Quiz - Equipment - J-Valve

[scubadada]

Last summer I let it go after it got hard to breath and yes there is point where at least I couldn't suck any more air until I opened the valve.

So, it looks like a J valve warns you of lower pressure with breathing resistance, and then delivers more gas after you pull the lever. The cylinder pressure below about 300 psi is not available to you until you turn the valve. I have left my answer as d, Await answer from @Pedro Burrito

Edit, so I see the answer has been released and I am wrong. I would argue with this as stated above

 

[wetb4igetinthewater]

If I remember my physics (don’t ask how long ago) springs compress linearly with force squared.

 

[AfterDark]

So, it looks like a J valve warns you of lower pressure with breathing resistance, and then delivers more gas after you pull the lever. The cylinder pressure below about 300 psi is not available to you until you turn the valve. I have left my answer as d, Await answer from @Pedro Burrito

Edit, so I see the answer has been released and I am wrong. I would argue with this as stated above

It is not extra air however.

 

[agilis]

If the rod was never pulled, would you be able to breath the cylinder to near empty, albeit with increased resistance, or would it quit delivering gas until you pulled the rod. I never tried this myself, so do not know the answer. I always pulled the rod when the resistance increased and did my ascent. If the latter is true, then it does, indeed, make more gas available.

It's a semantic question. "More" gas is not supplied. The gas that was always there becomes available by pulling the rod. If you don't pull the rod you can't get to the gas that you already paid good money for at your LDS.

 

[AfterDark]

It's a semantic question. "More" gas is not supplied. The gas that was always there becomes available by pulling the rod. If you don't pull the rod you can't get to the gas that you already paid good money for at your LDS.

And one could drown!

 

[Altamira]

This is a very interesting discussion. Although I used a J-valve from 1965-74, I never actually tried to breathe without opening the J-valve once the reg became really hard to breathe. I had always assumed that if I did not activate the valve, I was not going to get that "trapped" 300+ psi no matter how hard I sucked on the reg. Any experiences trying to breathe all the air in the tank without activating the J-valve?
Edit: Did not see @AfterDark and @agilis posts when I commented, but after reading their posts, I think my understanding of how my valve worked was correct when I was using it.

 

[agilis]

Here I am getting so picky again. You have to define what is meant by "extra air". If it's air that you don't (and shouldn't) plan on using, you COULD call that extra air. Much like a pony bottle. If you know it's in the tank, then you could say it is not extra air.
Bottom line here is most likely all of us discussing this know exactly what a J valve was for and what it did, and probably the reason it became outdated--invention of the SPG plus the valve being unknowingly opened before or during the dive.
The majority voted D, but it is not as overwhelming a result as the clearly worded questions.

The word 'extra' is the issue. I interpret extra as additional. It was always there, in your tank, bought and paid for, just waiting for you to, in effect, open a valve a bit more. My take on it is that it's not extra air, it's the same air you started the dive with.

 

[agilis]

This is a very interesting discussion. Although I used a J-valve from 1965-74, I never actually tried to breathe without opening the J-valve once the reg became really hard to breathe. I had always assumed that if I did not activate the valve, I was not going to get that "trapped" 300+ psi no matter how hard I sucked on the reg. Any experiences trying to breathe all the air in the tank without activating the J-valve?

Can't be done, not if the Jvalve is working properly.

 

[scubadada]

It is not extra air however.

No, just available air

 

[-JD-]

If I remember my physics (don’t ask how long ago) springs compress linearly with force squared.

Linear, not exponential.

The equation is: F = kx

Force = spring_constant * displacement