J-Valves are Supposed to Leak?

[surflung]

J-Valves Are SUPPOSED To Leak?
- I got re-interested in using the J-Valve reserves on my Twin 38s and Twin 42s after using the Twin 42 reserve successfully at Fortune Pond. But, the J-Valve on the yellow USD Twin 38s leaked so badly that I could breathe them down to empty without pulling the J-Valve.
- I decided to fix the problem but first wanted to test them. So, with the lever up, I opened the main valve to let the air out of the tanks... Hopefully to see if the J-Valve would hold back the 500 psi reserve until I pulled the lever down. I wasn't surprised that the USD Twin 38s didn't hold it. But the military surplus 38s also leaked... Though not as freely as the USD 38s. And even the Twin 42s, in which the reserve worked fine for me,... Also leaked.
- Well, repairing the USD 38s J-Valve was impossible because the threads were galled on the plug that retains the 500 psi reserve spring (That might also be why it wasn't working properly). I had a working valve and swapped it in... And that system worked fine for me at Wazee last weekend... Though I hadn't tested it for leaks like before.
- Then as I was re-reading the section on "Constant Reserve J Type" in Basic Scuba, I found this statement: "The air is never actually shut off short, but rather breathing resistance is caused to climb rapidly, jolting the preoccupied diver back into reality. The seat of this particular mechanism is thus made so that it can leak a little."
- So, my empty-the-tanks leak test doesn't mean the reserves on my surplus 38s and twin 42s are bad! I know the reserve on the 42 works. I just need to test the reserve on the surplus 38s in a diving environment. And, the repaired reserve on my USD Twin 38s was tested by diving it and worked last weekend.
- I don't know if I'm just dense or what... But it has taken me several times reading about this and experimenting with it to begin to understand it. Right now I'm wondering if it's best to leave the lever up (reserve "ON") all of the time to take the compression off the reserve spring and give the least leakage at the seat?

[SeaRat]

Eban,

I don't think Fred Roberts was completely correct when he made that statement (page 263-4 of Basic Scuba, second edition). My experience with the J-valve and twin tanks is that the reserve is not noticable until the tank without the reserve is empty. The cylinder pressure in the reserve tank is always 500 psig over the non-reserve tank due to the spring in the reserve. So when the non-reserve tank is at zero, the reserve tank should be at 500 psig. Tripping the lever then equalizes the two tanks and you can actually hear this air movement while underwater, and if listening so can your buddy.

Something I didn't realize for the longest time is that the actual 500 psig spring is not on the outside where the lever mechanism resides. Instead, it is on the inside of the valve, and only accessible by taking off the nut with an Allen wrench. The seat/retainer is on the inside of the doubles valve. This apparently has never been removed and cleaned on your valves. Or, I may be off and you know this already. If clean and set up correctly, it should not leak, but retain the 500 psig in the one cylinder until trippped. It may leak a tiny bit overnight, but not immediately. If it does leak readily, there is something wrong with the valve.

John

[nikeajax]

Also, if I dismember correctly, you can adjust them too, to a higher PSI: I always hear people complaining about j-valves and saying, "Well... that's a horrible idea, 300-psi doesn't give you much time at all..." Am I mistaken about the adjustment?

Jaybird

[SeaRat]

May 19, 2015 12:00:30 GMT -8 nikeajax said:

Also, if I dismember correctly, you can adjust them too, to a higher PSI: I always hear people complaining about j-valves and saying, "Well... that's a horrible idea, 300-psi doesn't give you much time at all..." Am I mistaken about the adjustment?

Jaybird

Jaybird,

Yes, you are mistaken for all valves except the Scubapro depth-compensated Reserve, which can be set for either 300 psig or (I think) 600 psig. I have several of these valves from Scubapro, and you need to turn a screw in the top of the valve to reset it, but it can be done with full pressure in the cylinder provided the regulator is not attached and pressurized. Other than that, the spring pressure is the only way to change it, by changing the spring itself. For instance, you can put a 500 psig spring for doubles into some single valves (Sherwood). 'Hope this helps.

