Deep vs shallow SPORTS diving... which is safer?

[Dennis Guichard]

Minor quibble: you refer to "Henry's Law of diving physics." Henry was a chemist, and the law of partial pressures is invariably taught in general chemistry classes.

In fairness, the subject is probably best classified as "Physical Chemistry".

How's the cuppa?

Yes you've got a point but he got me thinking which in itself can't be a bad thing!? They all seem to be largely just guessing at it anyway, I don't really know how us (me) common folk are really expected to understand much of it..!?
Cuppa was marvellous as only a cuppa can be... :-D

 

[dberry]

Thinking is definitely a good thing!

 

[Bowers]

sounds like youve got it figured out, but just in case; amount of air used does not correlate (in any consequential value) to the amount of saturation. Im going to use some made up numbers here so bare with me.. if i breath in 100 molecules of nitrogen and my body absorbes 10, then i breath out 90 unabsorbed molecules. if my buddy breaths in 200 molecules but only absorbes 10, then s/he breaths out 190 unabsorbed molecules. even though gas was breathed down twice as fast there will still only be 10 molecules absorbed. This is far more simplistic than the question you are asking but hopefully it adds some perspective

 

[boulderjohn]

So question... because I'm having a questiony deep-thinking kind of Sunday... if a diver breathes all their gas out with a higher breathing rate in say 20-minutes vs another with a slower breathing rate doing the same but over say 40-minutes... have they both in-gassed the same amount of inert gas at depth...?

I am having a lot of trouble understanding your questions, so if I misunderstood in my response, please forgive.

It seems to me at times that you are perceiving tissue saturation like pouring water into a bucket. It is not like that at all.

When you inhale, your lungs fill with molecules of air, and those molecules are distributed to the body through the blood. All the time, the molecules are just wandering around at random, going through the walls of the vessels, etc. That means that while molecules are leaving the lungs and entering the tissues, molecules are also leaving the tissues and entering the lungs. At the surface before you dive, you have, by pure law of averages, just as many molecules coming in as going out. You are 100% saturated.

Dive down to 33 feet of saltwater, and you now have twice as many molecules going into the lungs as you have in your tissues. Thus, by law of averages, twice as many molecules will be entering your tissues as leaving it. Wait 5 minutes, and your 5 minute compartment will be 50% of the way to becoming equal. But that does not mean your tissues are 50% full, like a bucket. It means they are 50% equal to the gas coming in. Go to 100 feet, and you will suddenly be far less than 50% at that pressure.

Eventually, things will slowly start working their way toward equilibrium--saturation. On a recreational dive, SOME of your tissues will become saturated, but most will still be working their way toward equilibrium. Then you start to ascend. Those tissues that were saturated are now supersaturated--they have more gas molecules in them than the air you are breathing. By law of averages, more molecules will now be leaving your body than entering it. You are offgassing in those tissues.

The rate at which you breathe does not matter. What matters is the comparison of the number of molecules (gas pressure) in the lungs to the gas pressure in the tissues. Whichever is higher will flow to whichever is lower.

 

[dmaziuk]

Where I dive it has become common practice to do the last dive deepest simply because people controlling the dives think that a short deep dive (31m for 40 minutes) is safer than a long shallow one (say 18m max for 60 minutes)...

Going by PADI tables, I would say one of them would put you in the fairly moderate pressure group P whereas the other would be a mandatory decompression dive. In fact, any dive to 31 m requires a deco stop, so... define "safer" in that context.

 

[tursiops]

Going by PADI tables, I would say one of them would put you in the fairly moderate pressure group P whereas the other would be a mandatory decompression dive. In fact, any dive to 31 m requires a deco stop, so... define "safer" in that context.

Safety stop, not deco stop. There are no deco stops in the PADI tables.

 

[boulderjohn]

I realize that I never responded to your core question about the difference between shorter deep dives and longer, shallower dives.

In recreational no-deco-stop diving, IS a deep short dive safer than a longer shallower dive, assuming all else is equal in terms of ascent rate control, safety stops, hydration, etc...?

When diving tables, the normal way of judging decompression stress is by looking at the final pressure group you are in., I will compare using something close to the examples you gave. My tables are imperial, but the idea is the same. I cannot do your example of 31 M for 40 minutes, since that puts you into serious decompression, so I will use the maximum NDL of 20 minutes for 30 meters (100 feet). Your dive to 18 meters for 60 minutes also puts you into decompression, so I will use 60 feet for 55 minutes.

A) One Dive to 100 feet for 20 minutes = pressure group O.

B) Dive to 60 feet for 55 minutes = pressure group W

So, yes, a single shallower dive to its maximum NDL does ineed put you in a higher pressure group than a single deeper dive for lesser time.

C) What happens if you do two such dives, with an hour surface interval? After an hour, your group W diver is now a group I diver. If that diver repeats the 60 foot dive, then the maximum time is only 30 minutes, and, once again, the diver finishes that dive near the end of the alphabet.

The figures you gave for the dive depths and times are pretty much off the charts, so are you you sure that is what you are doing? If you are doing multi-level dives to get those depths and times, then the table work I did here is meaningless.

 

[dmaziuk]

Safety stop, not deco stop. There are no deco stops in the PADI tables.

Sorry, I don't believe in "no deco" dives, and that a "required stop" is not a deco stop. You can call it "safety absence of upward motion" or you can call it "pink floyd with fucsia tinge", that's entirely up to you, to me: it's a stop and it is non-optional.

 

[dmaziuk]

Your dive to 18 meters for 60 minutes also puts you into decompression, so I will use 60 feet for 55 minutes.

Sorry, I looked at the wrong column yesterday. I claim presbyopia meets ADD, that's my excuse and I'm sticking to it.

 

[boulderjohn]

Sorry, I looked at the wrong column yesterday. I claim presbyopia meets ADD, that's my excuse and I'm sticking to it.

That is one of the most consistent problems with using tables. I used to have students use a card or sheet of paper to make sure they did not make that error. The overwhelming majority of mistakes on the final exam were of that nature.

You could thus attribute any DCS happening on a dive as a result of such an error to diver error, but I would assign the blame more to a system that makes such an error all too likely.