There is nothing written in TDI’s course standards that specifies the size of cylinder or volume of decompression gas a student must have to stay within limits for the agency’s Deco Procedures or Helitrox Decompression Course. There is mention that each team member should have sufficient gas to conduct all planned dives within the gas management guidelines mentioneoutlined in the appropriate course materials, but nothing more.
However, recommendations in published instructor guides are another thing altogether, and it may be worth the effort to share some of the thinking behind the almost universal suggestion that “beginning” tech divers – who are just cutting their teeth planning and executing staged decompression dives – use one 40 cubic foot (6 litre) aluminum stage for their dives.
When asked, the majority of student divers at this level will explain that the main reason the 40 cubic foot aluminum bottle is preferred has to do with its weight or its buoyancy characteristics. Actually, this really is not the case. Pound for pound or kilo for kilo, the big sister to the 40, the ubiquitous 11 litre aluminum 80, delivers a better return.
If they take a second stab and the correct answer, some student divers may explain that the smaller bottle is easier to swim with. Well, there is a slight difference in the inertia a diver has to overcome swimming with an 80 compared to a 40, but for most people, not enough to be a real issue. When one considers streamlining, with the correct rigging, an 80 offers very little additional drag, and since it can contain twice the volume and costs about the same as its little sibling to purchase, why then is the 80 not the default sized deco cylinder for newbie deco divers?
The real answer is gas volume. ALL the major tech agencies teach similar/same rules with regards decompression gas planning. Highly simplified this boils down to a small reserve put aside for a fudge factor and half of the remainder for the diver’s planned deco. The other half (plus that fudge-factor reserve) is for contingencies like the buddy’s deco gas being unavailable. This effectively means that using a 6L bottle restricts a planned decompression around a fixed maximum gas volume of about 580L or less than 20 cubic feet.
If we do the math, this available volume of gas is “Not a lot” and the logical result is a dive plan in which decompression times are kept to a maximum of 20 to 25 minutes or so. (The other assumption being that the 40 is used to carry a gas mix that the diver is going to deploy at a moderate depth… perhaps 21 metres or 70 feet.) The result is a nice learning curve. Or at least, that’s the projected result.
So in effect, a 40 cubic foot bottle essentially plays the part of a set of training wheels for tech divers. And for the record, there is absolutely NO mention of ANY TDI program requiring specific kit from a named manufacturer… with the exception of unit specific CCR classes of course! But more about the Brand L vs Brand C controversy another time.
Doppler's Tech Diving Blog 
The 40cf bottle is smaller. It is easier to carry, in and out of the water. It is easier to work around under the water, reaching for reels, pockets, etc. Perhaps as a diver gains experience, the larger 80cf offers more gas with equal hassle, but for divers who have not carried stages or deco bottles, the 30 or 40cf makes good sense. Also, for many dives the 40cf is the correct choice in terms of gas management.
skip
Steve,
I know the AL 40 is the most popular choice. Another option is the LP steel 45 as well. Options are good and you know some of the reasons I prefer a 45 over the 40. IMHO give new tech divers some choice and reasons on both sides to making that choice.
Bobby