Popped up too fast from safety stop

[JackD342]

At the risk of sounding pedantic (who, me?)...

This seems to be a very common mistake, or perhaps it's just a shorthand way of saying that the *percentage* pressure differential increases at shallower depths. The actual pressure differential is completely linear, irrespective of depth. It's about 1/33 ATA (atmospheres absolute, or 14.7 PSIA) per foot in salt water, and 1/34 ATA per foot in fresh water.

For example, ascending from 66 FSW to 33 FSW the pressure declines from 3 ATA to 2 ATA, a 33.3% decrease. Ascending from 33 FSW to the surface, the pressure declines from 2 ATA to 1 ATA, a 50% decrease. But in both cases, the pressure was reduced by 1 ATA.

However, the *volume* differential is not linear and it is greater at shallower depths. This is what increases the risk of lung overexpansion injury and makes bubbles grow faster for a given ascent speed when you're shallower.

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.

 

[Vicko]

Usually it does not matter, but there are factors to consider that are currently way outside of your diving experience.
Don't think that just because nothing happened now, or the first few times nothing ever will happen. Once you start doing deeper dives and start getting closer to your no deco limit chances of getting bent increase. Also there is a very good chance of you getting some sort of baroutrauma if you lose control of your buoyancy so practice, practice and practice more.

 

[divad]

Practice your SS without a line.
Use your lungs to accent/descend 3 feet..

In a swimming pool.

 

[Neilwood]

It sounds like you had a bit of a scare but managed to sort it out on your second dive.

Sometimes a scare is not a bad thing - it certainly reinforces things better than a number of people telling you things.

You have now become far more aware that good buoyancy is very important and that you should be adjusting it in small amounts. I make a point of adjusting my buoyancy with little bursts from inflator/deflator (never holding the buttons down). Buoyancy changes do take a short time to take effect so being aware of what is happening and the effect of your current actions is very important and becomes especially so the nearer the surface you get.

I have only ever felt the need to securely hold the line on one safety stop and that was due to surge where the waves were trying to corkscrew me. In normal conditions, you should be able to hold your stop without any assistance and make a slow ascent from there (it should be the slowest part of your ascent).

 

[wedivebc]

So be careful about announcing “common mistakes” when you don’t like the shorthand description - while you are technically correct, .

I think he was trying to be polite. Your assertion

Pressure differential per foot is greater at shallower depths.

is completely incorrect. Calling a technical "shorthand" is being generous.

 

[JackD342]

I think he was trying to be polite. Your assertion

is completely incorrect. Calling a technical "shorthand" is being generous.

Sigh....
Arguing about things we actually agree on....

I don't always write a novel when I contribute here, either due to time constraint or even just mood. In this case, the issue is what is meant/understood by "the pressure." I meant one of several things, @TrimixToo assumed something else.

I was not referring to the ambient pressure in the water column, and I was not referring to the absolute or the relative pressure inside the cylinder, or the absolute or the relative pressure in the first or the second stages. I was referring to the effective pressure in the lungs for a diver holding their breath.

The context was the danger of lung expansion injury. And again, the most important point is to NOT equate less depth with less risk, but rather the opposite in this and some other cases.

 

[jlcnuke]

Sigh....
Arguing about things we actually agree on....

I don't always write a novel when I contribute here, either due to time constraint or even just mood. In this case, the issue is what is meant/understood by "the pressure." I meant one of several things, @TrimixToo assumed something else.

I was not referring to the ambient pressure in the water column, and I was not referring to the absolute or the relative pressure inside the cylinder, or the absolute or the relative pressure in the first or the second stages. I was referring to the effective pressure in the lungs for a diver holding their breath.

The context was the danger of lung expansion injury. And again, the most important point is to NOT equate less depth with less risk, but rather the opposite in this and some other cases.

You can call it whatever you like, but it's volume, not pressure, that has a non-linear change based on depth. All pressure (absolute, relative, "effective" (what the hell is that supposed to be??) has a linear change based on depth. To say 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" is factually incorrect. Pressure changes the exact same for a 15 ft rise in depth, regardless of starting depth. If you're explaining it to your students like that, I wouldn't be surprised if they didn't understand the concepts correctly, since the explanation you're giving is incorrect.

 

[JackD342]

You can call it whatever you like, but it's volume, not pressure, that has a non-linear change based on depth. All pressure (absolute, relative, "effective" (what the hell is that supposed to be??) has a linear change based on depth. To say 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" is factually incorrect. Pressure changes the exact same for a 15 ft rise in depth, regardless of starting depth. If you're explaining it to your students like that, I wouldn't be surprised if they didn't understand the concepts correctly, since the explanation you're giving is incorrect.

@jlcnuke
Two questions:

1. Do you agree or disagree that the risk of lung expansion injury when holding a breathe is greater in shallower water?
2. Assuming you agree, can you please provide your succinct description of that situation suitable for imparting to a new diver? Who knows, maybe you will explain in a manner I liked and adopt for future use.

 

[jlcnuke]

@jlcnuke
Two questions:

1. Do you agree or disagree that the risk of lung expansion injury when holding a breathe is greater in shallower water?
2. Assuming you agree, can you please provide your succinct description of that situation suitable for imparting to a new diver? Who knows, maybe you will explain in a manner I liked and adopt for future use.

1. Sure, because it takes less being stupid, err holding your breath, before your lungs are likely to "overfill".
2. This is due to the rate of change of volume being greater at shallower depths. While going from ~100ft to 66 ft only results in the air volume increasing by about 32% for a given amount of air, going from 33ft to the surface doubles the volume the same amount of air takes up. So if your lungs were 2/3rds full and you started holding your breath at 99ft, then resumed breathing at 66 ft, it's quite possible you could get lucky and simply have fuller lungs and not a medical emergency. However, if you started with them 2/3rds full at 33 ft and held your breath to the surface, you'd be almost certainly having a medical emergency.

It's the same explanation you were trying to give I believe, but using volume instead of pressure as the non-linear changing variable..

 

[ystrout]

I think you're fine. You'd know if you have the bends, and as long as your didn't hold your breath, you won't get a lung injury.

I think all new divers get this scare a few times. I certainly did.

A year ago, I couldn't remember the frequency I was breathing at during a safety stop and floated up a few feet. I got scared I was holding my breath and potentially could have gotten a lung injury. In reality, I wasn't holding my breath at all, just not breathing super fast because I was so relaxed.

A bunch of people on this forum said that there's a difference between holding your breath and being at the 'inflated' part of your breathing. Holding your breath means closing some thing in your throat and being at the inflated part of your breath leaves it open. Air can move freely out of your lungs as it expands. I don't know the biology/physics behind it, so perhaps someone else can explain.

But as long as you didn't hold your breath, I'd say just treat it as a learning experience.