captain wrote: ↑
Sat Aug 25, 2018 10:03 am
Eben, I received your PM but my reply does not leave my out box. I sent you a email to your email address.
Thanks Tom, I got the email and sent payment by paypal this morning.
Basic Scuba on J-Valves
- I finally thought to look things up in Basic Scuba
and I got some very clear information. Apparently, it is normal for a J-Valve to leak. In fact, the whole time you are diving with tank pressure over 300 psi, the air you breathe is leaking past the J-Valve because the higher pressure can overcome the J-Valve spring that holds the seat closed. It is only when the tank pressure gets down in the 300 psi range that the J-Valve spring is able to "restrict" the air flow. This restriction is supposed to be dramatically noticeable by the diver, so that he knows he is almost out of air and begins to surface. If the diver is breathing hard, he will probably notice the restriction sooner than if he is breathing easy. Pulling the J-Valve physically opens the J-Valve seat, compressing the spring and giving the diver un-restricted access to the remaining air in the tank.
- There are a number of factors affecting exactly how much air pressure is held in reserve. Here are the two main ones. First is the spring... Basic Scuba
says the springs can vary +/- 100 psi. So a 300 psi single tank reserve might give you as much as 400 psi or as little as 200 psi. Second is the condition of the seat. Dirty, corroded, or degraded by age may leak enough not to restrict the air flow at all. Or, only when you're breathing hard.
- Ambient Pressure plays a role, too. This is kind of interesting. If I understand it right, the reserve valve delivers more air when you are deeper and less air when you are shallow. This is because the force of the air restriction is actually the power of the spring plus the power of the ambient air pressure. In the example given in the Basic Scuba
book, the reserve restricted at 300 psi spring + 44 psi ambient = 344 psi. Simply brilliant. For this reason and the never-completely-sealed aspect, and the depends on how hard you're breathing aspect... I don't think it's reliable to try and measure the reserve pressure with a tank checker nor an SPG. I've tried it and it made at least one J-valve look like it wasn't working when it was. The confirmation you seek is a moving target.
- I think the bottom line in using a J-Valve reserve regularly is to physically test it out by breathing it. Breathe your tank(s) down to nothing and see if the reserve actually gives you sufficient air to finish your dive. This isn't hard to do if you have a tank balancing hose. Simply fill the J-valve test tank from a full tank. Fill it to 350 psi and lift the reserve lever to "On". Hook up a regulator and start breathing until you feel a significant restriction. Breathe hard and soft so you can see the difference it makes. Then, flip the reserve lever down and start a stop watch. Now breathe through the regulator until the tank is empty and the stop watch will tell you how much time your reserve will give you.