How to Preform the Controlled Emergency Swimming Ascent (CESA)

Scuba Divers
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The Controlled Emergency Swimming Ascent (CESA) and Out-of-Air Emergencies

Photo of a scuba diver on a tropical coral reef
A diver who finds himself unexpectedly alone can use the Controlled Emergency Swimming Ascent (CESA) to surface safely in an out-of-air emergency. Image copyright, johnandersonphoto

Imagine that you are swimming peacefully underwater. Fish swirl around you in an ever-changing rainbow of color. Light filters from the surface and shimmers silver patterns on the white ocean sand. You are in your own world, calm, relaxed and . . . sluurrrp, out of air! Where's your buddy? No, really, where's your buddy? You look for your dive companion and his alternate air source and realize that he is nowhere near you. Perhaps he is off flirting with a turtle, or maybe he just swam away to check out an interesting coral head. Whatever the case, he is too far for you to reach his alternate air source in time. What do you do?

Obviously, a diver in this situation needs to make it to the surface. Instead of panicking and shooting up in a dangerous, fast ascent, a skillful diver would swim safely to the surface using a Controlled Emergency Swimming Ascent (C.E.S.A.). He does this by swimming slowly to the surface while exhaling and deflating his buoyancy compensator. Every certified diver learns the C.E.S.A. in his Open Water Certification Course, but most divers forget the skill because it seems complicated and is not practiced regularly. Here is a step-by-step guide to the C.E.S.A., an important emergency management skill that every diver should master.

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How Can You Safely Practice the Controlled Emergency Swimming Ascent (CESA)?

See a photo of scuba divers ascending during a controlled emergency swimming ascent
A student diver and a certified scuba instructor practice the Controlled Emergency Swimming Ascent (C.E.S.A.) in the ocean. Do not practice the C.E.S.A. vertically without the supervision of a scuba instructor. Image copyright, nataq

The Controlled Emergency Swimming Ascent (C.E.S.A.) can be a dangerous skill to practice. Do not practice swimming vertically towards the surface without a certified scuba instructor present. If the C.E.S.A. is performed incorrectly, a diver risks pulmonary barotrauma, decompression sickness, or drowning. Don't be too frightened! There are methods to avoid these risks. In fact, this is precisely the reason that the C.E.S.A should be practiced periodically – so that in the unlikely event of a real emergency, a diver will execute the skill correctly and reach the surface safely.

To practice the C.E.S.A. safely on your own, select a shallow water dive site (such as a swimming pool) with sufficient space to allow you to swim horizontally at least thirty feet. Start thirty feet (or more) from a wall or other visible marker and practice swimming toward that “goal” as if it were the surface ​without removing your regulator from your mouth. By swimming horizontally, a diver eliminates the risks associated with pressure changes such as pulmonary barotrauma and decompression sickness. As long as he keeps his regulator in his mouth, a diver has no risk of drowning. You will practice the skill exactly as you would vertically. You are just turning the whole exercise on its side.

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Step 1: Attain Neutral Buoyancy

A diver with neutral buoyancy
Instructor Natalie Novak of attains neutral buoyancy before beginning the Controlled Emergency Swimming Ascent (C.E.S.A). Natalie L Gibb

Before simulating a Controlled Emergency Swimming Ascent (C.E.S.A.), a diver should relax and make himself neutrally buoyant. (A good way to obtain neutral buoyancy is using a skill called the fin pivot.) Neutral buoyancy is an important step because a diver will not be able to swim freely if he is sinking down and hitting the floor. He will have similar problems if he is fighting positive buoyancy and floating up. In a real diving emergency, a diver would begin the C.E.S.A. neutrally buoyant, so the practice scenario will be most realistic and beneficial if a diver begins the exercise that way.

Once you have attained neutral buoyancy, take a moment to relax, visualize the steps of the C.E.S.A., and slow your breathing rate. As you move through the following steps take the time to execute each of them thoughtfully and deliberately. Remember that this is not a real emergency, and you will retain information better when you think about it and practice in a calm state.

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Step 2: Arms Up

Controlled Emergency Swimming Ascent CESA Deflate
Instructor Natalie Novak of lifts her BCD deflator above her head in preparation for the Controlled Emergency Swimming Ascent (CESA). Natalie L Gibb

Even during a Controlled Emergency Swimming Ascent (C.E.S.A.) a diver should attempt to swim up at a safe ascent rate. That is why the skill is called the Controlled Emergency Swimming Ascent. It would be unfortunate to safely reach the surface only to have suffered a nasty hit of decompression sickness from ascending too quickly. A diver maintains a safe ascent rate by venting expanding air from his buoyancy compensator (BCD) as he swims towards the surface. He lifts his deflator above his head so that he is ready to release small amounts of air from the BCD if he finds that he is ascending too quickly. (If you do not understand why you should release air from the BCD as you ascend, read more about buoyancy basics.)

