Here's an excerpt from a gas management article I wrote ... there may be some tidbits in there you'll find useful ...
Some tips for using your air more efficiently
Air consumption is often related to other aspects of your diving, such as buoyancy control, weighting, trim, your breathing pattern, and swimming speed. As overall skills improve, so will your air consumption ... often dramatically.
Here are some tips that can help you improve your air consumption, and in general get more enjoyment out of your diving experience.
Breathing
For most of us, scuba diving is the first time in our lives that we have ever actually had to think about breathing. And there is a technique for proper breathing on scuba gear. In general, you want to take long, slow, deep breaths. A complete inhale and exhale should take anywhere from 5 to 8 seconds ... sometimes longer for more practiced divers. Rapid breathing affects your buoyancy. Shallow breathing tends to build up carbon-dioxide in our body, which causes us to feel oxygen starved and breathe harder and faster. Practice long, slow, deep breathing on land ... and then try it in the water. You will often notice an immediate improvement in your buoyancy control, and over time will notice that as your buoyancy control improves, so does your gas consumption.
Weighting
Improper weighting affects your gas consumption considerably. Too much weight causes you to carry excessive gas in your BCD or wing to maintain neutral buoyancy, and even small changes in depth will cause large buoyancy shifts due to the expansion or compression of that gas. You should perform weight checks any time you get a new piece of gear, and occasionally as your diving skills improve, because even something as simple as becoming more relaxed underwater will often allow you to lose weights you thought you needed.
Conversely, underweighted divers will struggle to stay down ... especially toward the end of the dive as your cylinder loses gas and becomes more buoyant. All that extra work causes you to breathe harder and consume your gas supply at a faster rate.
Trim
Humans are psychologically oriented in a vertical position. After all, it's what we've done since we learned how to walk. When learning scuba we must teach ourselves to move about in a horizontal position. Proper trim is very important to good gas consumption. Water is 800 times heavier than air, and we cannot efficiently move through water in the same way we move through air. Maintaining a horizontal position means that as we move through the water, we have to push less water out of our way than we would in a vertical position. It also radically increases the efficiency of our fins to move us in the direction we want to go. Both of those are huge factors in terms of our air consumption, because it reduces the amount of work we need to do to move around.
Swimming speed
Many divers ... new divers in particular ... tend to swim rather quickly. While that will get you from point to point faster, it will also increase your air consumption dramatically. In fact, the faster you go the more air you will consume getting from one place to another. Slow down ... it's not a race! There are lots of tiny creatures (and even some large ones that are good at camouflage) that you will likely not see if you are swimming quickly. Going slow, and keeping your fin kicks relatively small, will not only improve your air consumption dramatically, it will help you get more enjoyment out of your dive.
... Bob (Grateful Diver)
