How does a J-valve work?

[jpsexton]

I just scored a couple used tanks and one of them has a J-valve on it. I'm trying to decide whether to try to find a used K-valve to replace it with when I have them hydroed or to just leave on and use it. Could someone give me a breif explanation? Should I replace it?

 

[herman]

They are fairly simple. In the down position they act just like a K valve . In the up position they will cause the resistance to breathing to go up considerably at around 500psi (assuming it's working correctly). Before SPGs, you started with the valve in the Up position and dove until breathing resistance got high, at that point you knew you were at 500psi and it was time to go. You pulled a rod than moved the valve down and returned the breathing resistance to normal. I have one on an older tank and choose not to remove it. I just dive it in the down position- unless I am messing with a new buddie. Two things to keep in mind if you keep it. In the up position, it does add some resistance to air flow, you SPG will fluctuate as you breath. I have not found it to be noticable but it will scare the dickens out of your buddy to see your SPG needle moving up and down as you breath. The biggest problem is running into fill personal who don't understand them, you CAN NOT fill them with the valve in the up position. The tank will "fill" almost instantly and appear to be full to the fill station but you will know different when you put a SPG on it. If you understand them, I don't see any need to replace them unless they have failed. If you intend to use it as designed I would highly suggest you test it. A lot of people have removed some of the internals of the valve and rendered them nonfunctional (works fine as a K valve, the reserve fearure is defeated). No doubt opinions will vary. If you decide to get rid of them, let me know I could use a couple for some vintage gear.

 

[DA Aquamaster]

The downside with using the J valve as a reserve in the good old days was that inevitably one when the tank got hard to breathe, you would reach back to pull the valve and release the reserve and would discover that it had already been unitentionally been pulled - meaning you were just plain out of gas.

In my experience, a J valve did not increase work of breathing until you got down to within 200-300 psi of the reserve pressure (usually 500 psi on single tanks and 300 psi on doubles.) And once it starts to get hard to breathe the effect is more or less linear until it stops delivering air.

 

[jpsexton]

Thanks, it sounds like if I can find a used K-valve somwhere I'd be better off just replacing it.

Anyone have a K-valve they'd like to trade for a J-valve?

Seriously though Herman, if I don't trade it and wind up buying a K-valve your welcome to it.

 

[Thalassamania]

The downside with using the J valve as a reserve in the good old days was that inevitably one when the tank got hard to breathe, you would reach back to pull the valve and release the reserve and would discover that it had already been unitentionally been pulled - meaning you were just plain out of gas.

In my experience, a J valve did not increase work of breathing until you got down to within 200-300 psi of the reserve pressure (usually 500 psi on single tanks and 300 psi on doubles.) And once it starts to get hard to breathe the effect is more or less linear until it stops delivering air.

There were different springs available to set the J-valve to either 300 PSI (for 2250psi steels and doubles) or 500 PSI (for 72 Aluminums). There was also the ScubaPro DCAR (Depth Compensated Automatic Reserve) that held back 300 or 500 PSI (depending on the spring) above ambient (as though ambient means anything in this pressure range). There were also some regulators (e.g. Dacor 300-J) that had a J-valve mechanism built into the regulator.

 

[Walter]

Thanks, it sounds like if I can find a used K-valve somwhere I'd be better off just replacing it.

Why would you think so? If you leave it down, it's the same as a K valve.

 

[jpsexton]

Why would you think so? If you leave it down, it's the same as a K valve.

I just thought if I was'nt going to use it as designed then I would'nt have to worry about whether it was in the up or down position. I really like to keep things as simple as possible. I'm not sure I'd like the guage fluctuations either. With that said though, if I don't find a used one I will just use it as is. I don't think it would be worth buying a new K-valve to replace it with.

 

[Walter]

Leave it down all the time - no guage fluctuations - no switching - simple as it gets.

 

[james croft]

I'm with Walter. I dive them all the time with no problem. I have a bunch of J valves and never had an issue with any of them.

 

[ClayJar]

Actually, I was on a class checkout with my LDS last month, and I noticed one of my buddy's rental tanks had an oddly shaped valve. I looked at it more closely and realized that it was a J-valve, but there was just a plug where the switch used to be. Apparently, you can rebuild a J-valve without the pull.