If you're correctly weighted, you should be neutrally buoyant—able to make a safety stop at 15 feet with 500 psi and no air in your BC.

How to Use Less Air

One of the most visible signs of our overall diving skill is air consumption, which reflects mastery of such basic techniques as relaxation, buoyancy control and fin propulsion. It's hard to hide the need to surface because you're running low on air, especially when your buddies or even an entire group must cut the dive short because of you. No one wants to be the first to give the thumbs-up. And then there's the inevitable post-dive braggadocio: "Yeah, I surfaced with 1,000 psi in my tank. What about you?"

Before you buy double tanks, here's some good news: Your rate of air consumption is not genetically encoded. Efficient breathing under water is a skill, one of the most important adaptations we make to scuba dive, and you can learn it. Here's how to start saving air on your next dive.

• Breathe deep. Your rate of breathing must be slowed down or you will move air without giving your body adequate opportunity to absorb oxygen. Slow, relaxed, deep breaths promote a more complete exchange of oxygen and carbon dioxide. The greater your depth, the slower and deeper your breathing should be.
  
• Be a mime. Because water is 800 times denser than air, you simply cannot move as often or as quickly under water without paying a substantial penalty in effort and therefore air consumption. When you do move, it should be like a slow-motion mime—easy and deliberate.
  
• Keep your hands to yourself. Don't use your hands to swim. Let your arms and hands float loosely at your sides, fold them lightly across your chest, tuck them in your weight belt or beneath your tank on your back.
  
• Get neutrally buoyant. You'll know it when you achieve it: a sense of weightless ease, of being perfectly buoyed by the water around you. Absolute stillness. See "How to Get Neutral".
  
• Stay horizontal. Keep your body parallel, as much as possible, to the direction of movement. Swimming at an angle to the direction of movement is one of the greatest wastes of energy and air by novice divers.

  Keep your body parallel, as much as possible, in the direction of movement. Streamline yourself and your gear by keeping your hands and arms close to your sides.

• Streamline your gear. Secure all hoses as close to your body as possible. Use the smallest possible tank. Wear the smallest, lowest-lift BC that is still adequate for your diving. Put gear such as slates in BC pockets. Get rid of all excess weight. Keep your hands free and carry only the gear you need. Leave your snorkel in the bag unless you'll be making a long surface swim. Each of these steps decreases drag, therefore reducing the energy you put out and the amount of air you use.
• Regulate your regulator. Use the highest performance regulator you can afford and are comfortable with. Wash your regulator properly and have it serviced and adjusted regularly—at least once a year, more if you dive frequently, and after a regulator has been sitting idle for an extended period. Set any diver-controlled adjustment to the easiest breathing position that doesn't free flow. You do not conserve air by using adjustments to increase resistance.
• Conserve tank air. When appropriate, swim on the surface using a snorkel, or on your back with the BC partly inflated. Keep in mind that you are not as efficient on the surface as under water, but the air is free.
• Stop spilling air. When your regulator is out of your mouth, detune it and turn the mouthpiece down to avoid free flows. If your regulator is poorly maintained, it can also leak air through the mouthpiece. Take the same precautions with your octopus, securing it in a position so the mouthpiece will not face up and leak air. Have your regulator and octopus serviced and adjusted regularly.
• Reduce workload. Swimming is the greatest part of your underwater workload, so do as little as possible. Ride with currents and use buoyancy control for ups and downs instead of kicking. Pull yourself gently along anchor lines instead of kicking.

  Good buoyancy control is key to breathing efficiently under water.

• Stay warm. It's a fact: Warm divers use less air. You lose body heat even in the warmest tropical waters, which are considerably below your body's core temperature. And remember, water robs heat from your body 25 times faster than air. To save air, start by increasing the thermal protection of your torso, particularly in the armpits and crotch. Next, give better protection to your head, feet and hands. Increase the thickness and coverage of your wetsuit or switch to a custom-made suit.
• Increase fitness. The higher your level of fitness, the better your body can utilize the oxygen in the air you breathe. You should eat well, rest well, reduce stress and get plenty of exercise, while avoiding bad habits, particularly smoking.
• Get experience and training. The more you dive, the more comfortable and efficient you will be. Additional diving courses provide even more experience under supervision, along with increased understanding of how best to dive.
• Relax. It's not a contest. People vary in size, lung capacity, metabolism, sex (women generally use less air than men) and state of fitness. So there is a point past which you can't improve your air consumption. Trying to be something you're not or using techniques that are unsound simply lead to frustration and, of course, increased air consumption.