J valve behavior?

[Slonda828]

Duckbill,

I drew a basic J valve on a sticky note at work from memory with a pen. I said that it was not exact. The references regarding forces were relative to that diagram that I drew. When I said "they all do that", I was referring to single tanks. He didn't really ask about doubles, so I did not see a need to overcomplicate and already complicated subject. Also, I really don't enjoy helping people, I just like Dale. It is also easier to clarify someone's work than to do your own.

You are correct about a set of doubles, but if you are too slow to verify that your j valve works, then you deserve to run out of air. Just like if you are too stupid to check and maintain your SPG, then you deserve to run out of air.

Dale, your picture looks pretty much like mine, which isn't bad considering I drew it from memory If you notice though, the reason that I said your SPG is dipping is true. The j valve is essentially in balance until you breathe, then your demand creates an area of low pressure on the western side (in your diagram) of the valve seat, causing the seat to move closer to the orifice and causing a decrease in pressure (IAW Bernoulli). When the air from your tank flows from that area of low pressure to equalize it, then the valve again achieves a balanced state (pressure is equal on both sides) and the pressure rises as the seat moves easterly (based upon your diagram) and pressure goes up.

So, after all of this, functionally, the SPG is supposed to dip on a j valve tank like yours. That was what I was really trying to say from the beginning, and that remains unchanged.

 

[Paladin]

Dale,
Two things:
1. The HP port on that J valve is positioned between the on/off valve seat and the reg. When the valve is off, the port is not pressurized. That valve was designed specifically to allow you to attach or remove an SPG at any time, whether the tank is full or not.

2. SPG HP fitting orifices are very tiny so that, in the event of a gauge failure, the air from the tank will bleed off as slowly as possible, allowing the diver more time to reach the surface. Add the restriction of the SPG orifices to the restriction of the J valve seat (when the reserve is on) and you will definitely see a noticeable dip in pressure at each inhalation on a single tank.

You can go ahead and remove the HP plug from the Dacor/Seamco J valve without fear. As I said, the plug is between the on/off valve seat and the regulator tap orifice. If the valve is off, then the HP tap is not pressurized.

 

[duckbill]

You are correct about a set of doubles, but if you are too slow to verify that your j valve works, then you deserve to run out of air. Just like if you are too stupid to check and maintain your SPG, then you deserve to run out of air.

Take heed. That J-valve had worked fine before. Are you saying one needs to check his J-valve's operation before each dive? That would require that you drain the cylinders (which were doubles for this to happen, remember?) with the J closed, then wait maybe 10 to 15 minutes (to allow it to leak if it does), put a gauge on the yoke and open the J to see what your reserve pressure really is. Even if the J checks out fine, it is no guarantee that it will work fine the next time. Again, this specific problem applies to doubles, and mostly to the older USD doubles manifold we all love so dearly.

Dale, your picture looks pretty much like mine, which isn't bad considering I drew it from memory If you notice though, the reason that I said your SPG is dipping is true.

Your diagram is nothing like the one Dale posted. It is the exact opposite! I know it is difficult to make out on the sectional diagram, but the passage from the yoke attachment goes to the chamber behind the J-valve seat. The passage from the cylinder goes UNDER the J-valve's seat through the "orifice". Your drawing shows the yoke passage going to the orifice under the seat, and the cylinder passage going to the chamber behind the seat.

The j valve is essentially in balance until you breathe, then your demand creates an area of low pressure on the western side (in your diagram) of the valve seat, causing the seat to move closer to the orifice and causing a decrease in pressure (IAW Bernoulli). When the air from your tank flows from that area of low pressure to equalize it, then the valve again achieves a balanced state (pressure is equal on both sides) and the pressure rises as the seat moves easterly (based upon your diagram) and pressure goes up.

I think you are confusing the operation of a regulating valve with that of a reserve valve. Your description is straight from an explanation of how a regulator seat functions in a piston or diaphragm reduction valve. A J-valve does not have a piston or a diaphragm and push pin.

The results of their functions are not identical. A reserve valve will let all but the set pressure past as long as demand is maintained, and the seat seals perfectly, which most do not, nor need to on a single cylinder. A regulator valve will let just enough air past to satisfy the demand of the set pressure (120-150 psi for 1st stage seat, ambient pressure for 2nd stage seat).

 

[Slonda828]

I understand the difference, apparently we are not speaking the same language. I am also done talking about this. I am not mad, but I am tired of talking semantics with you.

 

[duckbill]

I am also done talking about this. I am not mad, but I am tired of talking semantics with you.

It is physics, not semantics. But that is O.K. As long as others can leave the table having a working understanding of J-valves under their belts, then I am more than satisfied.

