J valve behavior?

[DaleC]

I was test driving a rig (St 72, functional J valve-with a modern 1st & 2nd stage and spg) in the pool last night and noticed something odd. when breathing, the spg would dip as it does when a K valve is just cracked open a bit. The J valve was open all the way and the reserve was set correctly.
Thoughts?
Does a modern reg set act differently on a J valve, was this a common condition with older valves or do I have a problem with the J valve itself?

Thanks in advance,
Dale.

 

[John C. Ratliff]

Dale,

This is a function of the restriction of the J-valve itself, and is the reason many divers decide not to use the J-valve. This will happen only on single tanks, as a double tank J-valve will not provide the restriction that the single tank has. The J-valve puts a 300 psig spring against the tank flow, and when it does this, will show almost that loss during each breathing cycle on modern regulators. This is actually a function of the J-valve design; I don't think you can get around it if you use the J-valve. Maybe some others with a greater understanding of the mechanical engineering involved can also comment.

SeaRat

 

[vintagediver]

I have found by accident, that is an indication that the reserve lever may be indexed incorrectly.
Check the function of the reserve on low tank pressure. You may find that the lever is in the wrong position.

The malfunction was on a Dacor J-valve.

 

[DA Aquamaster]

Back in the day, I do not recall seeing a drop in pressure on the SPG on each inhalation. The regs at the time would have been both a Mk 3 and a Mk 10, and both would still qualify as "modern" regs.

There a couple things to consider. Most regs today utilize a very small orifice leading to the SPG - and you can see this when you depressurize and immediately remove the reg from the valve - there is a fairly long slow hiss of air flow out the inlet from the air trapped in the SPG and hose bleeding back out through the small orifice. Simiarly, when you pressurize the reg, the SPG needle moves up the scale rather than just jumping to 3000 psi.

That said I have seen numerous regs over the years with manufacturing errors where the internal machining cuts into the bottom of the HP port, resulting in a much larger orifice. But that is not a serious issue as most modern SPG hoses also incorporate a restriction to limit gas loss in the event of an HP hose failure.

A J-valve does work with the supply pressure pushing against the seat and spring and it is an unbalanced design, but the pressure drop on each side of the seat in the J-valve is very small.

When inhalation stops, the second stge seat closes, the second stage stops drawing air from the first stage, the first stage seat closes and this in turn stops drawing air from the valve, so the J-valve seat also closes - but there will be air in the tank, the valve and the hP sections of the reg at all bascially the same tank pressure.

On inhalation, the second stage seat opens, draws air from the first stage causing 10-20 psi IP drop in the first stage, causing the first stage seat to open and this in turn then causes a drop in the valve that would cause the tank pressure to push the seat open.

Since the spring pressure is constant and designed to trap a 300 psi reserve in the tank, at high tank pressures, very little IP drop would be needed to open the valve, and the SPG will never register the pressure change, especially with the restrictors in the reg and hose. leading to the SPG. At lower tank pressures, more drop in pressure would be needed to open the valve, until you reach a point at 300 psi where a 300 psi drop in pressure is still not enough to open the seat in the J-valve.

That lower pressure differential may cause some drop in supply pressure to the regulatr at pressures under maybe 600-800 psi, but ore correctly it causes more lag in opening the valve. What this gets sensed as by the divers is a slightly elevated inhalation effort that gets progressively higher as the tank pressure reduces toward 300 psi.

But again the presence of the restrictors in the HP port and HP hose should prevent this moemtary drop in supply pressure from actually registering on the SPG.

My concern would be that the J-valve in question is in need of service and may have dirt, lack of lubrication, etc that may be delaying the opening of the seat.

In most J-valve I have seen, the largest restriction is not at the J-valve orifice but rather at the very tiny holes used in old dip tubes. rather than being open the full diamter of the tube, they are often pinched closed at the end of the tube with two fairly tiny holes drilled through the sides of the tube near the pinched section. That would be another area I would investigate, especially if you see the same trait with the J-valve de-activated with the lever in the reserve position.

And as noted above, it is not uncommon for the lever to be mis-indexed. Some J-valves were intended to be set with the lever on either the front or back of the valve, but not all of them were, and on most it takes some understnading of what is happening to know what the valve position should be realtive to the lever.

 

[AfterDark]

Sounds to me like you were seeing the draw of your inhalations.

 

[simonbeans]

especially if you see the same trait with the J-valve de-activated with the lever in the reserve position.

Everyone of my J-valves will show a large swing on the SPG when the reserve lever is UP. Put the lever down (researve activated) and there should be no swing. If you are using an SPG, you don't need to use the J-valve as a reserve anyway.

 

[DaleC]

Whatever the outcome thanks all for helping me to better understand my J valve.

It is a Dacor J valve with a built in HP port (though I was using the HP port on the 1st stage).

It was just serviced by a pretty knowledgeable guy but I have contacted him anyways to take another look at it.

It was exactly like I was watching the draw of my inhalations.

