This happened at Columbia Deep in Cozumel. Approximately 20 minutes into the dive and about 70 feet deep, I felt something touching my fin. At first I thought I had kicked someone so I turned to look, and this guy had grabbed my fin and was trying to get my attention. He then swam up to me and showed me his gauge which read zero PSI! I immediately asked if he wanted my spare by doing the share air signal. He didn't seem sure and I think he was instead thinking that his gauge was faulty. I got out my alternate and gave it to him and he started using it. He then properly positioned himself on my right side so that the hose was on his side and hung on to my tank. He was quite calm during the whole episode which was nice because my heart was really racing! The DM was about 20 feet in front of us and it was probably 1-2 minutes before he looked around and saw us. He then came back and put the OOA diver on his alternate and they went up for a safety stop. After the stop he went up to the boat and the DM caught up with us and we continued the dive.
So what happened? We had a lengthy discussion when the dive was over and it turns out the tank was not filled all the way. This of course would have been discovered had he checked his gauges before the dive. He was not a new diver and was very upset with himself that he had not checked it. The lesson here is pretty obvious - check your gauges! Another thing to mention is that he did NOT have a buddy. As a fairly new diver myself, I learned a lot from this dive. One, how I would react in an emergency, and two it reinforced the need for a buddy and to have a good pre-dive check.
So what happened? We had a lengthy discussion when the dive was over and it turns out the tank was not filled all the way. This of course would have been discovered had he checked his gauges before the dive. He was not a new diver and was very upset with himself that he had not checked it. The lesson here is pretty obvious - check your gauges! Another thing to mention is that he did NOT have a buddy. As a fairly new diver myself, I learned a lot from this dive. One, how I would react in an emergency, and two it reinforced the need for a buddy and to have a good pre-dive check.