OOA at very end of safety stop

[Chavodel8en]

Thank you all for your input.

From boulderjohn's calculations, and alot of your input, im wondering if one or both of 2 things also happened (besides my lax gas monitoring).

1) Inaccurate SPG. It is an old SPG. I bought it used ~14 years ago and it has served me fine for 100+ dives. No issues with accuracy that I've detected. And it seemed to be fine during the dive - I gradually used up my gas throughout the dive. And just to be fully transparent, I did a subsequent dive (maybe not a good idea) - after a good SI - and it seemed to work fine. I left myself a much larger margin for ascent on this second dive however, and did extend my safety stop.

2) I misread the SPG, either at 700 psi, or near 0 psi, or both. This is quite possible, as I feel I sometimes get a parallax view of the SPG unless I really take a good look at it. I look down at the SPG clipped to my hip. I generally use a Cressi Big Eye mask, which I like for a good downward field of view, but I lost it recently, and was using a mask without that good a downward view. That may have contributed to it. And when I saw the gauge "at 0" I didn't try to get a better look, I just immediately headed to the surface. I should have subsequently confirmed that it was really at 0, but I did not.

Also, I note in retrospect that when I broke down my gear, there was some air in the 2nd stage -- not the robust blast I usually get when I clear the reg, but some air, so I likely was not fully OOA - just too close for comfort. So to respond to a previous question, I probably could have at least partially inflated the BC from backgas if I really needed it.

I will pay very close attention to my SPGs accuracy. If necessary, I will replace it. I suspect it was my poor accuracy, rather than the SPGs.

 

[Caveeagle]

...Probably main lesson learned is more monitoring of gas.

I am sure that "more" monitoring of your gas would have helped, but think you have a procedural problem. Working with a goal of "getting back on the boat with 500psi" is a recipe for problems. (just my opinion). Checking it often.. or more often is not really a reliable way of deciding the "turn point" in your dive. When you look down and see 600-900psi, does that drive you to do anything specific? Or as long as your above 600-700psi, do you carry on "as you were". I have a hypothesis, that looking at your gauge ~20 times during a dive with an "as you were" reaction tends to develop sort of a programmed response. So all it takes is 1 or two misreads or minute of being distracted, and you are in big trouble.

I know that to most people, diving to a rule of 1/3rds seems like overkill, but I do thing its a good practice to agree on a "turn pressure". That was you and buddy or team can agree on a set point where you STOP looking at fishies, and begin an ascent or navigation back to the anchor line etc. This way, it you happen to get distracted or forget to check for a minute or two, you are way less likely to down below 500psi and be surprised by a very short breath.

 

[2airishuman]

Inaccurate SPG. It is an old SPG. I bought it used ~14 years ago and it has served me fine for 100+ dives. No issues with accuracy that I've detected. And it seemed to be fine during the dive

I buy older regulators, service them, and dive them. Frequently they come with SPGs. It is rare that I find an older SPG that I consider safe to dive. Typically the seal between the housing and the crystal fails, they flood, they corrode, and then they start to stick.

This it the most dangerous thing about analog gauges. They stick. That is, they read accurately overall, but sometimes the needle stays in one place despite the fact that the pressure has changed, and then it doesn't move until the pressure has changed enough to overcome the friction that made it stick, or it is subject to a light impact or vibration.

Based on your narrative, I would conclude that the most likely explanation is that your SPG was stuck at 700 PSI even though your cylinder pressure was somewhat lower, perhaps only 400 PSI.

The way you check for this is you put your regs on a full cylinder and then very gradually (25 PSI a second or so) reduce the pressure by feathering the purge button while watching the SPG. If it makes a gradual, continuous descent to zero, great. If it makes some 50 PSI jumps then it's probably still OK. If the jumps are much more than that, it's time for a new SPG.

You might also want to check the accuracy of your gauge at lower pressures by testing it on a cylinder that you know has 500 PSI in it, either because you've checked it with a known good gauge, or because you've weighed it completely empty and with its present contents and done the math.

