"Hot" First Stage

[pescador775]

Kev, we started this thread speculating that the tank held sufficient pressure but valve was partially closed. However, later, Chris dropped a bomb that the tank (upon surfacing from 30 fsw) contained 150 psi! Yet, a couple minutes earlier in the dive the gauge read 900 psi and fluctuating, dropping to zero on an inhalation. Mattboy gave more attention to gauge failure but did not specify what kind of failure. Earlier, I thought that the regulator first stage could be screwed up and described a possible fault but now we hear that the tank pressure was probably low during the dive, and that the gauge reading in the water could have been a false positive. The gauge could have been "stuck" but why would it fluctuate on inhalation? There is a problem with taking a diver's observations and subjecting it to analysis. The observations could be scewed or lack certain facts. It is very difficult to tease out a result with high confidence. For example, did the gauge drop to a low level and return to 900 psi or did it just drop? Did it happen once, twice or ever as described? How long was the interval between doffing the gear and double checking the tank pressure? Why didn't he say at the outset that the tank pressure was low? If it was low why did he have the regulator checked? How does a reg come fresh out of the shop with a "high" intermediate pressure? Right now, this is messed up.

Personally, I think that this story would make a good basis for an essay at an instructor or technician course. No right answer, reviewer would judge on the basis of original thinking.

 

[halocline]

I don't know exactly what was wrong with the gauge, but I do know that it stopped reading tank pressure accurately. It stayed at 900 while tank pressure dropped way below that. After that point I'm not sure it matters too much; you can't trust it. Once the HP chamber reached lower than IP and the stage stayed open, air flow and pressure changes in the IP chamber would also occur in the HP chamber. This means that the SPG was subjected to variances in pressure with inhalation/exhalation, which would explain it reacting to breathing. It does not explain why it would go back up to 900 PSI, and I have no idea about that. Maybe since that's where it "stuck" that's where it went, who knows.

I think the important thing is that the reason the OP had trouble on the dive was because he ran out of air, not a reg malfunction. He ran out of air ostensibly because the SPG failed, although I bet after this he'll pay closer attention to his own feeling about probable consumption. (He said in the beginning of this thread that he thought he was using much less air than usual)

Your other questions, which are valid, about the analysis of the problem, are cause for discussion.

 

[h90]

It is more of an solution to a problem that doesn't really exist. The idea was to prevent someone thinking it was closed and try to open it with too much force that could possibly damage the valve. If you open your own valve open it all the way, you will know its open. If someone volunteers to check it say no thanks it is already open.

NO, it comes from old valve design. But maybe so old that it was before scuba diving.
Maybe only a problem when you have big temperature differences (like for steam) But it had a reason....

 

[beanojones]

When I finish jamming and the tank is full I place a piece of tape over the orifice. Once, a guy hooked up his reg to a tank without removing the tape. He completed the dive that way. I did not realize what had happened until time to refill the bottle. The tape was still in place with a neat, round hole in the center.

Once? That is pretty much every tank I fill. I get the feeling people think the tape is there to keep water out.

(The tape is there to mark it as full.)