Planning a dive around air supply and time

A dive plan that keeps air and time from running short rests on three habits. Check how much gas you have before you enter the water, decide in advance when you will turn back, and keep a reserve so the return is never rushed. These habits are simple, but they only work if you make the decisions before the dive, when you are calm, rather than during it, when the current, the view, or a buddy's signal can pull your attention elsewhere.

The ideas here are general planning principles for scuba diving. They do not replace the gas planning taught in a certified course or the guidance of your instructor and dive operator. Gas use and decompression limits depend on your training, your equipment, and the site, so treat these ideas as a framework rather than a complete procedure.

Checking gas and equipment before entry

Before you enter, check that the cylinder valve is open, the pressure gauge reads what you expect, and the regulator breathes freely. Note the starting pressure and write it on a slate or record it in your dive computer. Many cylinders are filled to about 200 bar or 3,000 psi, but the actual starting pressure depends on the fill, so do not assume a number.

Check the other parts of the system at the same time. Confirm that the second stage and any alternate air source work, that the inflator and dump valves respond, and that the dive computer has enough battery and is set to the correct gas mix. Your buddy should check the same items on your equipment. A gas check on the surface is far cheaper than discovering a problem at depth.

Knowing your own breathing rate helps too. Surface air consumption, the volume of air you breathe per minute at the surface, is a number many divers learn in training. Combined with depth and the pressure gauge, it lets you estimate how long a cylinder will last on a planned dive and whether the plan is realistic for you.

Using the rule of thirds

A common planning rule is to use one third of your starting gas for the outward part of the dive, one third for the return, and keep the last third in reserve. Under this rule, the turn point falls when about one third of the starting gas has been used. The reserve is there for emergencies, such as a delayed ascent, a buddy who needs air, or a current that makes the return longer than planned. Because breathing rate rises with cold, stress, or effort, the same plan can use gas faster on some dives than on others.

The rule is a starting point, not a guarantee. Your instructor may teach a different reserve based on the type of dive, and some dive operators use fixed pressure figures instead of fractions. Recreational training also commonly sets a surface reserve, meaning the gas you aim to still have when you reach the surface, often around 50 bar or 500 psi, or whatever figure your operator specifies. Use the method you were taught, and adjust it when conditions change.

Setting a turn-around point

Set a turn-around point before the dive, expressed as a time, a pressure, or a location. The right number depends on the dive, and it must leave enough gas for the return and for the surface reserve you intend to keep. Turning back early is almost always easier than turning back late, because the return needs both gas and attention.

Turn around early if the current picks up, visibility drops, the group splits, or someone's gauge reads lower than expected. Do not wait until the air is nearly gone to head back. A plan you follow is safer than one you try to improve during the dive, when the margin is already small.

When diving with a buddy, plan around the diver with less gas. Agree on one team turn pressure before the dive, based on that diver's gas, and use the same number on both gauges. Both divers turn when the diver with less gas reaches it, so the return begins with the planned reserve intact.

Keeping time and planning the ascent

A dive computer tracks depth and time and shows no-decompression limits, which change with depth and with how long you stay there. Keep a margin below those limits rather than planning to reach them exactly. Deeper and longer dives reduce the time you can spend at the bottom, and the computer updates as you go, so check it throughout the dive rather than relying on the figure you saw before entry.

Plan the ascent as carefully as the descent. Many training agencies recommend ascending no faster than about 9 meters or 30 feet per minute, and a safety stop of about three minutes at around 5 meters or 15 feet is commonly taught. Build the stop into your plan, watch the ascent indicator, and keep your buddy in view as you rise.

The safety stop and the slow ascent also use gas, so the plan should include them from the start. A diver who plans to spend the full bottom time and then add a safety stop may find the reserve is smaller than expected. Working the numbers before the dive is the simplest way to avoid that surprise. A calm ascent at the end of a planned dive is the best way to finish with gas and time to spare.