• Welcome to ScubaBoard


  1. Welcome to ScubaBoard, the world's largest scuba diving community. Registration is not required to read the forums, but we encourage you to join. Joining has its benefits and enables you to participate in the discussions.

    Benefits of registering include

    • Ability to post and comment on topics and discussions.
    • A Free photo gallery to share your dive photos with the world.
    • You can make this box go away

    Joining is quick and easy. Login or Register now by clicking on the button

J-valve question

Discussion in 'Vintage Equipment Diving' started by broncobowsher, Jul 14, 2019.

  1. broncobowsher

    broncobowsher Solo Diver

    1,005
    533
    113
    Having never dove one, but think I have the principle behind it, I think I know my answer.
    If you put a pressure gauge on a J-valve tank it would read ~500 PSI low while in Dive. Until the reserve is pulled and then it would read correctly.

    Do I have that correct?
     
  2. Compressor

    Compressor ScubaBoard Supporter Staff Member ScubaBoard Supporter

    # of Dives: 1,000 - 2,499
    Location: NYS
    2,855
    1,323
    113
    If you put the pressure guage on a tank with a a J valve, then until the you pull the rod, the pressure reading will be underestimated. I'm not 100% sure but in the days I dove with a J Valve, I recall that looking at the SPG when I pulled J valve, the SPG reading would go up about 300-500 psi.
     
    agilis likes this.
  3. rhwestfall

    rhwestfall Solo Diver

    # of Dives: 200 - 499
    Location: "La Grande Ile"
    12,326
    11,033
    113
    Lets say you start with a normally full tank at 3,000. You hook up your regulator and turn it on, it will read 3,000. You do your diving, and the pressure drops with consumption. At some point, lets say 500 psi, you decide to pull the reserve. Your SPG will rise by the amount of the reserve, typically in the 300-500 psi range, giving you 800-1,000 psi...

    Traditionally, folks really used to run them low, and use the reserve to surface. Thing is, the reserve needs to be monitored for position. Many a time, something would bump/snag the valve, and when pressure got really low, pulling the rod did nothing, and things got "interesting".

    Those that dove/dive them have this habit of reaching back and giving the rod a push to make sure it had the reserve engaged properly....
     
  4. KWS

    KWS ScubaBoard Supporter ScubaBoard Supporter

    # of Dives: 500 - 999
    Location: SE TEXAS
    4,303
    987
    113
    I dont think so. the J valve is basically or i should say functionally 2 valves in series one controlled by the valve handle and the other by the rod. the one by the rod is a spring controlled closeing valve. if you have 2200 in the tank your gage reads 2200 when you get to 500 you read 500 when you get to 499 the valve shuts fully and you have to over ride the closing spring to get it open again by pulling the rod.. With the gage on the regulator you will read 0 psi after you take your last breath at 500 psi. As long as the psi is above 500 your gage will read tank presure. it is the air flow rate / cu ft/min that is reduced more and more as you near 500 just as if you were breathing a tank down on a K valve and you start approaching 100 psi. Here is a link showing how the override of hte spring valve works.

     
    John C. Ratliff, AfterDark and Bowers like this.
  5. rhwestfall

    rhwestfall Solo Diver

    # of Dives: 200 - 499
    Location: "La Grande Ile"
    12,326
    11,033
    113
    actually, yeah... pulling the rod with more than 500 only gives you a very slight incremental increase....

    Good post @KWS
     
  6. tursiops

    tursiops Marine Scientist and Master Instructor ScubaBoard Supporter

    # of Dives: 2,500 - 4,999
    Location: U.S. East Coast
    7,818
    5,474
    113
    No. The SPG reads the full tank pressure through the J-valve unless the pressure drops below approx 500-800, then the J-valve shuts down and the gauge reads zero. Pull the rod, the valves opens again, and the gauge sees the tank.

    We used to take each other's pull-rods off underwater and use them as (illegal) tickle sticks, then put them back on again to surface. In retrospect, I can't believe we did that.
     
    captain and rhwestfall like this.
  7. Bob DBF

    Bob DBF Solo Diver

    # of Dives: I just don't log dives
    Location: NorCal
    6,393
    6,437
    113
    I just use the SPG port on the j-valve, when on the reg the SPG bouncing around because of the restricted flow when the reserve is engaged drives me nuts.



    Bob
     
    dead dog likes this.
  8. KWS

    KWS ScubaBoard Supporter ScubaBoard Supporter

    # of Dives: 500 - 999
    Location: SE TEXAS
    4,303
    987
    113
    Thanks BOB I was trying to avoid that configuration because i was not sure J valves Could be riggeD with a SPG ON THE VALVE ITSELF. Its been the 60's since I had a J valve. Were those ports later used for blow plugs.
     
  9. Bob DBF

    Bob DBF Solo Diver

    # of Dives: I just don't log dives
    Location: NorCal
    6,393
    6,437
    113
    The burst disc port was seperate and always there. Some have an SPG port and some don't, mine does.


    Bob
     
    John C. Ratliff likes this.
  10. herman

    herman Divemaster

    # of Dives: 500 - 999
    Location: Raleigh,North Carolina
    9,491
    2,052
    113
    There is a bit more that goes on in a real world J valve. In a perfect theoretical J valve, the pressure would read 300-500 less than tank pressure, depending on it's setpoint and close off completely at the setpoint. Pull the rod and you get the remaining pressure and full flow. In practice the J valve seat almost never seals completely but rather greatly restricts the flow of air. If you watch a SPG hooked to a J valve you will see it dip to some much lower value, rise quickly to 300-500 below the tank pressure then slowly creep up to/close to full tank pressure. How much it dips and how slowly it reaches full tank pressure depends on a lot of variables but mainly how good the J valve seat is....most are not that good. A really good seat will shut off the air and keep the SPG low at or below the setpoint but those are rare these days. How they feel when you breath depends a lot on the valve/spring condition. Most will get harder and harder to breath from as tank pressure gets close to the setpoint but won't shut off completely. That said, I have had a few with good seats that get pretty darn hard to breath from at the setpoint.
     

Share This Page