How does a J-valve work?

[Ed Palma]

Up means the reserve is off, ie it is closed. Down means the valve is open, or "on." Remember, when filling, the valve has to be open, then once filled, the lever should be put in the up position.

 

[meesier42]

OK I just got my tanks back from Hydro. The J valve says Off in the up position and On in the down position. I know you guys said just leave it down but I gotta know what Off/On means. It seems backwards to me. Apparently it dose'nt mean reserve on or off.

read ON as "ON RESERVE"

The valve works as a simple, its just a spring. With about a 500 psi preload on it. So it takes a 500psi difference for air to overcome the spring and let air flow. Which means on a single tank, your SPG will act really crazy, I can't even image what an AI computer would think.

Then when your bottle starts going below 500psi, the pressure at the regulator starts dropping and it gets hard to breath. until you go "On reserve"

On doubles it don't act as weird, since the J-valve only works on the left jug, so you breathe off the right jug until its' 500 psi below the left then both start dropping with the 500psi differential. Then when you go "ON RESERVE" you hear the sound of the jugs equalizing and you end up with about 250-300psi afterwards.

Hope that helps,

 

[Tom Winters]

I dived with J-valves when I was a kid. If you hydro your tanks, get yourself some nice new shiny Thermo valves or whatever to replace them. Then take the old valves and throw them in the garbage. Sure they're salvageable and rebuildable - so just get rid of them and do yourself a big favor.

 

[Thalassamania]

I dived with J-valves when I was a kid. If you hydro your tanks, get yourself some nice new shiny Thermo valves or whatever to replace them. Then take the old valves and throw them in the garbage. Sure they're salvageable and rebuildable - so just get rid of them and do yourself a big favor.

While I do not disagree, you're a bit short on "why."

 

[Tom Winters]

The time it takes to rebuild an old valve plus the parts' cost sure starts paying a nice chunk of a new valve. For yoke replacement valves which these old J-valves all are, I have seen the great Sherwood 5000 K-valves going for $10 to $15 when the owner upgraded to DIN. I have had 4 sitting in a box for years now that just aren't worth ebaying or listing for sale.