Let me see if I have this down, you teach your students to get the dive started look at your air gauge and now determine what dive profile you are going to do. what does the buddy do or team do to join in this fun, I just do not know how that works in your kind of diving.
VDGM, I'm going to give you the benefit of the double and assume this is an honest question.
Last night, three of us went diving. One had a partially full HP130, I had a full LP95, and the other had a full HP100. We planned a dive to 60 feet. I know, from prior computation, that it takes 20 cubic feet of gas to get two people up from 60 feet, using the ascent profile I use. As we sat and talked through the dive plan, I quickly figured the psi in the tank required to reserve 20 cubic feet for each of us. For the guy with the HP100, it was 700 psi. For me, and the diver with the HP130, it was 600 psi. The guy with the HP130 had 3000 psi to start with, which left him with 2400 usable. That's about 90 cubic feet of gas. At his SAC rate of .5, at 3 ATA, he's using 1.5 cubic feet of gas a minute, so he's got 60 minutes at 60 feet before he hits his gas reserve pressure. The dive we were planning was only 45 minutes long, and wouldn't all be spent at 60 feet, so he's not going to hit that reserve pressure during our dive.
I had 1900 usable, or about 67 cubic feet. My gas consumption is lower than my buddy's, and I wasn't going to hit reserve pressure by the end of the dive, either.
The third diver, with the HP100, had 80 cubic feet usable, and his gas consumption rate was unknown. He was a relatively novice diver, though, and it was likely that, of the three of us, he would gas limit. I told him to let me know when he hit 2000 psi, as that would be when he had used half of his usable gas, and at that point, we needed to be turning the dive.
All this took about a minute and a half, and we had a team gas plan, matched for dissimilar tanks, and all three of us knew how long, how deep, and who we needed to watch.
I do this at some level for every dive I do, although a lot of the time, it's more like, "I've got stupid amounts of gas" and we let it go.
