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A slow SPG is a bad thing?

Discussion in 'Basic Scuba Discussions' started by Nathan Doty, Oct 13, 2019.

  1. hsinhai

    hsinhai Nassau Grouper

    # of Dives: 500 - 999
    Location: Taipei, Taiwan
    Until next time it is..
  2. divad

    divad Solo Diver

    And that's applicable to everything...and nothing
    wKkaY and happy-diver like this.
  3. happy-diver

    happy-diver Skindiver Just feelin it

    # of Dives: 2,500 - 4,999
    Location: same ocean as you
    There's a tiny hole chamber stuff leading to the bourdon tube similar in size to the spool hole

    I grab a syringe and squeeze some phosphoric acid in or vinegar, flush with water and then air

    twist a soaked cotton bud in there to clean the lumps out of burnt grease and various other scum

    This is called revitalising the classic older stuff that is no longer available for posterity and to be used

    Then there's also these little buggers

    a crapped out spg is a crapped out spg but a blocked spg is far from a crapped out spg
    Bob DBF likes this.
  4. АлександрД

    АлександрД Manta Ray

    # of Dives: 100 - 199
    Location: Moscow, Russia
    it could be much more simple problem.

    good HP hoses and some SPG has tiny hole for air to prevent air loss due hose or Bourdon tube exploding.
    (in the middle)

    after some years of use it could be a little oxidized (carbonized etc), and this pinhole, initially small, could became more smaller :)

    So... once I`d offered very simple method to repair it :)
    (common disclamer for this forum and some countries : All written below is just information, and not a solicitation or guidelines for action. All written below you can do just on your own risk)
    Sam Miller III and divad like this.
  5. g1138

    g1138 Marine Scientist

    # of Dives: 500 - 999
    Location: Charleston, SC
    Would not recommend repairing an SPG just by soaking.
    This is what's behind the dial. Inside SPG.jpg

    It's a metal bourbon tube that flexes with pressure. The flexing effects the spring coil, which turns your needle.

    Now let's ask a question: Does corroded metal have more or less flexibility? Less
    Can corroded metal be restored back to original flexibility? Not really.

    Also for knowledge of SPG testing, you need to test reliability not just for max pressure, but also the range of pressures. Corrosion will effect the reliability of accurate reading in a way that makes the needle move inconsistently.
    Take for instance my worst SPG bench test against a calibrated master gauge.

    Master Gauge reading / SPG reading
    3000psi / 3000 psi
    2500psi / 2500psi
    2000psi / 1900psi <- reading low
    1500psi / 1600psi <- reading high
    1000psi / 1200psi <- high
    500psi / 800psi <-High!
    200psi / 500psi <-No...just no

    So at 500psi reading, your actual tank pressure would be 200psi. I took that SPG out of service. Moral of the story, don't trust just the max tank pressure reading. Bench test your gauges at multiple critical pressure readings.
    tridacna likes this.
  6. Zef

    Zef Divemaster

    The guy did not run out of air because his needle was stuck at 1000psi, he ran out of air because he relied on a piece of gear that was telling him something and he did not know how to interpret the message.

    If one looks at their SPG and note the reading and then note the reading a few minutes later one should note a change. It is expected that throughout the dive the amount of air carried will diminish, at least unless one is holding their breath the entire time between checks....if the needle has not moved when the spg is subsequently checked it is an indication of a problem. If the diver ignores this then they are likely to experience and OOA emergency.

    Did the SPG fail? yes, was it the cause of your partner's ooa issue? nope.

    In the US we say: "I dropped the egg and it broke", when we drop an egg on the floor. In one of the foreign countries I have lived in the expression in this situation translated from the local dialect would be: "The egg fell from my hand and broke itself". There is a stark cultural contrast of responsibility/accountability with regards to how the egg became broken. Just like your partner running out of air....it was not the SPG, despite it being buggered.

    chillyinCanada and FreeFlyFreak like this.
  7. divad

    divad Solo Diver

    Sounds like you're just blowing liquid deeper into the bourdon tube.
  8. Nathan Doty

    Nathan Doty Barracuda

    I thought of the airline being an issue as a well, it got the same cleaning as the gauge. Then I swapped gauges between my other lines. All 3 gauges sweep to the same levels and at the same approximate speed. I used 2 tanks at different pressures, both are below 800 psi, I need to fill... I'll check them full once they are filled.

    It will get permanent decommission if it acts twitchy.

    I've lost track of dangerous hobbies I've had. The correct thought process is what will I do WHEN my equipment fails, not IF it fails.
  9. g1138

    g1138 Marine Scientist

    # of Dives: 500 - 999
    Location: Charleston, SC
    Why not concentrate efforts on preventing equipment from failing? Prevent wear, routine maintenance, & gear replacement when warranted. The risk VS reward matrix for an SPG you don't fully trust swings into the negative for me.

    The industry doesn't service SPG's for a reason, we only check accuracy and call it "calibration". But it is a standard to replace when not accurate.
  10. happy-diver

    happy-diver Skindiver Just feelin it

    # of Dives: 2,500 - 4,999
    Location: same ocean as you
    Very extreme example and you do have to suck it out again, that depends
    how much liquid gets past the hole if any, and if you use a needle or a drill
    or if you soak in a vessel or a vibrating machine with only 10mm liquid in it
    soaking the threaded part only not the entire gauge although you can do it

    and if you think about it, admirably as you did

    and working with very hot water promotes evaporation


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