Popped up too fast from safety stop

[TrimixToo]

Your physics/math is correct, and you agree with my point but not my phrasing of it, as I chose not to go into detail but to keep it brief.

When I explain to students, (with a little bit of rounding) I explain that pressure will change one-sixth on a 15 foot rise from 60 feet, but will change one-third from 15 foot to the surface.

For a new diver all the non-linear effects of shallower depth variance are what they need to have drilled into their consciousness until it is second nature - increased risk of lung expansion injury, increased difficulty of bouyancy control, and increased air time remaining.

So be careful about announcing “common mistakes” when you don’t like the shorthand description - while you are technically correct, you are emphasizing the wrong practical message - many casual readers will absorb the absolute wrong take away and continue to think in terms of the shallow depth as “only” 15 feet.

I'd agree if you said "volume" rather than "pressure." Volume changes nonlinearly with depth. Pressure does not.

I phrased my post carefully. I'm afraid that saying it's the pressure that behaves in a nonlinear way with depth and not the volume *is* a very common mistake among instructors and other divers alike. The hazards you describe are real. The behaviors you are trying to train are the right ones. I think it's worthwhile to tell people the actual basis for both, but what do I know?

 

[JackD342]

Okay, everyone had me questioning myself on the increased risk of lung expansion injury and got me working back to a source.
That's okay, it's fine to be challenged, it's how we grow.
First, here is a link to a DAN article that covers the topic very well:
Scuba Diving Pulmonary Over-Inflation Syndrome — Medical Dive Article — DAN | Divers Alert Network

If you don't have time to click it and read in full, here is the most salient portion:
"For a diver at 15 fsw/4.6 msw, the total pressure acting on his body is 1.5 atmospheres (one atmosphere at the surface, plus an additional 0.5 atmospheres exerted by the water column). A sudden ascent to the surface would therefore result in a 30 percent pressure reduction, and assuming a compliant chest wall, a volume increase of 50 percent. Lung injury may result.
Actual volume changes may be less than this because of the effect of the surrounding chest wall to provide some rigidity and protection for the lung. However, if the same vertical change occurred from a depth of 66 fsw/20 msw, the 0.5 atmosphere of depth change would only result in a 16 percent reduction in pressure and a 20 percent increase in lung volume, and would be less likely to cause lung injury. Boyle's law thus explains why abrupt changes in depth while in shallow water can be far more hazardous than equivalent changes of depth in deep water."

And as long as we are at it, another very important line from the end of the article:
"It is important to note that a breathhold ascent from a depth as shallow as 4 feet of sea water (fsw)/1.2 meters (msw) may be sufficient to tear alveoli sacs, causing lung tear and one of these three ailments."

So I feel VERY comfortable standing by my statement that the pressure changes by one-sixth when ascending from 60ft to 45ft, and by one-third when ascending from 15ft to the surface. Feel free to quibble about my describing that as an increased "rate of change," but I need my divers including the 10 year olds, to quickly understand the concept and the point being made without whiteboarding a physics lesson at the surface of the pool.

Anyway, for anyone with doubts or confusion on the subject, please refer to the DAN article at the above link.

 

[EFX]

Okay, everyone had me questioning myself on the increased risk of lung expansion injury and got me working back to a source. And as long as we are at it, another very important line from the end of the article:

"It is important to note that a breathhold ascent from a depth as shallow as 4 feet of sea water (fsw)/1.2 meters (msw) may be sufficient to tear alveoli sacs, causing lung tear and one of these three ailments."

When I first heard that you could suffer AGE (arterial gas embolism) from a rise of just 4 feet I was surprised and somewhat doubtful. A few years ago my LDS sponsored a trip to a hospital with a barometric chamber. In the chamber they took us "down" to 180 ft and back up. Some of us brought our dive computers to monitor the depth. Prior to the "dive" the technician spoke about his work at the chamber. He talked about one call he received about a patient being brought in from a suspected AGE incident. A husband and wife team were practising buddy breathing 6 ft down in a pool by sharing a regulator. The procedure called for 2 inhales then the person with the reg gives it to their buddy. Apparently, the husband got greedy with the reg, the wife panicked and promptly stood up on her way to the surface, and immediately suffered an incident. 911 was called and off to the hospital she went. The diagnosis: AGE. So, it doesn't take much.

 

[caruso]

Apparently, the husband got greedy with the reg, the wife panicked and promptly stood up on her way to the surface, and immediately suffered an incident. 911 was called and off to the hospital she went. The diagnosis: AGE. So, it doesn't take much.

Thanks for that. Now I'm afraid to take a bath.

 

[Graeme Fraser]

Thanks for that. Now I'm afraid to take a bath.

Taking a bath is fine as long as you make a continuous ahhhhhhhh sound! Might get suspicious looks from the other half though

 

[60plus]

Having too much weight can cause "pop ups" because to compensate for the excess weight you will have too much air in your BCD which will expand rapidly as you ascend, causing sudden excess buoyancy. One boat DM I dive with insists that both descent and ascent is done with fully deflated BCDs using hand over hand on the line. He says overall it is the best way of keeping the group together and avoids wasting time with divers drifting about, thereby maximising dive / bottom time to the benefit of the whole group. Also it minimises the possibility of a pop up and someone injuring themselves on the underside of the aluminium dive boat.

 

[Neilwood]

@60plus has a good point about the excess weight. Having too much lead means managing a larger air bubble in the BCD which, unless you are careful, can cause rapid ascents. The trick if you need the weight (such as a thick wetsuit), is to use the deflater little and often.