I've consulted 4 texts, all of which are authorative, about what you say about the J tank holding back air from the very beginning at full pressure, and they all say that the valve is open until it hits 500 psi. This includes the "big actors" such as the U.S. Navy Diving Manual, Diving for Fun[/B] by Joe Strykowski, Currier's Dive, the complete book of skin diving,[/B], and they all say the same thing. And, that was what I was trained throughout all my diving instruction.
They all were wrong, and Duckbill is right. I conducted a small experiment with my twin 45s, breathing off them and letting the reg free flow until I used about 200 psi. I then tripped the J valve, and heard the equilizing. This was at 1800 psi to 1600 psi (approximately--no gauge), a full tank set.
Duckbill, thanks for that explanation. You got it, and the rest of the diving world is simply wrong.
Now, I'm curious. Does this mean that, for that first 500 psi, the new gauges like my Suunto Cobra dive computer, which calculates air consumption, will be off until we get to 1300 psi (for my 1800 psi tanks) and both cylinders start feeding? I don't think anyone has addressed this question in the scuba literature that I've seen.
Good question about the computer. I'm curious too. I don't use one, but that would be a good brainstormer.
The texts are somewhat correct when talking about single cylinders. The J-valve is open in the sense that it will allow air to push past the spring pressure at each breath until the spring completely overcomes the set reserve pressure and keeps the seat closed. I think that is the best way to explain it and I would read it that way. Doubles just throw another piece of the puzzle into the mix.
I have a pressure gauge permanently mounted on my twin 50s (my avatar). The port is at the J-valve side and the gauge is fed air from the reserve cylinder. On a J-valve ported gauge (assuming the port gets air from the cylinder side of the J-valve), here is what you would see during the dive. The pressure reading drops at first, but only because of the temperature drop when you hit the water. Then is stays the same until the reserve starts to bleed over (starting pressure minus 700 psi reserve pressure on mine). From then on, the reading drops at the rate of useage. Breathing resistance is felt when the gauge shows the reserve pressure (again, 700 psi on mine). Then the reading drops to nearly half (400 psi) when you trip the J. The readings from a J-valve ported gauge as on my 50s can be disconcerting at first, because if you are not aware of when the breathing resistance will begin, you wonder what the heck is happening. "Why am I having trouble breathing when I have 700 psi?!" Then you trip the lever and you have less than you thought (400 psi)!
That is different than the more common configuration where the gauge reads the pressure at the yoke (for reference). There, you see the pressure drop throughout the dive, while there is 500 psi more (700 in my 50s) in the other cylinder. (Maybe that's the real reason double hose divers swam with their left shoulder down- more pressure/weight on that side! I'm kind of kidding, but 700 psi in my twin 50s equates to 2.8 pounds more on that side than the other!!!). The yoke-mounted gauge drops to near zero and then rises when the J is tripped.
What is really fun, though, is to use those twin 50s with the reserve tank gauge AND a gauge on a banjo. Then you can watch what is happening in both tanks simultaneously! Fun stuff!
But, I'm afraid none of this is helping treasureman fix his problem. Treasureman, I hope that once you isolate the problem that you can then get it fixed up. Please let everyone know what you find out. Someone may then know how to help.
Last Edit: Nov 26, 2006 2:12:36 GMT -8 by duckbill
Silly humans, fins are for fish. Mammals use flippers.
Treasureman, do check out your J valve, and give us a report. That's the most important thing now.
Duckbill, I cannot spend a lot of time on the computer question, but will probably about Wednesday. I have noted that my projected dive time with my twin 45s is a bit off early in the dive, and I think the computer is only seeing the one cylinder's drop in pressure over time for that period. It then levels out, but I think more information is needed to determine what happens to a computerized projection of dive time based upon air consumption when using doubles with a J valve.