This may be a bit long, but bear with me.
After 28 years of diving, every time I get asked about doubles I ask back "what do you want to do?" Doubles let you do two things that a single can't- stay longer at shallower depths and go deeper. Both of these bring on DECO problems very quickly. If you are not prepared for DECO, then stay with the singles and NDL diving. If you want some extra gas, a small 20/30/40 bottles as a pony or stage work well.
Now what do you do with the extra gas - most divers will use the gas for exploration of some type be it wrecks, caves, reefs, or photos. At this point the diving has become a tool that lets you do that other thing.
For anyone who wants to go further and get into doubles. I always recomend starting with the old standard aluminum 80's. Why?
Less cost - for your first set of doubles picking up 2 80's used can be done for less then $160 (sometimes much less say $50 each), add a manifold (new 200/used 100) bands (new 65-100/used 50). Total cost 300-350. Or a little over what you might pay for a single steel tank. Also, as you travel around the world you will most likely only find aluminum 80's. So if you plan to travel, get used to using them.
Weight - I think you can figure this part out. Steels on land suck and for boats, it is not getting off that you worry about (gravity will see to that) it is getting back on. A fully riged set of 120's will be a ***** to control on a marginal day and bucking ladder.
Now go diving - a lot.
Is this what you realy want to do? Do you want to put the time and money into it? Will you get bored in 2 years? The sad fact of it is that many, if not most, of the people who post on this board will drop out of the sport in 3 years or less.
OK, if you made the jump to deep/long diving and need more gas then the 80's. Break up the set, the 80's are now called stage bottles, and get the steel tanks you need to do that thing you want to do. The manifold can be reused and maybe the bands.
Now you have a large set of steel tanks, 2 80 stages, and most likely a 20-40 foot small bottle. You now have the tanks to jump 200-250+ feet or go a long way back into a cave- if you want to.
Many will question the 80's because of trim, but for a beginner (and for all your sets) do the following and trim the tanks.
1) Take a filled set of tanks into the pool and weigh them in the water - use a 0-20 pound fishing scale. See how negative the tanks are and how they float in the water. Does the bottom of the tanks float up or drop down when you hold the manifold lightly.
2) Now dress the tanks, put on the regs, BC, back plate, canister light and anything else you may have on the rig (lift bags, john lines, knifes, etc.) make sure all the air is out of the BC and everything else and reweight. How heavy/light are they now and how do they float?
3) Now dump all your air out to about 300 PSI. What do they weigh now and how do they float. Most 80's doubles will be just about neutral but have a large amount of lift at the bottom. They will lay on the bottom on the manifold streight up and down. At this point take some small weights and place them on the tanks untill they trim out level. For 80's it should take about 3 or 4 pounds. These weights can now be mounted between the tanks with long zip ties. When they are all trimed out, how much are they negitive/positive? 80's shoould be about 4 pounds negative.
You now will know how your rig changes with air consumption. You will also know how much weight needs to be added/subtracted from your weight belt to ajust for the tank swing at the end of the dive.
4) Now put on your wet/dry suit and jump in the pool and weigh your self till you are neutral.
At this point you know how much weight you need to add for your suit and how much you need to add/drop for the tanks and should be able to calculate the total required to be neutral with 300 PSI in the tanks with no air in the BC.
In the above, if the tanks are 4 pounds negative and you need 25 pounds to sink your wet suit, the total weight needed to be neutral and trimed out at the end of the dive should be 21 pounds.
Now go diving, - A lot. Play with your rig, adjust the BC/back plate and compensation weights till you get what works for you There is no substitute for experiance.
Finaly have fun.
Pete Johnson
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