 

[John C. Ratliff]

Duckbill has done an excellent job of explaining the J-valve function, and how it influences the SPG reading. I have a couple of other things to say about it though. First, I have this diagram from the Sherwood catelog some years ago which shows even better the air flow through the valve.

As can be seen, the seat is directly against the flow of air from the tank.

This evening I did a couple of short experiments to see what the influence of a J-valve on the regulator has on the SPG reading. Well, it was a mixed bag. It depends upon the regulator first stage design. I seem to remember that my AMF Voit Trieste, which accepts a J-valve onto the regulator (it is a Titan first stage) did not show the fluctuation that the regulator showed when using the tank's J-valve. (By the way, this is one feature that has never been duplicated on any double hose regulator: the ability to accept a J-valve into the regulator itself.) I have my Trieste, but it is not set up right now with a J-valve, and I'd have to spend some time I don't have to put it back on (break down the Voit J-valve, and install it onto the regulator). So I went to two of my single hose regulators, the Calypso J and the Sportsways Navy Unit. The Calypso J did not cause the SPG to fluxuate during the breath, even when the J-valve was in the dive position (up). The Sportsways Navy Unit did cause this fluxuation. This is a function, as Duckbill stated, of where they placed the SPG port in relationship to the J-valve. It appears that the USD Calypso J was upstream of the J-valve for the HP port, while the Sportsways Navy Unit was downstream.

Duckbill talked about one set of twin 38s where the J-reserve malfunctioned and he was left without reserve. One of the things that can happen, especially with doubles, is that the J-valve is bumped. If it is moved even just a little bit, the air can leak like Duckbill described, and cause the diver not to have a reserve. If you look at the catalog above you can see how vulnerable the double set is to being bumped. This particular problem is one of the downfalls that caused the J-valve to go out of favor. But there are things that can be done to remedy this situation. Here are two Navy divers who have modified their twin tanks to have the J-valve on the other side, and pointed toward their shoulder.

This will keep the J-valve from being bumped to the "on" position. Here are two other Navy divers, and one was modified:

I have set mine up in this manner:

Tonight, in my experiments, I have two single tank J-valves by Scubapro, and these are ones where the spring pressure can be modified. One is set at 350 psig, and the other is set at 600 psig (this was for cave diving, in its infancy). The Scubapro valve can also be reversed to alleviate the bumping situation.

The interesting thing is that I could see the difference in the spring tension between these two valves by watching the SPG, and its fluctuation. With the 300 psig valve, it fluctuated 350 psig; with the 600 psig valve, I watched the fluctuation go 600 psig, which is very, very distracting. So keep that in mind.

Slonda, thanks for the input. Your contribution has sparked a good discussion for this public forum, where many divers come to see the history of our diving equipment.

One other thing about diving a J-valve with doubles and triple tank systems. Duckbill, I can breathe my UDS-1 down to zero tank pressure, and not feel the resistance. I have actually sat on the bottom at about 15 feet and watched the SPG go to zero, and I still had three breaths left. This is because the valve openings in the UDS-1 valve system are huge in comparison to other systems (doubles, for instance), and zero on the SPG may not mean there is actually no pressure behind it, just that the gauge reads zero. I cannot do that with my twin tank configurations and valves (Sherwood and USD double tank valves). I think it has to do with the internal openings the air must pass through in these valves. When the J-valve is tripped on my UDS-1, it equilizes between the other two cylinders. USD stated that the reserve was 1000 psig (12 cubic feet of air), but when I trip it I am almost at 500 psig on the SPG.

Finally, Fred Roberts in his book Basic Scuba described the advantages of having the reserve on the regulator rather than on the tanks in a twin tank configuration. He said that the spring on the twin tank configuration was set at 500 psig, which when equalized gave 250 psig in each tank for the reserve. But if the 300 psig J-valve is on the regulator, between the two tanks, then it gives you as a reserve of 300 psig in both tanks. You gain 50 psig with this configuration. I have gone so far as to have both at times, which gives me 500 psig in one tank (equivalent to 250 psig in both) and 300 psig in both tanks. Added up this is 550 psig (it is done sequentially though, so I don't get those readings unless I pull both at the same time).

SeaRat

 

[Slonda828]

Nevermind.

 

[captain]

The reason you didn't see a fluctuation on the regulator mounted J is most likely because the SPG is reading pressure up stream of the J reserve seat rather than down stream when the J reserve is in the valve.

 

[John C. Ratliff]

The reason you didn't see a fluctuation on the regulator mounted J is most likely because the SPG is reading pressure up stream of the J reserve seat rather than down stream when the J reserve is in the valve.

Thanks Captian,

I think that's what I said about the Calypso J:

It appears that the USD Calypso J was upstream of the J-valve for the HP port, while the Sportsways Navy Unit was downstream.

SeaRat