John

[surflung]

- Yes I agree John, I'm figuring he must have meant very minor leakage if at all. The only one I've had apart is the USD J-valve I swapped in... And with the old gasket crumbling and corrosion debris I can see how the seat could get dirty and not seal very well. This one I swapped in I had completely apart and cleaned it all up real nice. It'll be a good one to compare the other two against.
- Jaybird, the book says these are not adjustable other than to be set by the strength of the spring. On the other hand, they ARE depth compensating to some degree. At 100 feet deep, the reserve would include the additional 44 psi from 3 atmospheres (3 x 14.7 psi) plus the reserve psi. My doubles reserve spring is 500 psi. Pull the lever and it shares the 500 psi with the empty tank... Making 250 psi in each. The 44 psi from 100 ft ambient air depth compensation brings it to 294 psi in each tank... If I understand it correctly.
- If you use the reserve to end your dive and come up, I think that few hundred psi will last quite a long time at shallower depths. I've breathed down 300 psi just picking up sticks around the dock and I swear it took 10-15 minutes before it finally ran out. Figure a safety stop at 10 feet and I bet you'd get awfully bored if you sat there long enough to use up all of the reserve air.

[SeaRat]

Just so everyone knows, here is the part we are talking about:

[nikeajax]

John, does that nut, looks like part No.1 on your diagram: does that have any adjustment to it, no-right? It's just a holder-onner: sorry to baffle everyone with that overly tech term

Jaybird

[duckbill]

What he means by "leak a little" is that the air is not cut completely off at the set reserve pressure. Starting around 300 psi, for example, the air flow while taking a breath is restricted more and more as the cylinder pressure continues to drop. This air flow past the restriction is what I think he means by a "leak", as the air is leaking past the reserve seat. BUT, this should only happen under demand, i.e. when a breath is being taken. When a breath is not being taken, the reserve seat should be sealed shut by the biasing spring, just as at any other time that there is no demand.

On a single cylinder, you can get away with a bit of leakage past the reserve seat when closed, as you will still feel the restriction once the reserve kicks in. BUT, it is very important with doubles that there be NO leakage (as in while under no demand, which is what we are now refering to), as you found out the hard way. Especially for those that sip air slowly, a reserve seat leak will allow both cylinders to remain equalized, and by the time you start to feel a restriction it is because BOTH cylinders are near empty! This leak equalization occurs while you are slowly exhaling.

300 psi is plenty for the intended design. A reserve was NEVER meant to be a means to tell you when to turn around and head back. It was always intended to alert the diver that it was time to surface directly.

The pressure at which a reserve seat activates is based on the tension of the biasing spring. The only difference between a 300 psi reserve, a 500 psi reserve, and a 700 psi reserve is the spring. Change the spring, and you change the amount of reserve. A shim washer can be added to increase the reserve pressure as well.

[SeaRat]

May 19, 2015 18:03:24 GMT -8 nikeajax said:

John, does that nut, looks like part No.1 on your diagram: does that have any adjustment to it, no-right? It's just a holder-onner: sorry to baffle everyone with that overly tech term

Jaybird

Jaybird,

No, part #1, which holds the reserve lever itself does not influence the spring tension on the other side (where the arrow in the above diagram). Similarly, the other nut is must be tightened all the way down (#13 in the above diagram). Duckbill mentioned putting a washer into the system on the spring, and that would change the reserve pressure.

By the way, I checked my Sherwood twin manifold today. The twins were sitting with around 350 psig, so I made sure that the reserve lever was up, then emptied the other tank. When no more air would flow, I shut off the valve (I have a twin post system), then opened the reserve. The other cylinder stayed at the initial pressure, then equalized when I pulled the reserve rod.

