Quiz - Equipment - J-Valve

[wetb4igetinthewater]

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.

Now my understanding is that when the pressure of the cylinder reaches a low pressure, it then engages the spring, so it does take an action (restricting air flow) when a condition occurs (low pressure). Pulling the lever disengages the spring and you can resume breathing as effortlessly as previously.

 

[AfterDark]

I am a little young and inexperienced and never dove a j-valve, although learned about it enough to know how it works. I understand why it was used in 50-60-70’s but why would anybody use J-valve nowadays? Is this for situations when the visibility so low you can not read your depth gauge or a computer? Or is this a historical question?

Personally, I decided to try a double hose regulator for picture and video taking. It puts the exhaust bubbles behind the divers heads which make it less startling to the critters because, IMO they don't feel the pressure wave ahead of the bubbles and probably don't see them either.

Last season I put a J valve I have on one of my tanks to use with the double hose. The double hose doesn't have HP or LP ports so it's back to the past. It's more of a back up in case I get too distracted. This also requires I orally inflate my wing, back to the past again.

This year I bought a banjo fitting that will allow me to get back to the 1980's using a SPG. It goes between the DBL hose reg and valve and allows a tap for HP air and SPG use. Will still need to blowup my wing. Unless I want to tote a pony bottle.

 

[AfterDark]

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.

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.

 

[AfterDark]

Now my understanding is that when the pressure of the cylinder reaches a low pressure, it then engages the spring, so it does take an action (restricting air flow) when a condition occurs (low pressure). Pulling the lever disengages the spring and you can resume breathing as effortlessly as previously.

The spring has more strength than 300 psi can overcome so it closes until it is opened by the diver.

 

[justadyver]

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.

 

[AfterDark]

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.

300 or 800, I have one in the junk box can't get parts for the built in gage. I used the 800psi setting for ice diving..let that sink in.

 

[TMHeimer]

I guess my point was not clear. In my artificial example, yes, (d) is correct. But (a) is more correct than any of the others. It is a way to get people to think rather than overthink....or underthink.

Agree with you completely on that. On these type of questions you certainly do think. I've always been a "memorizer" dating back to my days as a student. I'll get the 100% on the test because I was great at memorizing junk-- not because I was smart or a great thinker. An 11th grade teacher said to us "no memorizing dates & stuff here--I'm gunna get you all to think". I thought "Oh Sh!t".
Guess we'll just have to end it in disagreement. I don't like questions that have correct and "more correct" answers. That's just me.

 

[TMHeimer]

A is the only correct answer. The J valve does not give any 'extra' air and certainly does not allow 'monitoring' of the air supply. I dived with J valves for a couple of decades. I still have one, a Dacor. What a J valve does is create a spring blockage to the air supply at pressures below five or six hundred PSI. As you got close to that level of pressure you could feel some difficulty in drawing a breath. You then pulled the lever downward with the attached actuating rod, removing the spring blockage and enabling you to use the remaining volume which was always there anyway. No 'extra' involved. No monitoring either. You had no idea of how much air you were using or still had until the spring blockage kicked in.

The J valve simply let you know when you were down to the last roughly 20% of the cylinder's volume. The biggest problem with these J valves was an accidental lowering of the lever during the dive, thereby removing the warning pressure block, so users learned to check the actuating rod regularly by reaching behind their back and pushing upward on the rod. I sometimes still, after all these decades, automatically reach behind me at the start of a dive to check that the non-existent rod is in the up position. I fail to understand how anyone familiar with these devices could come up with any answer other than 'A'.

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.

 

[Pedro Burrito]

Spoiler: Answer

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

Thank you for participating in these quizzes.

 

[tursiops]

Actually it could the opposite. While in abstract math 2+2 makes exactly 4, in the real world it makes approximately 4. The chance that the result is bounded between 4-epsilon and 4+epsilon diminishes as epsilon goes towards zero.
On the other side, the chances that 2+2 makes something larger than 3, or smaller than 5, are order of magnitude larger.
Hence B and C are much more true (or, better, probable) than A.

Glad I'm not taking a class ftom you!