The theory part of a scuba course

Pressure and volume

At the surface you live under about 1 bar of air pressure. Seawater adds about 1 more bar for every 10 m (33 ft) of depth. A flexible air space, such as your lungs, your mask or your BCD, shrinks as pressure rises and expands as it falls. Physics students know this as Boyle's law.

The table comes from the same data as the gauge on the home page. Two lessons stand out. First, the biggest change in volume happens in the shallows, so equalizing and buoyancy need the most attention near the surface. Second, you breathe denser air at depth, so a tank that lasts an hour in shallow water lasts far less time deeper down.

Equalizing and the never-hold-your-breath rule

Your ears and sinuses contain air spaces that you have to equalize as you descend, usually by gently pinching your nose and blowing against it, or by swallowing. Courses teach you to equalize early and often, and never to force it. If an ear won't clear, you rise a little and try again, or end the dive.

The most important rule in the theory is to breathe continuously and never hold your breath on scuba. If you hold your breath while rising, the air in your lungs expands with nowhere to go, which can cause serious injury. Your course explains this in detail, and your instructor will drill it in the pool.

Breathing gas and nitrogen

Most recreational diving uses ordinary compressed air, which is about 21% oxygen and 78% nitrogen. Under pressure your body absorbs more nitrogen than usual. Ascend slowly, and it leaves your body gradually. Stay too long or ascend too fast, and it can form bubbles, which is decompression sickness.

That is why divers plan their time at each depth using dive tables or, more often today, a dive computer. Both work from limits on how long you can stay at a depth and still ascend directly to the surface. Your course teaches you to read whichever your agency uses. Some courses also introduce enriched air (nitrox), which needs its own training.

Planning a dive

Theory lessons cover how to agree a plan with your buddy: the maximum depth and time, when to turn around, how much gas to keep in reserve, hand signals, what to do if you get separated and how you will get out of the water. "Plan your dive and dive your plan" is a phrase you will hear often.

The underwater environment

You also learn how conditions affect a dive: currents, waves and surge, visibility, temperature and how water absorbs color and sound. Courses explain how to treat aquatic life and the reef: look, don't touch, keep your fins clear of the bottom and don't take anything away.

Handling problems

Part of the theory deals with things going wrong: running low on air, a free-flowing regulator, cramp, getting tired at the surface, losing your buddy. Each has a practiced response, and you will rehearse most of them in the pool. Knowing the steps calmly in advance is what keeps a small problem small.

Getting the most from e-learning

1. Study in short sessions and take the quizzes seriously, even when you could skip ahead.
2. Write down every question you have and bring the list to your instructor.
3. Learn the hand signals and the buddy check before the pool session, so pool time goes on skills.
4. Finish the theory close to your in-water dates, so it is fresh.
5. Remember that reading is only the first part. You are not trained until an instructor has seen you do each skill in the water.

Theory FAQ