Quiz - Equipment - J-Valve

[TMHeimer]

Here the reserve was set to 50 bars, in a 200 bars tank. This is 25% of total volume. Which indeed was 4000 liters, as the standatd tank, here, was a 10l+10l twin cylinder.
So 1000 liters of reservoir. And this for the one with higher SAC rate, as the rule was that as soon as one did pull the reserve, all the group has to end the dive and surface. And it was not possible to cheat, as the noise made by air going from one cylinder to the other ("tweeeiiing") was a clear indicator to everyone that it was time to ascend.

Right. I was thinking of solo diving.

 

[EFX]

I don't quite get:

a. serve as a warning device, alerting the diver when tank pressure is low.

How does it alert the diver? It doesn't do anything. Breathing just becomes more difficult as it is running out until you open up the reserve. I don't see such an inaction as an action.

Back in the 70's I dove with j-valves. I remember reaching the "reserve" pressure and my (second stage) reg would start to hum. Today, I dive with a Aqualung Titan reg (with no j-valve) and it usually does the same thing when I breathe it on the surface.

 

[TMHeimer]

I googled J valve to see if I could get something "official" as to whether use of the reserve air was planned as part of the dive or was something to get you out of an unplanned LOA /OOA situation. Found nothing. Only thing relevant I found on SB was from a 2001 thread by Rick Murchison, who described it as an "emergency........"

 

[John C. Ratliff]

But, but, but, an AI computer might warn you if you have your alarms on

...and don't dive in cold water with a 1/4 inch (oops, 7mm) hood.

SeaRat

 

[John C. Ratliff]

I don’t know why we are discussing AI computers which are not relevant to the question. While I agree that an AI dive computer can be programmed to warn you when gas pressure is low, we are taking about a valve that has a lever to open another small chamber. That’s it. It doesn’t do anything else, certainly not warn you. It is the lack of gas available that tells you to open up the reserve

There is no "small chamber." The J-valve simply hold back 300 psig in the cylinder (or 500 psig in one tank for doubles, or 750 psig for one tank with triples).

SeaRat

 

[John C. Ratliff]

I may be mistaken I think back in the late 60's scubapro had a j valve you could set for either 300 or 600 psi reserve
called the dcar valve.

That is correct, and you can change it even with a full tank. It was requested by early cave divers.

SeaRat

 

[John C. Ratliff]

Hooke’s Law. Just looked it up.

There is one other aspect not mentioned. And that is the surface area of the opening. This influences the amount of force that the spring needs to apply.

SeaRat

 

[John C. Ratliff]

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.

That is very interesting, as your reserve on the twins was 100 bar, or 1470 psig. That is much, much higher than USA J-valve on a doubles manifold, which was set at 500 psig.

SeaRat

 

[John C. Ratliff]

Yes, you are absolutely accurate with regard to a single stage DH like a USD Mistral with an upstream design - the lower the tank pressure, the easier it is to breath. However, with a downstream design such as the single stage Voit VCR-2 50 fathom, just the opposite is true. They get more difficult to breath as tank pressure drops.

As a side note, the single stage Healthways Gold Label DH (a upstream design) has a small disk with a hole in the center called a K-restrictor. It's purpose is to reduce the flow of air when tank pressure drops making it more difficult to draw air on each breath as a warning to end your dive. There is no "reserve" so to speak, you just go from a very very easy breathing reg as tank pressure drops to having to suck harder. It was designed to use on a K-valve tank (i.e. no reserve). Sorry for the history lesson, this stuff just facinates me.

I have a number of regulators with the restrictor orifice on them, the Healthways Scuba, Scuba Deluxe, and Gold Label could be bought with either a restrictor or non-restrictor opening. The Healthways Scubair, and Scuba Star could have eight too for single stage regulators. There was also a U.S. Divers Company R-valve, which had a restrictor setting.

The restrictor was used to tell you by breathing resistance when your air supply was about gone. It did this at first by taking the Venturi assist off the regulator (if it had one--this was before that was incorporated into regulators). Then it's each breath it would get harder to breath. It would, quite literally, force the diver to surface to receive easy-breathing air. That was the design, but it had a big disadvantage. If the diver had to go deeper (think caves, or wrecks), the breathing would get very difficult, or impossible. For that reason, the U.S. Navy never allowed either R-valves or the restrictor orifice for their diving.

SeaRat

 

[Couv]

I may be mistaken I think back in the late 60's scubapro had a j valve you could set for either 300 or 600 psi reserve
called the dcar valve.

The SP DCAR valve also had a small indicator pin to check the approximate gas pressure in the tank. For more info and a diagram, see this excellent post by @Luis H
Scubapro DCAR tank valve with gauge.