Diving with a J valve

[2airishuman]

I'm intrigued by vintage diving and wonder how the J valve was/is usually used in practice. Is it usual to continue the dive until actually hitting the reserve, or is it more common to end the dive based on bottom time and use the J valve reserve mechanism as more of a safety?

 

[tbone1004]

depends on how deep you are and how good your consumption is, and your level of risk aversion. J valves typically are activated at 500psi, and since they were used on lp72's, that represented about 14cf of gas left. For rough gas planning based on a 200psi actual reserve, and a SAC rate of 0.6cfm, and the fact that these were likely used with double hoses, so you didn't plan on air sharing, that's good to about 80ft by my book, but I'd push it to 100ft with no problem or 60-70ft in an AL80.

CF matters in gas planning for ascent, not psi, so the J-valves are perfectly fine for single tank solo gas planning with a LP72 to most depths, but it all depends on the volume of the tank, and anything less than about 12 liters I wouldn't go deeper than 60-70ft

 

[Altamira]

It also depended on what kind of dive environment you were in. Open water, benign conditions, fairly shallow dive, we would generally head to the surface when it started to get a bit hard to inhale with the old unbalanced single hose Voit Avalon regulators we used, knowing the J-Valve would give us an additional 500 psi margin. Same set up in cave diving, we planned to be out of the water well before the J-valve would need to be activated. In those conditions, if we had to use the J-valve, we had seriously screwed up, and could easily have died. Good dive planning and totally focused, conservative diving enabled us to avoid that pitfall.

 

[NAM001]

When the j valve came out most did not have a SPG. the valve was the only indication of running low on air. Some of the wealthier divers had spg's on a valve called a K valve. So much for memory lane......

 

[USdiver1]

As a vintage equipment diver who regularly uses a J valve equipped 72 for non-deco O/W diving, I commonly dive using the constant reserve deployment as my cue to surface, so I guess I breathe my tank down to the limit. It is my bottom timer for shallow water dives, when there are no possibilities of having to go into deco. Depending on the regulator I am using, I may or may not have access to a SPG, so the J valve serves a very useful purpose of preventing OOA situations; and when I rebuild my Js, I always rebuild the constant reserve mechanism as well. When filling, remember to open the constant reserve mechanism so as not to force air into your tank against the spring loaded reserve. That said the J valve does not replace an SPG, watch or bottom timer, it just provides additional safety to avoid OOA. I recommend the use of a J valve in all O/W diving, if only for the added safety of the constant reserve, it can be especially useful for photogs and spearos who may get "lost" in the moment and fail to monitor their SPG.

 

[Akimbo]

In my sphere of visibility, everyone just pushed the reserve lever down because it was unreliable -- especially diving in kelp. We had reserves on doubles in the Navy but I can't remember actually getting low enough on gas to use one.

 

[John C. Ratliff]

The J-reserve on a single tank was set to 300psig, not 500 psig; that 500 psig was for twin tank manifold's, and after equalization between the two tanks, it was 250 psig. Therefore, on twin tanks, using a regulator with a 300 psig reserve was actually a better option.

There was one exception, the Scubapro J-valve which could be set ti either 300 or 600 psig.

There was a way of reversing the J-valve lever which makes it much harder to inadvertently be tripped too.

SeaRat

 

[John C. Ratliff]

The J-valve reversal is quite easy to do on double tanks. Here below are some photos of that, showing me and my twin 42s with the J-valve reversed. One photo is from a book showing U.S. Navy divers in a submarine hatch, and one has the J-valve reversed.

To accomplish this reversal, simply assemble the center section of the manifold facing the other direction, and then move the harness bands so the harness is on the other side of the twins. For single tank J-valves, the reserve lever itself needed to be removed and placed in the reverse direction. Note, only a few single tank J-valves had this feature; Scubapro and Dacor were ones that could do this, if my memory is correct. The others (USD, Voit, etc.) could not be reversed as there were guides for the levers, and they only allowed the lever to be placed in the "out" direction.

I also have the ad for the Scubapro J-valve that can be changed from 300 to 600 psig.

When I was using J-valves, we would normally use the tanks to the reserve, then surface. This was never in an overhead situation (caves, ice or wrecks), but simply as a part of normal diving.

SeaRat

 

[captain]

The J valve can be reversed quite simply. With the valve in the up or down position simply remove the slotted retainer screw and spring, pull the J lever off and replace it pointing in the opposite direction, replace the spring and retainer. Now up becomes the reserve off position and down become the reserve on position. At the end of the dive the rod is pushed up to access the reserve rather than pulled down..
The tank must be filled with the reserve off. Reversing it as described above may result in not getting a fill if the person filling the tank is not made aware of the change. With the valve reversed it is harder to accidentally bump the reserve off.