Quiz - Equipment - J-Valve
- Thread starter Pedro Burrito
- Start date
Please register or login
Welcome to ScubaBoard, the world's largest scuba diving community. Registration is not required to read the forums, but we encourage you to join. Joining has its benefits and enables you to participate in the discussions.
Benefits of registering include
- Ability to post and comment on topics and discussions.
- A Free photo gallery to share your dive photos with the world.
- You can make this box go away
Scuba Lawyer
Contributor
No worries, but a more accurate statement is that the spring is not "triggered" when the tank pressure drops to around 300 psi. The spring is constantly trying to push the J-valve seat onto the volcano orifice to close it off and the only thing preventing that is tank pressure above 300 psi (ish). Activating the reserve by pulling the rod simply moves the seat back away from the orifice allowing air to flow freely once again. At least that has always been my understanding from having rebuilt a handfull of J-valves. If someone has a better explanation I'm all ears.
Wrong.
The Hook's law states that for a perfect spring there is linear proportionality: F=k*x, where k is the spring stiffness, in N/m.
It is called an air reserve. Same concept as in the gasoline tank of a motorbike, the valve has three positions, close, open and reserve. In open position, a couple of liters of gasoline remain trapped in the tank, and can be used only turning the lever to "reserve".
It is exactly as a pony bottle (but with less safety, as already discussed). It is air you carry with you and it is a reserve in case you finish the main tank.
Conceptually it is like having a smaller main cylinder and a pony. Unfortunately, it appears that American J-valves had a bad tendency to be opened by error, vanishing the safety of such a reserve.
Italian J-valves could not be opened by error before the time, and always carried a double regulator post, so they were close the safety of a true pony (if fitted with a double regulator, as it was standard practice here).
It must also be said that in Italy no one was diving a single cylinder. The Aralu tank I posted earlier was a twin set, and the reserve was acting on just ONE of the cylinders, and tuned to 100 bar. Below that pressure, the cylinders are separated, and you breath only from the left one. When it is empty (and the pressure reduces progressively, giving you ample time for reacting) you pull the rod, the reserve opens, and the 100 bar on the right cylinder partially travel to the left one, making a typical sound "tweeeinng"....
The right cylinder becomes cold, and the left cylinder becomes hot (you feel this if diving naked, as it was common in summer here in Italy).
If you had a SPG, you will find the pressure raising from almost 0 bar to 50 bar when opening the reserve.
I did never consider the reserve fully safe on a single cylinder, without also having an SPG.
It was fully safe only on a twin cylinder with the Technisub valve, which in facts was the standard at the time.
AfterDark
Contributor
No ya'll missed the word extra. There is no extra air, just air held back by the J valve to warn you it's time to surface; without by the a way SS, because the amount of air was insufficient.
That is absolutely correct, and when I tested the first one I rebuilt, I used a modern reg with an SPG to breathe it down on land and check where it took effect (happened to be around 400 psi). A side effect of the function you describe is that the SPG would drop significantly with each breath, with a pretty quick recovery.... almost like having a tank valve barely open will do. Feeling wise, there was no change from a normal valve until it got down to 400, where it started to get hard to pull a breath. I'm sure if I breathed it down far enough, it would have cut off completely until pulling the rod.No worries, but a more accurate statement is that the spring is not "triggered" when the tank pressure drops to around 300 psi. The spring is constantly trying to push the J-valve seat onto the volcano orifice to close it off and the only thing preventing that is tank pressure above 300 psi (ish). Activating the reserve by pulling the rod simply moves the seat back away from the orifice allowing air to flow freely once again. At least that has always been my understanding from having rebuilt a handfull of J-valves. If someone has a better explanation I'm all ears.
Respectfully,
James
CT-Rich
Contributor
Early regs were not the same as modern high performance regs. I haven’t used a double hose with a J-valve, but, to my understanding, DH performance would start to drop letting you know it was time to head up and then you would pull the rod.
The experience using a j-valve with a modern regulator will be different because you will go From fine to not fine with much less warning.
The experience using a j-valve with a modern regulator will be different because you will go From fine to not fine with much less warning.
AfterDark
Contributor
Actually with a single stage DH reg breathing gets easier as tank pressure drops. The reason is the main spring has less pressure to overcome in order to open. A 2 stage DH reg is like a modern single hose it operates the same at all tank pressures because of the 2 stages. The J-valve operates and performs the same on any regulator.
Put the latest and greatest single hose reg on a J-valve and at around 300psi breathing will become hard.
Similar threads
