I attended ERAU at the Prescott AZ campus a long time ago...
I'll take a more moderate approach to the doubles thing.
1. I agree that it makes sense to just buy what you need once rather than buysomething, then buy something else.
2. From that perspective double AL 80's make a great deal of sense - especially if you forsee yourself diving in a 3mm wet suit. AL80's are slightyl positive (about 4 lbs each) when empty, so unlike most steel doubles they are comparatively bouyant. That tends to make them a bit more stable and eases the problem of swimming them up from depth in the event of a wing failure - a consideration if you are diving a wet suit as it compresses and leaves you more negative at depth and it does nto function as redundant buoyancy like a dry suit.
3. Double AL 80's use 7.25" diameter bands and the bands and manifold can be tranfsferred to many higher capcity steel tanks such as X7 series worthington tanks in the 100 - 120 cu ft range. The AL 80's can then be used as singles and configured as stage bottles so they do not become useless or a wasted purchase.
4. Assuming you have a reasonably good sac rate in the .6 range on a normal working dive, double AL80's will allow you about 25 minutes of bottom time at 150' with a 1/3rd reserve. That is enough gas to get you all tthe way through advanced nitrox and deco procedures, Cavern, Intro to Cave, etc.
5. Doubles are not rocket science. You do not need a specialized class to dive them. You have two regs but the overall configuration is not hard and once explained it makes sense and is fairly intuitive. YOU DO NEED at a minimum someone to mentor you show you the ropes, help with equipment selection and square you away on the basic configuration (stick with a basic DIR approach as most instructors will not find serious fault with it.) I encourage divers to get very familiar with and comfortable in doubles prior to taking a tech course as adding the learning curve for doubles on top of the curve for a tech class can be daunting and results in much frustration and little actual learning.
6. The opposing arguments are that doubles are somehow hard to manage and or sacred tech territory. They are not. Doubles were very popular in the 60's, 70's and 80's with recreational divers - modern versiosn just add an isolator manifold and an extra first stage. They offer more capability and flexibility but can still be used in a recreational setting. The other concern is that the 154 cu ft of gas in double AL80's is more than enough to get a new diver in trouble. If you tuirn on gas rather than time you will get bent. If you understand NDL's and have the good sense to stay within the NDL's the extra gas is not going to harm you at all. I am big on personal resonsibility and see arbitrary restrictions on who can dive doubles as being counter productive and just plain stupid.
7. So...find a local technical diver who knows his or her stuff (avoid the wannabees and the guys with big egos and or stong opinions they canot support with real world experience or anything other than "thats what the book said, so and so, etc said." If a dier really knows his or her stuff they can articulate exactly why they do something and exactly why it is a solid idea and can explain the pros and cons of a given approach or configuration. Some people will seriously object to that mentoring approach but that is exactly how DIR used to be taught - diver to diver in a mentoring relationship long before GUE was even thought of.
8. Do lots of homework, know the physics, know how the systems work, know your limits and know that you are mortal and are far from bullet proof. I did a lot of stupid $--t in airplanes and was very lucky to survive long enough to figure out that even if very talented, bold pilots never live to be old pilots. The same rules of the universe apply to diving.
9. In general, AL 80's with a modern 200 bar DIN isolator manifold, and decent 7.25" tank bands is a good first set. A one piece harness on an alumium or stainless steel backplate is a good choice. (in most cases, aluminum is -2 lbs negative, SS is -6 lbs negative).
10. What plate you use shoudl be determined by your weight requirements. You can figure that pretty closely by adding in the extra bouyancy of the second AL80 (about 4 lbs when empty) to what you currently use with a single AL 80. Then substract about 5 lbs for the manifold and bands, and the weight of the plate -2 to -6 lbs) and the weight of an extra second stage (maybe -1 lbs). For example, if you currently dive with 10 lbs of weight with a single AL 80 in a 3mm suit, add 4 lbs for the tank, subtract 12 lbs for the manifold, bands and reg, and you will be in the 1-2 lbs of weight needed range. Test it in a normal open water dive. Do the first dive (ending at no less than 1500 psi , then do the second dive and plan on making your safety stop with 500 psi. Ideally you will hold at 10' with no air in the wing with 500 psi in the tanks and be neutral. Add or remove weight to achieve neutral buoyancy in that condition.
11. In most cases a good doubles wing in the 40-50 lb range will get you where you need to go in technical diving. There are several models that work well. Avoid bungees and other gimics. I am not a big fan of dual bladder/dual inflator wings, but some instructors will insist on it if you are not diving a dry suit.
Do your homework, and use the above as a guide but most importantly find a competent local tech diver or a competent technical diving instructor to get you moving in the right direction. It is not something you can learn to do on the internet. Then do not exceed any recreational or certification limit until you complete the required technical classes.
Tobin sells great stuff and I would not be afraid to buy anything from him.
