The two big reasons for using double tanks are to provide redundancy for dives where the surface is not a good option, and to provide adequate emergency reserves for deep or penetration dives, where those reserves are required. Doubles ARE obviously more gas . . . but to use them safely, you have to learn to cope with the increased complexity that comes with having two regulators and a manifold. Otherwise, you have just increased your chances of having a gear failure leave you out of gas.
As has already been mentioned, there are a lot of ways for a new diver to decrease the initial, horrendous gas consumption. (It's an almost universal issue!)
First off, get yourself properly weighted -- and it may take a few dives to sort this out. One of the problems with new divers is that their anxiety causes them to hold a LOT of air in their lungs, thus making them think they need a ton of weight to sink. Once they are underwater for a few minutes and begin to relax, it becomes clear that they are overweighted, because they have to inflate their BCs a lot to get neutral. This large air volume is difficult to manage, because especially on beginner dives which are shallow, even small changes in depth cause large changes in buoyancy, which results in constant adding and dumping of air. That's one of the reasons a weight check is often better done at the end of the dive, when the diver is more relaxed. Dropping the weight that a weight check tells you you can MAY result in what seems like some problems descending; better technique and breath control will eventually solve that.
Once you are properly weighted, the next thing is to try to distribute your weights so that you can hold a horizontal position in the water without much effort. If, as many people do, you put all your weight on a belt, it may more or less obligate you to float feet-down. In that position, if you kick, you drive yourself upward in the water column. When you notice this, you will vent your BC until you are negative enough not to rise -- but of course, you are then exerting a lot of your kicking effort just not to sink, and every movement you make is costing you some gas. When you can float in a horizontal position, than any movement of your fins will push you forward, which is what you want.
After you have solved the basic problems of weighting and trim, you can begin to work on reducing the amount that you move. Muscle effort generates carbon dioxide, and CO2 is the primary drive for breathing, so the more you exert yourself, the more volume you have to put through your lungs. There is no getting around that! At this point, you've gotten rid of the need to kick just to stay in place, but you may still be wasting effort if your kick is ineffective. Many new divers utilize a "bicycling" motion for kicking, which involves flexing and extending all the joints of the lower extremity -- hip as well as knee and ankle. This results in a lot of movement but not much propulsion. Learning to hold your body flat from the shoulders to the knees will result in transmitting a lot more force to the fins, whether you use a long-leg flutter-type kick, or whether you bend the knees and utilize a modified flutter or frog kick. In addition, you should work to stop the use of the hands. Instability, especially on the roll axis, often results in a lot of hand usage for new divers, and the hands are very inefficient tools underwater. It takes a lot of arm-waving to accomplish what one can do with a twitch of a fin!
In this same vein is overall slowing yourself down. Water has a lot of resistance, and it goes up exponentially with the speed at which you try to travel. Going fast on scuba is a LOT of work, and work generates CO2. In addition, many underwater animals survive through effective camouflage, so moving fast means you see less. A lot of new divers swim constantly and much too rapidly; some of that is due to instability in the water, because swimming can hide a lot of issues with buoyancy and balance. If you do the weighting and trim work, you won't need to do that, and you will be free to do more hovering and watching the world around you . . . which has the happy side effect of lowering your gas consumption (and the unhappy side effect that you get cold faster
).
In the process of accomplishing all of the above, you will probably have solved the final problem, which is the ineffective breathing pattern that new divers typically use. As I said above, the biggest thing driving the amount of air you have to breathe is the need to get rid of CO2. CO2 levels in the blood are directly related to the volume of gas that passes through the small air sacs in the lungs, but those air sacs lie at the end of a long distribution system of air passages that DON'T participate in gas exchange. All the air you move in and out of your trachea, for example, does nothing for your CO2 levels. So, if you breathe in short, shallow, rapid pants, you move a LOT of air, but do little to exhaust CO2. That is why you so often see people talk about a slow, deep breathing patterns for scuba. There is a lot of truth to this, but if you make your respirations TOO deep, you will find you have a lot of buoyancy issues, especially if you are a larger person. The ideal breathing pattern is one you should have gotten a bit of an idea of in your OW class, when you did the fin pivot or neutral buoyancy exercises. There is a pattern of breathing where you inhale slowly, and just as you begin to rise, you exhale. The rhythm is such that you go up and down only a couple of inches with your breathing -- this is the most efficient breathing pattern for diving. It is typically a bit slower and deeper than breathing on land, more like the breathing you might use for yoga or meditation, but it is not huge breaths.
I hope this information is useful for you. Mastering these ideas will not only reduce your gas consumption, but it will also help make you a more efficient and more comfortable diver.