If by indexed incorrectly it is meant that the reserve is off when it should be on (or visa versa) that wasn't it. I felt the increased resistance and the immediate woosh of decreased resistance when I pulled the rod at about 300psi.

Unfortunately I did not watch the spg after I activated the reserve.

DA, I have that exact type of dip tube but I can't remember if it's on this, or my other, 72. I'll check it.

I'm not concerned, if this behavior is normal. I just remember seeing the same thing happen when I accidentally only cracked my K valve and dove it once. I'm currently using the spg to get a sense for how the J valve "feels". Once the bugs are out I will remove it and use my SP Mk 5/109 or a doublehose (when I get one). I'll edit in a pic in a second...

Here it is:

I may pretend to be a NAUI diver if it will help impress the girls:

 

[captain]

The SPG will fluctuate on inhalation when the J in in the reserve position. Tank pressure above 300 psi does not hold the J open constantly, it opens at every inhalation. On some J valves it it is possible to tell if the the reserve is up or down by the sound during inhalation of the air rushing by the J valve seat.

 

[Slonda828]

Dale,

I have double 72s, double 50s, a steel 72, and an early AL 80. They all do that. You have to think about how a J valve functions from an engineering standpoint. When you breath on your regulator, you are creating an area of lower pressure on the opposite side of the J valve seat (provided it is in the functional position). So when you suck air through your regulator it actually acts to suck the j valve shut against the orifice and stop air flow more (hence your SPG indicates a pressure drop. Then, the pressure of your tank (if above the spring rate of the j valve spring) will force the seat back away from the orifice as the force acting against it is higher than the force acting to close it. I will say that the better a j valve works (the MORE it seals) the more of a pressure drop will occur during inhalation with a SPG mounted. The spring is rated to stop acting against the cylinder pressure at a specified point and to slam shut against a volcano orifice and restrict airflow. Then you go "on reserve" and the seat is mechanically moved away from the orifice allowing air to flow again. It's wonderful and simple.

Keep in mind that in most, if not all J valves (all of the ones that I have rebuilt) the air flows from the dip tube to an orifice behind the j valve seat then acts to help close the seat (in the off reserve or up position). When you actuate the lever (on reserve) then the seat is mechanically moved away from the sealing surface an air is allow to flow from the dip tube to the orifice, past the j valve and through its orifice, then finally to the mating surface where your regulator sits. I hope I didn't explain this poorly but I am at work so I do not have pictures here.

By design, a properly serviced j valve is very safe and reliable provided that it is functional. I have about 50 dives on them now, and have never once had a failure. I have yet to find anyone besides Thalamassamania ( he had one physically fail) who has actually had a real failure and not a "my buddy's buddy saw one break" story or a "I was too stupid to put the reserve lever up before diving" story. Interestingly, unless you have a piece of brass physically break into two pieces, there is nothing inside most j valves to fail. There is one o-ring, typically sandwiched between two PTFE washers, a stem, a seat, a bonnet, that's about it. I just rebuild mine personally, then I verify that they cut air at the appropriate pressure about every ten dives. I feel perfectly safe with them. I wouldn't dive in overheads or deco with them, but that isn't what they were meant for anyway. I write my reserve pressure under my service pressure on the shoulder of the tank, as some of my valves actuate at 500PSI while others actuate at 300PSI.

I will say that doubles with a j valve function the same, with the exception that only the cylinder with the j valve on it holds reserve gas. So the other tank literally empties, then when you go "on reserve" the gas flows through the manifold and equalizes into the other tank. It is neat if you are using an SPG, because pressure will go to 0, then back up to 300 PSI when you pull the lever. Underwater you will hear a sound similar to what a tank makes when you fill it at a dive shop. Cool beans.

Cheers.


EDIT: here, I drew this for you.

You can see how the action of breathing air from the regulator serves to create that low pressure area near the orifice of the j valve, causing the spring to push j valve seat closer to the orifice when the lever is up. This causes pressure to drop as the j valve seat gets close to the orifice, limiting air flow slightly. Then, the spring drives the seat back into its original position, and the SPG swings back up to where it was as the restriction is removed. Remember, Bernoulli's principle states that as air enters an area of restriction speed increases and pressure decreases. This process is transparent to your lungs, as they cannot detect this minute pressure drop. Your SPG, however, can. This is a very basic picture, so don't get all PE on me, it's not like I have access to CAD stuff here. I am just trying to be as clear as possible. Please feel free to ask me any other questions that you may have.

Lastly, the reason that a K valve tank shows a dip in pressure when the valve is only slightly open is a real world example of how Bernoulli's principle works. The air is flowing through the dip tube then hitting a restriction where the valve seat of the K valve is impeding air flow (because it is not open enough). The speed of that air increase, and the pressure decreases, which is indicated on your SPG when you breathe.

 

[DaleC]

Excellant post Slonda!

Perhaps we should sticky this thread ( or at least remember it the next time someone asks this question). Great discussion on the topic.