Recently, I have had a few friends who recently took their open water course tell me that they had a runaway ascent scenario and asked me why that happens.
To set the record straight, a Runaway Ascent can be a very dangerous scenario! Decompression illnesses aside, you could easily be hit by a passing boat!
Once you realised you are in a runaway ascend, immediately do the following:
1. Flare out horizontally. Stretch out your limbs to create as much drag as possible to slow down your ascend.
2. Pull on your BCD dump valve. If your BCD has multiple dump valve, pull on the one nearer to your legs. Orientated yourself such that the dump valve is at the highest point of your BCD. Certain BCD designs (especially jacket type), requires you to manually squeeze out pockets of trap air in the BCD.
3. Adopting a head down / leg up position, try swimming down if possible, again to slow down your ascend.
4. Breath out. Take in only little slurps of breath but breath out more.
5. if you are near the bottom when that happens, try grabbing hold of rocks or other features. Do be careful of what you grab.
To answer their question on why they have run away ascends. From my experience, there are two main reasons why it happens, and both are related.
Reason one - Swimming Posture
Unknowingly to most beginners, they swim in the heads up posture. Which means that they are constantly swimming upwards at a slow rate.
According to Boyle's law, when you ascend, pressure decreases, the air in your BCD expands. This means that now you are more buoyant. The more buoyant you are, the more you float/ ascend. The cycle repeats.
Beginners don't usually realise this until they are very close to the surface.
The solution to this is to adopt a horizontal swimming posture. Not only will it prevent you from a runaway ascend, you will fin easier, expand less energy and diving becomes more enjoyable.
Reason two - Being over weighted
Being over weighted also means that you are more likely to adopt a heads up swimming posture, which increases the likelyhood of reason 1 happening.
Being over weighted means that you will need to have more air in your BCD to achieve neutral buoyancy at depth. With more air in your BCD, any change in depth/pressure will result in a larger change in buoyancy.
For example, assuming a normal diver needs only 0.5L of air in his BCD at 10m depth to maintain neutral buoyancy. Being 2kg over weighted means he needs roughly 2L of additional air in his BCD (bringing the total to 2.5L) to maintain neutrally buoyant. A change in depth to 5m for the normal diver would only cause the air to expand to 0.67L (or 0.17L more), but for the over weighted diver it would expand to 3.33L (or 0.83L more)! For the over weighted diver, the air has expanded around 5 times more than it would be for the normal diver. This means that the over weighted diver would be at a higher risk of run away ascend than a properly weighted diver.
To solve this problem, get your instructor to ensure that you are properly weighted. There are many reasons why instructors will make a student over weighted in their first few dives, but leaving them totally over weighted without trying to teach them about proper weighting is absolutely unacceptable. but that's another post for another time!
Till next time, Dive safe and dive fun!
