Several of us were diving a "circuit" where the line loops back on itself. We were in teams of two. My buddy and I were the last team.

The plan was to place a jump reel where the line looped back, so we would have a continuous line. We were to turn back, if we hit thirds before the halfway point. One team hadn't been in before, but most had been in at least a couple of times before. The guide had been in many times. My buddy and I had been in a year earlier, and had plenty of gas to make the whole circuit at that time.

Just after the halfway point, my buddy showed me his SPG, he had already used two thirds of his gas. I let him go in front, just in case. The guide waited at the jump, to pick up the reel, and my buddy indicated his problem. We passed up one team, so we has extra gas in front of, and behind us.

In a few minutes, by buddy showed me his SPG, now in the red. Since I wasn't sure how much further we had to go, I unwrapped my long hose. I went on my backup, and held my main regulator, with the hose coiled. At about 200 psi, he held out his hand, and I gave him my regulator. He calmly put it into his mouth, and stowed his own reg.

I knew we had a couple of tight spots ahead, and started thinking about how we were going to get through them. None of this made any sense, because we had done this dive before with no problem. Then I remembered that when we geared up, my crossover was closed. I reached up and tried his crossover. I turned it, and got a long hissss, as the gas equalized. He looked back, and I showed him his SPG. He tried his regulator (holding onto mine) and once he convinced himself he was ok, gave mine back.

When we got back to the surface, I asked if anyone else had a closed isolator. It turned out the two other divers had found theirs off, which made over half of the team.

This makes the third time in 30 years that I have had to give up my regulator in a cave, and (knock on wood) none were really an OOA situation.

Analysis: My buddy should have monitored his SPG closer at the beginning of the dive, and we could have exited sooner. If he breathed the long hose, instead of stowing it, his SPG would have never moved, and that should have told him his isolator was off.

Moral: Check the isolator before every dive! While I agree, they rarely ever get shut off, there is always the possibility that a "trainee" may fill it, and close it by mistake. Practice buddy breathing once in awhile, you never know when you may need it, and it is much easier when it "second nature".