manuals as "nominal" just from a mathematical point of view; or is it simply adjectival or both, referring to the lowest tolerable number for function, and allowing for some variance?

    Some manufactures call for a single number and testing at two extreme -- high and low -- tank pressures; while others chart an acceptable range, usually within about 10 bar, at one. Considering most analogue IP gauges, even those routinely used in shops, call for a bit of guesswork, how many of the techs and DIYers, out there, play a bit with the numbers? What would many consider a safe margin of error?

    A friend, who is just beginning to work on her own gear, is currently obsessed with two high-quality IP gauges, both of which show tiny variances among them; and almost did a spit take when I gave her a digital one, for Christmas. When I suggested that she simply guess, at that tiniest hashmark between 8.5 and 9 bar, on the older gauges, she looked at me as though I had a bout of crazy eye.

    How would you all allay her fears --- aside from the fact that I am still breathing, after futzing with the stuff, for years?
     
  2. Perryed

    Perryed Barracuda

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    No gauge is 100% accurate unless you spend an ungodly amount of money on it. There is a good cushion built into scuba regulators to protect them from lawsuits. It really just depends on how anal the user is. Whatever works for you. I would worry more about percentages than 2-3 lbs of pressure.
     
  3. Bigbella

    Bigbella ScubaBoard Supporter ScubaBoard Supporter

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    I said as much to my friend, who truly a child of the digital age, where some tolerances, at her work, for example, seem to be several miles to the right of the decimal point . . .
     
    Perryed likes this.
  4. tbone1004

    tbone1004 Technical Instructor ScubaBoard Supporter

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    they use nominal especially on piston regulators because you can only ever get them so accurate.

    Couple things that are reasons for IP not being exact
    Piston regs for the most part can only have "close enough" IP because they use shims. Those shims don't work in 1psi or less increments, so if it says 145psi and you have 146, that's fine. Most piston reg manuals have a range to allow for this.
    No regulator is perfectly balanced. At 300psi the reg will have a different IP than it will at 3000psi, and that's different than at 4000psi. You may not also have the same supply pressure on the bench, or for at home tech's on the tank in your house, so it's only going to be "close enough".
    The HP seats are going to wear in and over the course of the first several hundred cycles will change IP as they settle in for a long winters nap, so if you check it right when you rebuild, then breathe a normal 20ish cycles and check again it will be different, and then after the first dive it will be different again. Not often by a lot, not often by enough that most gauges will be able to see, but it's different.

    Why does this not matter? Most second stages are balanced and they really don't care if they're fed 135 or 140psi and they're tuned to match their first stage. Unbalanced second stages care a bit more, but again, they're tuned to their first stage and the breathing is not going to be any different if it's tuned at 135 or 145psi. Just have to make sure that they're tuned together.

    There are times to fret about precision, regulators of any variety are not those times....
     
    halocline, Kharon, Bob DBF and 2 others like this.
  5. Bigbella

    Bigbella ScubaBoard Supporter ScubaBoard Supporter

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    I believe it is just that relativity that you and I had already mentioned, which bothers her the most . . .
     
  6. tbone1004

    tbone1004 Technical Instructor ScubaBoard Supporter

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    tell her it's a dynamic system with tolerances build into the specification, same as literally every other engineer specification. This just has tolerances build in integer increments instead of fractional increments.
     
  7. couv

    couv Instructor, Scuba

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    Simply show her the manual. E.g. a quote from the SP MK20/25 manual,
    "Intermediate pressure range: 125psi to
    145psi at 3000psi and 300psi supply
    pressure."
     
    FreeFlyFreak likes this.
  8. Bigbella

    Bigbella ScubaBoard Supporter ScubaBoard Supporter

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    She was A bit more preoccupied with her regulator, which insisted upon a single number, 8.5 bar . . .
     
  9. lowviz

    lowviz Solo Diver ScubaBoard Supporter

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    If she is all wonked out with numbers, maybe suggest she religiously keep it at the upper 3/4 of the range for the first stage. I like half, maybe breathes a bit harder, but no biggie. Lots of room for creep.

    Or set the first stage hot at the upper end of the IP and her secondary to anticipate her next breath. That should keep her busy but breathing comfortably...
     
  10. herman

    herman Divemaster

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    God help you if the reg creeps a little. :)
     
    Bigbella likes this.

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