What kind of valve is this?

[guruboy]

Here's a picture of the rest of the setup. Looks like the band does have a clip for the cable.

 

[nimoh]

This got me curious... With the switch in the up position, what does an SPG read? The actual tank pressure, or ~500 psi less than that?

I believe the tanks pressure is the same whether the lever is up or down.

With the lever up, if the tank pressure gets down to 500psi, breathing resistance will increase and at that time flipping the lever down returns the resistance to normal. At no point does it actually completely block a portion of the gas.

 

[g1138]

The way it's been explained to me. The J-valve switch on-"UP" activates a spring loaded one way valve that will open for any pressure greater than 500psi, or your reserve pressure. So your SPG will read the tank pressure all the way up until the reserve pressure is met, at that point your SPG will probably drop to zero until you flip the switch; because the valve is no longer letting any air through.

So theoretically you're breathing-breathing; 1000psi, 800psi, 600psi, zero. Flip the switch, 500psi.


Never tried it though.

 

[iamrushman]

this rig brings back vintage memories...i started with the j-valve and the steel 72...oh those were the days of not being politically correct but we still felt safe....(smile) ( lol)

 

[captain]

This got me curious... With the switch in the up position, what does an SPG read? The actual tank pressure, or ~500 psi less than that?

Actual tank pressure, a J valve does not suddenly shut off your air at 300 or 500 PSI, it just becomes increasingly harder to get air until you pull the lever down to acess the remaining 300 or 500 PSI. Also the SPG pressure will begin fluctuate with each inhalation as you get nearer the reserve pressure.
The J valve a simple and reliable mechanism. Usually single J valves use a 300 PSI reserve and double tank J manifolds use a 500 PSI reserve on one tank, when you pull the valve down you can hear the air from the left tank flowing to the right tank until both reach 250 PSI.

---------- Post added July 25th, 2013 at 10:39 PM ----------

Here's a picture of the rest of the setup. Looks like the band does have a clip for the cable.
View attachment 161182View attachment 161183

Yes, that is the clip, it is not a cable it is a stiff stainless steel or aluminum rod. If it was a cable you would only be able to pull down and not push up. Pushing up to be sure the valve wasn't accidently bumped into the down position, a check periodically done just as you periodically check an SPG.

---------- Post added July 25th, 2013 at 10:50 PM ----------

Back when my mom started diving she had a faulty J-valve that led to her running out of air and having to buddy breath with my grandfather.

Faulty, probably not, most likely failure to check that it hadn't been accidentally bumped down, which was the common problem just like failure to check an SPG.