We have been talking about the difference between tech computers and rec computers, but maybe there is more properly a middle ground--the "tech lite computer." What I mean is what is described above--a 3 gas computer that does not do helium. There are quite a few on the market, and they seem to be growing in popularity.
What is the purpose of such a computer? When would you use 3 different gas mixes on a dive?
The only good reason I can think of for 3 gases is for significant planned decompression, with the diver switching to higher oxygen concentrations during a prolonged ascent. For example, the diver could do an extended bottom time between 100-150 feet breathing anything from air to a light nitrox mix. During that time the diver would rack up the need for considerable deco with multiple stops. At the 70 foot stop, the diver might switch to EANx 50. At the 20 foot stop, the diver might switch to pure oxygen. That is how a 3 gas, non-helium computer might be used.
So what does this require? First of all, it will take a lot of bottom gas to accumulate that much deco time, so the diver will should be diving doubles or a high capacity single tank with a valve allowing for redundancy in case of a valve failure.
Next, the diver will need an advanced nitrox certification in order to get those higher fills, and the diver will need to have decompression procedures training to know how to plan the ascent profile. Advanced nitrox courses are very different from recreational nitrox courses. In recreational nitrox, the diver will normally be using a single tank, so there is no difference in the dive itself. That is why many agencies do not even require a dive for certification. With advanced nitrox, the only reason to be using such high oxygen mixes is to accelerate decompression, meaning the diver will have multiple tanks. Switching tanks safely is more complicated than it seems. Beginning tech divers might make a switch at 70 feet and be down at 100 feet and dropping or up at 30 feet and rising by the time they are done. Neither is a good thing during decompression. Either could be fatal. You also have to be trained in what to do when you owe lots of decompression time and something goes wrong that makes your planned decompression gas unavailable to you. In Advanced Nitrox courses, then, the in-water training is crucial.
In other words, this "tech-lite" is really still technical diving, and it requires proper training to do it safely. A recreational diver doing it without proper training may be fine, but it is a significant risk.
Now let's return to the bottom time on our sample dive. What I described as the bottom time usually falls under the title "deep air" or "extended range." It is becoming less and less popular as divers feel the need for helium at those depths. The agency for which I teach, TDI, has an extended range course, but it is rarely taught these days. A lot of tech divers will only do deep air or extended range when helium is not available, and some will not do it under any circumstances. Too dangerous.
Consequently, I think it will be a small subset of divers who will be interested in using a 3 gas, non-helium computer as it is intended. How many divers have bought such computers intending to use them for that purpose? I would suspect not many.
What is the purpose of such a computer? When would you use 3 different gas mixes on a dive?
The only good reason I can think of for 3 gases is for significant planned decompression, with the diver switching to higher oxygen concentrations during a prolonged ascent. For example, the diver could do an extended bottom time between 100-150 feet breathing anything from air to a light nitrox mix. During that time the diver would rack up the need for considerable deco with multiple stops. At the 70 foot stop, the diver might switch to EANx 50. At the 20 foot stop, the diver might switch to pure oxygen. That is how a 3 gas, non-helium computer might be used.
So what does this require? First of all, it will take a lot of bottom gas to accumulate that much deco time, so the diver will should be diving doubles or a high capacity single tank with a valve allowing for redundancy in case of a valve failure.
Next, the diver will need an advanced nitrox certification in order to get those higher fills, and the diver will need to have decompression procedures training to know how to plan the ascent profile. Advanced nitrox courses are very different from recreational nitrox courses. In recreational nitrox, the diver will normally be using a single tank, so there is no difference in the dive itself. That is why many agencies do not even require a dive for certification. With advanced nitrox, the only reason to be using such high oxygen mixes is to accelerate decompression, meaning the diver will have multiple tanks. Switching tanks safely is more complicated than it seems. Beginning tech divers might make a switch at 70 feet and be down at 100 feet and dropping or up at 30 feet and rising by the time they are done. Neither is a good thing during decompression. Either could be fatal. You also have to be trained in what to do when you owe lots of decompression time and something goes wrong that makes your planned decompression gas unavailable to you. In Advanced Nitrox courses, then, the in-water training is crucial.
In other words, this "tech-lite" is really still technical diving, and it requires proper training to do it safely. A recreational diver doing it without proper training may be fine, but it is a significant risk.
Now let's return to the bottom time on our sample dive. What I described as the bottom time usually falls under the title "deep air" or "extended range." It is becoming less and less popular as divers feel the need for helium at those depths. The agency for which I teach, TDI, has an extended range course, but it is rarely taught these days. A lot of tech divers will only do deep air or extended range when helium is not available, and some will not do it under any circumstances. Too dangerous.
Consequently, I think it will be a small subset of divers who will be interested in using a 3 gas, non-helium computer as it is intended. How many divers have bought such computers intending to use them for that purpose? I would suspect not many.
