J valve behavior?

[Slonda828]

Thanks Dale, since you are a fellow vintage diver I never mind helping!

 

[drbill]

Interesting... of course in the days I dove J-valves I didn't have SPGs.

 

[Slonda828]

I hear you Doc, I am a bit younger than you are, but I still don't use SPGs when I dive J valves. There's so many heresay stories about J valves killing divers that people are paranoid to use them in most cases. When you look at the parts diagrams it is pretty hard to imagine how one could fail unless one was too slack-jawed to set the lever in the up position.

 

[Darnold9999]

The problem was not so much that you set it wrong at the beginining of the dive as the lever got pulled as you brushed by something. Never happened to me, but I know I had the habit of checking the lever position near the end of the dive and any time I was in kelp.

 

[Slonda828]

I also check my lever, about as often as I would check an SPG if I was using one. I have never had one pulled, and I've been in some tight cracks in Ginnie and Devil's Den.

 

[duckbill]

Slonda, it is obvious that you put a lot of time and thought into your post, and that you really enjoy helping people. So, as courteously as I can, I have to say you are wrong on several points. It would be better to make corrections now than to have people going off not really understanding J-valve operation.

Dale,
I have double 72s, double 50s, a steel 72, and an early AL 80. They all do that.

Doubles should not show a dip on the SPG when the J is not activated and the cylinder pressure is well above reserve or IP.

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.

False. The air coming into the J-valve from the cylinder enters from under the seat. The spring holds the seat against the orifice, and the back side of the J seat communicates with the yoke, not the cylinder. When a regulator is attached and the air valve is open, the J-valve seat is still shut against the orifice as long as no air is being taken through the system. The only reason the J-valve opens against the spring pressure is because the pressure drops behind the seat and the pressure in the cylinder forces it's way past the J-valve seat against the reserve pressure of the spring. Taking a breath does not "suck the J-valve shut against the orifice". It sucks the pressure from behind the J-valve and causes the seat to open due to the pressure entering under it from the cylinder.

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.

This part is correct, because the air enters from under the seat, not as you have it in your diagram.

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.

Again, your diagram is backwards. The air from the cylinder enters the J-valve from under the seat. The chamber behind the seat communicates with the regulator through the yoke. The spring never stops acting against the cylinder pressure. The cylinder pressure simply overcomes the spring pressure when the pressure behind the seat is reduced when a breath is taken. The seat is shut on its orifice until a breath is taken. Taking a breath causes the seat to move away from the orifice, not "suck the j valve shut against the orifice".

Then you go "on reserve" and the seat is mechanically moved away from the orifice allowing air to flow again.

Correct.

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).

Untrue. As stated before, the air flows from the dip tube to an orifice under the j valve seat then acts to open the seat (in the off reserve or up position) against the spring pressure as long as air is being taken from behind the seat as during inhalation. Air flow from the dip tube to the orifice under the j valve seat does not "act to help close the seat". It does the opposite.

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.

True.

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.

Add one to your only exception. I had a J-valve fail on me once on my twin 38s, and I have never relied on it since. The problem with twin setups is that if the J-valve leaks even a little, the pressures between the cylinders will continually equalize during the course of your dive until you start feeling the resistance, activate the J-valve, and find you have no reserve afterall. The old USD doubles J manifolds with the steel ball seats are notorious for this.
This same problem will not happen with a single tank, since you are breathing any slight leakage at each breath and would still feel the resistance once the pressure in your cylinder came to be lower than the reserve spring pressure.

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.

You will feel the resistance before you get to 0 psi in your non-reserve tank, since your IP at 0 psi is 0 psi.

It should be noted that moisture filtering is especially important when using doubles with a J-valve. The reserve cylinder gets really cold due to the sudden rush of air out of it when the reserve is pulled, and any moisture therein is much more likely to condense at that time.

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.

Again, the spring is always pushing the seat toward the orifice. Taking a breath causes the cylinder pressure to overcome the spring pressure and open the seat, not "push it closer to the orifice".

Redraw your diagram with the air passages the other way around, and then I think you'll understand this better.

 

[DaleC]

So DB, what is causing the spg dip?

 

[duckbill]

So DB, what is causing the spg dip?

Your inhalations.

To reiterate what Mr. Ratliff said, you are seeing a function of the restriction of the J-valve itself.

If your J-valve was in the ready mode (non-deployed, not open, reserve setting), the air valve was all the way open (or not, wouldn't matter), you are using a single tank, and your SPG is somewhere between the J-valve and 1st stage seat, then you should be seeing the spg dip at each breath. If you are not, then I'd have to wonder why. In that case it is more than likely that the J-valve is actually in the open position.

Double hose, single hose, vintage, modern, all the same. The SPG is usually attached somewhere in the air passage between the J-valve and the regulator's 1st stage valve on all. The dip you are seeing is due to the restriction of the J-valve when it is closed (reserve setting). It is normal and expected. A twin setup will not exhibit this same phenomenon until reserve pressure is approached. An SPG attached to some valve HP ports will also not exhibit this behavior.

If you did not notice this happening while using the HP port on your Dacor J-valve, then it is because the port on your Dacor J-valve is connected to the cylinder pressure ahead of (upstream from) the J-valve.

If the HP port was always under pressure when the cylinder had pressure (i.e. you had to drain the tank to remove or install an SPG), then you know the airflow order is either:
cylinder>HP port>J-valve>on-off valve>yoke, or...
cylinder>HP port>on-off valve>J-valve>yoke

If the HP port was not under pressure when the regulator was disconnected (i.e. you could remove and install an SPG on a full tank), then you know the airflow order is:
cylinder>on-off valve>HP port>J-valve>yoke
(I know this last is the order on the Dacor "Low Boy" valve according to the schematic)
--------------------------

In short, Dale, you are seeing this happen because it should be happening now that you have the SPG behind (downstream from) the J-valve (in airflow order). I suspect the Dacor HP port you are used to using places the SPG ahead of (upstream from) the J-valve, so it does not see these fluctuations at each breath.

Just for s&g, try diving this same setup (SPG attached to regulator) AND have a second SPG attached to the J-valve's HP port. See how each SPG acts differently. I predict one will show steady, while the other "dances" at each inhalation. If your SPGs are calibrated the same, you will also see the difference of your J-valve's reserve spring pressure.

 

[captain]

To maybe clarify Duckbill's post which was very good consider the J valve in the reserve(up) position as a pressure relief valve that is set at 300 psi. When you are not inhaling the tank pressure and the pressure on the down stream (regulator) side of the J valve seat are equal.
As soon as inhalation starts the pressure on the regulator side drops and tank pressure forces the J open but only to the point of allowing the regulator side pressure to build to 300 psi so during inhalation the regulator first stage never sees more than 300 psi so the SPG swings just as it would if the the tank was about empty.

 

[DaleC]

Thanks all again. I actually had the spg hooked to the 1st stage as I was afraid to unscrew the valve HP port in case it was pressurized as DB suggested. I have two spgs and am bleeding the tank on Sunday (then I'll hook up the valve spg) so I'll post back in with the results using both next week when I retest it in the pool.

Here's a pic of a J valve I found in Jeppesen's Sport Diver Manual , 1975 ISBN 0-88487-013-8