I have another USD twin manifold not currently on a set of tanks, and when the reserve is in the "up" position, I cannot blow any air through it. In the lowered position, air will flow through when I blow on the dip tube.

This is also consistent with what the U.S. Navy Diving Manual, March 1970, states about the function of this reserve.

John

[duckbill]

Those older USD reserves are notorious for leaking. The seat is a metal ball that seals in a conical seat. No rubber as in the newer, improved design with the plastic handles. I learned about this when the reserve on my twin 38s kicked in while I was swimming amongst the branches of a fallen oak tree in the river. I pulled the reserve and found I had none! Being out of air, getting snagged or tangled in the branches or fishing line on the way up could have proven disasterous. I do not trust reserves on twin tanks any more.

[surflung]

Duckbill,
- Your explanations have cleared up a lot for me. Thanks for putting it in layman's terms.
- As I understand it now, the suction of breathing from the empty tank can overcome the tension of the reserve spring, opening the reserve seat and allowing reserve air to flow without having pulled the lever. I understand (now) that this is the leakage that Fred Roberts was referring to... Suction opens a downstream seat.
- My plan has been to use a banjo SPG in combination with the reserve and this may explain my erratic results when diving. I see the SPG go down to zero psi but I'm able to keep breathing. I have felt breathing resistance at zero psi with the Sportsways Twin 42s J-Valve and the new plastic knob USD single tank J-valve... And last weekend with the swapped in repair USD Twin 38 J-valve. But as you say, I still get air even though the reserve lever has not been pulled to open the reserve... Definitely NOT a confidence builder.
- Clarification on depth compensation: I got a call from Luis last night to clarify that the depth compensation doesn't come from the J-valve but rather the demand regulator... It's the natural result of the regulator cutting off at ambient pressure... An empty tank is 44 psi higher at 100 feet than at the surface. Re-read the section in Basic Scuba this morning and that's exactly what it says... Thanks Luis.
- Finally, breathing time at 300 psi... I'm diving Twin 38s that start a dive at 2,000 psi. That's about 80 cf of air at only 2,000 psi. So, for those tanks, 300 psi is still 12 cf of air... About 10-12 minutes in shallow water.

[duckbill]

May 20, 2015 2:50:07 GMT -8 surflung said:

- Finally, breathing time at 300 psi... I'm diving Twin 38s that start a dive at 2,000 psi. That's about 80 cf of air at only 2,000 psi. So, for those tanks, 300 psi is still 12 cf of air... About 10-12 minutes in shallow water.

This is something that most don't know. We were taught to leave 500 psi in the cylinder at the end of the dive, but that was with an aluminum 80. That 500 psi in an AL80 leaves about 13 cubic feet as a safety margin. However, that 13 cf margin in different pressure cylinders will be at different pressures.
Equivalent 13 cf safety margins:
Steel 72 = 450 psi
Twin 38s (1800 psi) = 300 psi
Twin 50s (1800 psi) = 260 psi

I have gotten evil glances from tank "technicians" when bringing my twin 50s in for visuals and they find 250 psi remaining. Firstly, water will not enter the cylinders even at ANY pressure above ambient (i.e. I don't run out of air), so 250 psi isn't an issue at any rate, and, secondly, I have had to explain to them that 260 psi in the twin 50s is the same as 500 psi in an AL80.

You might want to recalculate your 300 psi reserve for remaining air in your twin 38s. Remember, that "300 psi" reserve is only held back in one cylinder. Once you pull the reserve rod the actual, equalized pressure is more on the order of 150 psi between the two cylinders, giving approximately 6.3 cf (assuming 1800 psi cylinders).