Because you are practicing the C.E.S.A. horizontally, pretend that whatever object or wall that you set as your goal is the water's surface. Extend your BCD deflator towards the “surface” just as you would if you were using the skill in the open water. The only difference is that you will be extending the deflator horizontally in front of you instead of up because you have turned the skill on its side. This allows you to maintain the same body position as you would if you were ascending vertically in the open water.

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Step 3: Look Up

Look Up During the Controlled Emergency Swimming Ascent CESA
Instructor Natalie Novak of looks up to avoid surfacing under a boat or other hazard during the Controlled Emergency Swimming Ascent (CESA). Natalie L Gibb

While reaching the surface is the goal of the Controlled Emergency Swimming Ascent (C.E.S.A.), a diver will not benefit from swimming upwards directly into the bottom on a boat, diver, or other objects. The next step of the C.E.S.A. is to look where you are going! Once you get your arms and deflator into position, look towards your goal, or “surface” and get ready to swim.

Looking up has the added benefit of allowing a diver to watch the small bubbles he exhales (more on this in the next step) rise to the surface. The smallest bubbles will float upwards at the rate of about a foot per a second. Since it is unlikely that a ​diver will be monitoring his depth and timing device in a real emergency, he can use the rising bubbles to gauge his ascent rate. If he begins to ascend faster than his bubbles, he needs to slow down.

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Step 4: Swim Up

Exhale and swim slowly to the surface during the Controlled Emergency Swimming Ascent (CESA)
Instructor Natalie Novak of swims toward the "surface" while continuously exhaling during the Controlled Emergency Swimming Ascent (CESA). Natalie L Gibb

Now it is time to swim for the surface! Maintaining your body position, take a deep breath and swim slowly (no faster than one foot per a second) towards the “surface”.

Do not take the regulator out of your mouth!

Although you are “out of air” the regulator will prevent you from inhaling water. In a real emergency, you would keep the regulator in your mouth for this reason. Furthermore, if you have difficulties completing the skill the first few times you try it, you can resume breathing from the regulator as long as it is safely in your mouth.

There is just one catch – because a real Controlled Emergency Swimming Ascent (C.E.S.A.) is conducted while swimming up, a diver must slowly breathe out as he ascends to allow the expanding air in his lungs to escape. Otherwise, he risks a pulmonary barotrauma.

To simulate this situation, take a deep breath and slowly exhale as you swim horizontally towards the object you have designated as the surface. A good way to control your exhalation is to make a quiet “ahhh” noise. A person is accustomed to controlling his breath using his voice, and making a soft, humming sound as he exhales will help him to extend his exhalation time.

To reach your goal, you will need to exhale for at least thirty seconds. This may take some practice, but by maintaining a slow swimming pace and using your voice to control your exhalation, it is possible! The good news is that if a diver can complete this exercise horizontally, he will have absolutely no problem using the C.E.S.A. in an actual out-of-air situation. In a real emergency, a diver swims upwards and the air in his lungs expands. Even though he is exhaling, his lungs remain full from the expanding air, and therefore he will not run out of breath.

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Step 5: Establish Positive Buoyancy at the Surface

A diver remember to establish positive buoyancy after completing the Controlled Emergency Swimming
Instructor Natalie Novak of touches her weight belt to remind herself to drop her weights after completing a Controlled Emergency Swimming Ascent (CESA). Natalie L Gibb

As you reach the “surface” prepare to make yourself positively buoyant. In a real emergency, you would need to float with your head above the water to breathe. Remember that in this exercise you have run out of air, so there is no air remaining in your tank to inflate your buoyancy compensator. In this case, the easiest way to make yourself float on the surface is to drop your weights.

To simulate this during the skill practice, touch your weight belt (or integrated weight release) and imagine removing your weights. Don't actually release them (this would make you float up quickly), just remind yourself that this would be the next step.

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Good Job!

The Controlled Emergency Swimming Ascent CESA
Instructor Natalie Novak of has successfully completed the Controlled Emergency Swimming Ascent (CESA). Natalie L Gibb

Now you know how to safely reach the surface using the Controlled Emergency Swimming Ascent (C.E.S.A.). You have avoided decompression illness by maintaining a safe ascent rate – you watched your bubbles rise and released air from your BCD if you started ​to pass them. You have avoided a pulmonary barotrauma by exhaling continuously as you swam upwards, and you did not drown because you kept your regulator in your mouth the entire time and released your weights to float on the surface.

The C.E.S.A. is an important emergency management skill that allows divers to safely reach the surface on their own in the unlikely event of an out-of-air emergency. Divers should keep up to date with the C.E.S.A. and all other emergency management skills. However, keep in mind that an out-of-air situation is unlikely if a diver properly prepares his equipment, completes a pre-dive safety check, and monitors his air supply. A good buddy will also minimize a diver's chance of needing to use the C.E.S.A. If buddies stay close together, an out-of-air diver can simply use his buddy's alternate air source.

Special thanks to Natalie Novak of for taking time out of her busy schedule of teaching and guiding in Mexico to help me with these photos.