The Open Water theory questions new divers get wrong
In short
The Open Water theory questions new divers get wrong have tempting answers borrowed from daily life: pressure at 20 m (66 ft) is 3 bar, not 2; held breaths are most dangerous near the surface; equalize before it hurts; the NDL is time at depth, not air; thumbs up means “go up”; search a minute for a lost buddy.
At 20 m (66 ft), the pressure is 2 bar. That sounds right, and it’s wrong: the same slip later makes your tank seem to last half again as long as it does. In each of the six questions below, the wrong answer borrows from everyday life: a gauge that starts at zero, a thumbs up that means “great,” a number on a screen that looks like a fuel gauge.
All six come from the Octo app, and two appear below as questions you can answer. Pick your answer before you read on.
1. What is the absolute pressure at 20 m (66 ft)?
The absolute pressure at 20 m (66 ft) of seawater is 3 bar (3 ata): 2 from the water and 1 from the air above it. The tempting answer, 2 bar, counts the water only, the way a gauge that reads zero at the surface does.
Try a question from the app
absolute pressure
What is the absolute pressure at 20 m in sea water?
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Answer B. 20 m of sea water adds 2 bar, plus 1 bar of air at the surface: 3 bar absolute. Forgetting the air above gives the tempting wrong answer.
Every gas calculation starts here. At 3 bar, a flexible bag holding 6 liters (0.6 cubic feet) at the surface shrinks to 2 liters (0.2 cubic feet), and a tank that would last 60 minutes at the surface lasts about 20. Halve instead, as the 2 bar answer suggests, and you plan on 30 minutes of air that runs out at 20. More on Boyle’s law and absolute pressure.
2. Where is holding your breath most dangerous?
Holding your breath is most dangerous near the surface. The tempting answer is “at maximum depth, where the pressure is highest,” but what tears lungs is a change in volume, and volume changes most in shallow water.
Try a question from the app
where the risk is greatest
Where is the risk of a lung overexpansion injury greatest?
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Answer D. Volume changes most in shallow water, so a held breath is most dangerous there, even in a pool.
From 10 m (33 ft) to the surface, pressure halves, so the air in held lungs tries to double. From 30 to 20 m (99 to 66 ft), it grows by only a third. Divers Alert Network (DAN) notes that lung barotrauma can happen in a shallow swimming pool. So breathe continuously, and if your regulator leaves your mouth, blow small bubbles until it’s back. More on lung injuries and equalizing.
3. When should you start equalizing your ears?
Start equalizing at the surface, then often on the way down, before you feel anything. The tempting answers are “when it hurts” or “when you first feel pressure, around 3 m (10 ft).” By the time it hurts, the pressure difference may already have squeezed your Eustachian tube shut.
DAN’s guidelines say to equalize early, before your head goes under, and often, without waiting to feel pressure. Ear barotrauma can happen as shallow as 2 to 3 m (7 to 10 ft), and nearly half the calls to DAN’s emergency hotline concern ears. If an ear won’t clear, don’t blow harder: signal, go up a little, try again gently.
4. What does your computer’s NDL actually show?
Your no-decompression limit (NDL) is the time you can still stay at your current depth and then ascend directly, without mandatory stops. It says nothing about your air. The tempting answer reads it like a fuel gauge.
Two more traps: the NDL gets shorter as you go deeper, because you absorb nitrogen faster, and when two computers disagree, the pair follows the one with less time left, not the average. See dive computers and the NDL.
5. What does a thumbs up mean underwater?
Underwater, a thumbs up means “let’s end the dive and go up.” It does not mean “everything’s fine”: that’s the ring made with your thumb and index finger. The tempting answer comes straight from life on land.
A related trap: an OK from your buddy is a question. Answer with OK, or with the signal for what’s wrong, such as being cold. Reply with a thumbs up and you’ve just asked to end the dive.
6. How long do you search for a lost buddy?
Search for up to about a minute, then ascend and meet at the surface. The tempting answers feel loyal: keep looking until you find them, or wait on the bottom. Both burn air and can pull you further apart. Turn slowly while you look: bubbles are often easier to spot than the diver.
If your buddy hasn’t surfaced after a short wait, alert the boat, the shore team or the guide at once. Don’t go back down alone to search: you could become a second victim.
Why do the wrong answers sound so right?
The wrong answers sound right because each one borrows a habit from land or a gut feeling that doesn’t hold underwater.
| Question | Tempting answer | Right answer | Why it tempts |
|---|---|---|---|
| Pressure at 20 m (66 ft) | 2 bar (2 ata) | 3 bar (3 ata) | Gauges start at zero |
| Held breath | Most dangerous at depth | Most dangerous near the surface | Deeper feels riskier |
| Equalizing | When it hurts | At the surface, then often | Pain feels like the cue |
| NDL | Minutes of air left | Minutes at this depth without stops | It looks like a fuel gauge |
| Thumbs up | Everything’s fine | Let’s go up | It means OK on land |
| Lost buddy | Search until you find them | About a minute, then surface | It feels loyal |
Rereading the chapter won’t change a gut feeling. Picking the wrong answer somewhere safe and reading why it’s wrong will, which is why Octo draws its wrong answers from real divers’ mistakes. Why you forget dive theory explains the research.
How do you prepare for these questions?
You prepare by learning the reason behind each answer, then testing yourself until the reason comes first.
- Learn the reason with the number. “3 bar, because the air above counts” survives a reworded question.
- Say why each wrong answer is wrong. A trap you can explain is much harder to fall into.
- Practice with situations. “Your ear hurts at 3 m (10 ft) on the descent line” tests the same rule as “when do you equalize?”
- Spread it out. Short sessions over several days beat one long evening.
See our guide to studying for the Open Water exam, what the Open Water course covers, and one question per lesson on the Open Water practice page.
Frequently asked questions
Why do new divers get pressure questions wrong?
Most pressure mistakes come from forgetting the atmosphere. A gauge reads zero at the surface, but you already have 1 bar (1 atm) of air on you. Absolute pressure is the water plus that air: 3 bar (3 ata) at 20 m (66 ft). Gas laws, volumes and air consumption all use absolute pressure, so one missing “+1” spoils every step that follows.
Does a thumbs up mean OK in scuba diving?
No. Underwater, a thumbs up means “let’s end the dive and go up.” OK is the ring made with your thumb and index finger, and it’s a question that needs an answer. At the surface, divers show OK to a boat with a big ring made with their arms over their head, while waving an arm means “I need help.”
Is holding your breath dangerous in a swimming pool?
Yes. The biggest volume changes happen near the surface, so even a short ascent with a held breath can injure your lungs. Divers Alert Network notes that lung barotrauma can occur in a shallow swimming pool if a diver holds their breath while ascending. Breathe continuously on every ascent, whatever the depth.
Does the NDL on my dive computer show how much air I have left?
No. The no-decompression limit shows how long you can stay at your current depth and still ascend directly without mandatory stops. Your air is on your pressure gauge, in bar or psi, so check both often. The NDL gets shorter as you go deeper, and you should end the dive well before it runs out.
How long should you look for a lost dive buddy?
Search for up to about a minute, turning slowly and looking for bubbles, then ascend and meet at the surface. Longer searches waste air and can separate you further. If your buddy hasn’t come up after a short wait, alert the boat, the shore team or the guide right away rather than going back down alone.