This is why twin cylinders typically have 500 or 700 psi reserves. A 500 psi reserve in 1800 psi twin 38s will give 10.5 cf reserve, and a 700 psi reserve will give 14.8 cf- which is nearer our 13 cf "safety margin" goal. That is why "reserves" should NEVER be viewed in terms of time. The goal is to end the dive with your safety margin. Immediately surfacing once pulling the "reserve" is the only proper course of action. Of course, we all fudge a little from time to time

[SeaRat]

Eban,

Your calculations would work if you were using a Scubair-J, Calypso-J, Scubair 300 or Scubapro Sonic regulators. This is because the J-valve is then placed on the center post, and reserves for both cylinders. But Duckbill's point is very important, in that we only have a reserve on one cylinder with the twin tank manifold. So a 38 cubic foot cylinder (at 1800 psig) gives us a 6.3 cubic foot of air reserve when the J-valve is pulled. If it was on the center post, it would give us 12.6 cubic feet of air, a point that Fred Roberts makes in his discussion on page 265 when he discusses the Voit centrally-mounted reserve.

I have one set of tanks with regular J-valves which are hooked together by a cross manifold. Since each tank has a 300 psig reserve, I have the equivalent of a 600 psig reserve on a twin manifold. I can use these two J-valves independently, and plan to do so on my next dive.

I also used to dive twin 50s, at 3000 psig service (before I mothballed them). This set has a 500 psig reserve on one cylinder. Doing the math:

500 psi/3000 psi = X/50 ft3

(50 ft3 x 500 psi / 3000 psi) = 8.33 ft3

So on that set, I have 8.33 cubic feet of air available on reserve.

If I put the reserve down, and put on my Calypso-J regulator, the math is:

300 psi/3000 psi = x/100 ft3

(300 psi x 100 ft3) / 3000 psi = 10 ft3

Using this configuration, I have 10 cubic feet of air when on reserve.

John

[surflung]

- Duckbill, again you're making perfect sense to me except my twin tank reserve spring is not 300 psi, it's 500 psi (isn't it). So when the lever is pulled they balance to 250 psi in each tank, not 150 psi.
- John, you're using the same calculation as me but I think your figuring of the total air in both of MY cylinders after pulling the lever is wrong. Here's where I'm coming from: The USD Twin Tank reserve spring is tensioned for 500 psi. Pull the lever and the 500 psi reserve tank balances with the empty tank so that each tank now has 250 psi. I think we agree on that.
- Ambient pressure at 100ft deep is about 44 psi on both tanks... At that depth 44 psi is the empty condition. So the 250 psi in each tank is actually 295 psi in each tank... Though the regulator treats them like 250 in each tank at the 100 ft depth. I believe this is what Fred Roberts described as the ambient pressure effect.
- If I swim up to 10 feet deep, that 295 psi is now being treated by the regulator as nearly a true 295 psi because of the reduced ambient pressure.
- My 38 cf tanks are 38 cf at 1800 psi. At ~10% over working pressure, I pump them to 2000 psi. Two 38s at 2000 psi is 41.9 cf times 2 = 83.8 cf. I had rounded it to 80 cf for simplicity above. But from this point we'll use the same calculation method as you...

X = cf at 10 ft deep

X = 295 psi times 83.8 cf divided by 2000 psi = 12.36 cf

[duckbill]

May 20, 2015 14:07:37 GMT -8 surflung said:

- Ambient pressure at 100ft deep is about 44 psi on both tanks... At that depth 44 psi is the empty condition. So the 250 psi in each tank is actually 295 psi in each tank... Though the regulator treats them like 250 in each tank at the 100 ft depth. I believe this is what Fred Roberts described as the ambient pressure effect.
- If I swim up to 10 feet deep, that 295 psi is now being treated by the regulator as nearly a true 295 psi because of the reduced ambient pressure.

You'd actually have more than that in the cylinders, as you start to feel the restriction before the set reserve pressure is reached.

Anyway, in practical application you won't be using the differences provided by ambient reduction, or what additional above reserve pressure is remaining when you pull the rod at the first feel of restriction. That 44 psi ambient difference from 100 feet to the surface amounts to less than 2 cf, and that 2 cf will more than be used up by the safety stop(s) (at minimum) you should do for having dived that deep.