When we came up, we started discussing this and we both wondered what could cause this extremely low consumption. We also put on my buddy's regulator and SPG just to see if the pressure was same, and it was. I thought that maybe since a guy at my club had filled my tanks, he may have accidentally put 300 bars (4350 psi) in my tanks and that I was a bit sloppy when I read the gauge before entering the water. I could remember that it was just above the major marker and I asked myself, was it just above 200 bars or 300 bars (3000 or 4000 psi)? To me this might explain such apparently low consumption.
We completed our surface interval and kitted up again; the pressure in my tank was 180 bars (2600 psi) and that was more than enough for a second dive. We planned the second dive to a max depth of 18 msw (59 fsw). An open water diver asked to join us which was, of course, no problem. The plan was that I would stay in front as I wanted to do some photography, the new diver would stay in the middle, and my buddy who is an instructor would stay in the back. The dive was uneventful for about 25 minutes when suddenly, I felt resistance when I was breathing.
I asked myself, "What is happening now?" Almost at that same time, I was suddenly unable to take a breath. Quickly, I turned around to find my buddy who was just 1-2 meters (3-6 feet) behind me. I gave the out-of-air signal and he grabbed for his octopus. It felt like forever, but the time from when I gave the signal to when I got my first breath from his backup could not have been long. For the first few breaths, it felt like his 1st stage did not deliver enough air; but when I managed to calm down more, breathing became easier. Even though I think we both reacted calmly and did not panic, I felt stressed until I was sure I could get air from my buddy's backup.
We signaled to the third diver that we would start the ascent to the safety stop. We did that and on the way I was starting to think about my backup around my neck. I purged it and there was still air coming from that. So at the safety stop, I switched to my backup and let go my buddy's octopus. We finished the safety stop and ended the dive.
At the surface, we started talking about what happened. My buddy thought, at first, that I was just "testing" him because I started using my backup in the safety stop. But when we started to look at my gear we found out what was wrong.
The isolator valve on the manifold was closed so I breathed the tank on my right side till it was empty and the amount of air that was used on my SPG was just the air I had inflated my drysuit with from the tank on my left side.
When I thought about it later, all the signs should have been enough to understand what was happening, but we still overlooked the real issue. Thanks to our practice of staying close together when we do cold water diving where the visibility is sometimes very poor, my buddy was not far away this time to help me out. We have talked about it several times after and we think we reacted in a good way when the situation became real. It shows that if you keep calm and don't panic then it is much easier to solve the problem you are presented with. The open water diver told us when we came up, he thought it was just a drill and not a real incident. What I have learned though is to always check everything and don't assume that things are as you left it.
COMMENT:No-one expects to run out of gas but sadly, every year a number of divers panic when it happens unexpectedly even if their buddy is nearby. This diver signaled his buddy and secured an alternate air source before commencing his ascent. Later, he realized the cause of his apparent ‘out-of-gas' incident. No doubt when he opened his isolator valve a fraction then he would have heard the air moving from his left tank into his right tank until they were at equal pressure.
This is not an uncommon problem with isolator valves in manifolded doubles and a common method to prevent this problem is to perform shut-down drills before descending. Known as S-drills, this is where each diver reaches behind the head to shut down then fully re-open each valve. In this particular case it appears the divers did not check all their valves and inflating the drysuit, (plus possibly cooling of the tanks), caused the pressure in the left tank to fall slightly during the dives, further masking the problem.
When wearing a set of manifolded doubles, divers might remember this advice: if your air consumption appears too good to be true, then check your isolator valve because it probably is.
~ Peter Buzzacott, MPH, PhD