Otherwise you're stuck with a skills solution to a gear problem

 

[Chavodel8en]

I buy older regulators, service them, and dive them. Frequently they come with SPGs. It is rare that I find an older SPG that I consider safe to dive. Typically the seal between the housing and the crystal fails, they flood, they corrode, and then they start to stick.

This it the most dangerous thing about analog gauges. They stick. That is, they read accurately overall, but sometimes the needle stays in one place despite the fact that the pressure has changed, and then it doesn't move until the pressure has changed enough to overcome the friction that made it stick, or it is subject to a light impact or vibration.

Based on your narrative, I would conclude that the most likely explanation is that your SPG was stuck at 700 PSI even though your cylinder pressure was somewhat lower, perhaps only 400 PSI.

The way you check for this is you put your regs on a full cylinder and then very gradually (25 PSI a second or so) reduce the pressure by feathering the purge button while watching the SPG. If it makes a gradual, continuous descent to zero, great. If it makes some 50 PSI jumps then it's probably still OK. If the jumps are much more than that, it's time for a new SPG.

You might also want to check the accuracy of your gauge at lower pressures by testing it on a cylinder that you know has 500 PSI in it, either because you've checked it with a known good gauge, or because you've weighed it completely empty and with its present contents and done the math.

Otherwise you're stuck with a skills solution to a gear problem

Thanks, this is very helpful information. I didn't really know how to find out if my SPG was off, but your description of needles sticking and the procedure you suggest for checking that makes alot of sense. Will do this. I may need to invest in a new SPG.

 

[Akimbo]

I think the bottom line is any piece of gear can fail, or worse mislead. Divers need a plan A, B, and C. It can be as simple as exhaling on your way to sunshine or as complex as multiple independent backup gas systems. Of course, packing lots of extra gear through the surf at Monastery is a risk that must also be considered.

 

[Chavodel8en]

The way you check for this is you put your regs on a full cylinder and then very gradually (25 PSI a second or so) reduce the pressure by feathering the purge button while watching the SPG. If it makes a gradual, continuous descent to zero, great. If it makes some 50 PSI jumps then it's probably still OK. If the jumps are much more than that, it's time for a new SPG.

Update. I was finally able to utilize the method 2airishuman describes above (Thanks for the info!).
Purged my tank gradually. At 1500 psi, the needle stopped, jumped around alot, but stayed at 1500 despite my continual purging.

So Im not sure if the SPG was truly the cause of my OOA situation (would the sticky needle manifest at different locations?) -- but I do know that I need to invest in a new SPG ( actually just bought one from DGX).

The SPG was part of the original equipment I bought ~ 14 years ago (I bought it used - it was in the LDS' rental fleet) and it has served me well during that time and ~120 dives. Farewell my now undependable friend -- youve been replaced!.

 

[drbill]

I dive solo 95% of the time so monitoring my air supply is even more important for me. I often dive the same profile for monitoring and filming purposes so I am aware of where I should be re: gas. Years ago I experienced issues with an older SPG that would indeed stick at the lower range. Bought a new one about 8 years ago and have tested it's accuracy a number of times finding it very accurate. Of course back when I started diving we had no SPGs and had to rely on our J-valve reserve (assuming the rod hadn't accidentally been pulled down during the dive!).

 

[Eric Sedletzky]

Something you might want to consider. Maybe you should switch to an unbalanced first and second stage. They gradually get harder and harder to breathe starting at about 500 psi and give you plenty of "heads up" that your tank is running low. Balanced first and second stages allow a diver to breathe down a tank to the very end with normal breathing and give almost no advanced notice of an OOA tank.
Or, upgrade your SPG and pay attention to it.
I wouldn't worry about it, you lived. Just remember it and learn from it. We've all had some sort of close call. You technically didn't run OOA since the reg was still breathing. How do you know the tank didn't still have a few hundred PSI in it but your SPG read zero?
I've breathed down tanks to dead empty just to test regs to see how they behave when you go OOA. You'd be surprised how long an SPG can sit on zero before you actually run out.