Diving for 2 minutes to 20 feet or 10 feet and coming to the surface

[DJ457]

Scubadude - it's so hard giving someone advice when they have already made up their mind that this is what they want to do, which it seems you have already made yours up. I would listen to the advice that everyone has given you man.

As stated above, these are the two best choices IMHO:
1. If you want to continue descending and ascending just put on a wetsuit, mask and snorkel, fins and do it that way. You should be able to hold your breath long enough to collect shells from the bottom at 10ft with no problem.
2. If you want to use scuba, no problem, have a collection bag and just stay down until you've collected all the shells you want and then surface.

That's what I would do at least.

 

[scubadude223]

Fair enough.

I I just don't know what is safe if I don't do it.

 

[nimoh]

Scubadude - it's so hard giving someone advice when they have already made up their mind that this is what they want to do, which it seems you have already made yours up. I would listen to the advice that everyone has given you man.

As stated above, these are the two best choices IMHO:
1. If you want to continue descending and ascending just put on a wetsuit, mask and snorkel, fins and do it that way. You should be able to hold your breath long enough to collect shells from the bottom at 10ft with no problem.
2. If you want to use scuba, no problem, have a collection bag and just stay down until you've collected all the shells you want and then surface.

That's what I would do at least.

I don't believe he was looking for advice on his profile. He was looking for an answer to a seemingly simple question without realizing there was no simple answer.

 

[spectrum]

It's safe to do if done in good form but you could come to regret it.

Effective exhaling will eliminate the risk of a lung expansion injury. As other have explained the percent change per foot is greatest near the surface so you will be repetitively exposing yourself to the greatest hazard. If you are in good health (clear lungs) and exhale all will be fine.

A similar risk exists for your ears, you will be repetitively clearing.

The other half of the equation is DCS... The reason you don't find anything is that for all practical purposes there is nothing. This will not even register on any chart I have seen. Even if you found a chart that ran out to cover this the progression to a deco obligation would be long beyond belief. Some dive planning programs may let you tally something.

The thing I will offer is that you want to make your ascents exquisitely slow. You WILL be absorbing some amount of gas in your blood each time you go down and it will want to escape as you ascend. The more you aggravate this with hasty ascents the the worse it will be and finding your self profoundly tired when it's all said and done is probable. This is sometimes cited as sub clinical DCS. If can happen on intense and poorly executed training dives where students are up and down doing skills and getting coaching of otherwise yoyo diving.

Pete.

 

[dumpsterDiver]

This thread is cracking me up....LOL.. Going down to 20 ft for 30 second or a minute and then shooting to the top in 15 seconds is what a freediver will do. I often do 30 or 40 dives like that a day to depths of 40 or 60 feet or so (but it takes a little longer to swim up from 60 feet).

There should be no real difference if you are holding your breath (freediving) or scuba diving with respect to the effects of nitrogen. There is no difference on the impact to your ears or sinuses.

If you don't hold your breath on ascent, I would not be too worried about it. HOWEVER, freedivers will typically spend a decent surface interval between dives, often double their down time. If you are REALLY? going to be shooting down and back dozens of times per day to depths past 15 feet or so, I might be a little more conservative with the ascent rate.

I personally knew a guy who got pretty seriously bent (decompression sickness) when doing commercial inspection of a seawall. They were stupid, they swam up and down like 1 to 25 feet, over and over in cold water for three tanks.. He woke up partially paralyzed. theoretically you can dive all you want between zero and 33 feet, but in reality.. you might want to be a little careful.

The more productive means to do your diving would be to tow your float or kayak or whatever, and have a bag or basket clipped to the tow line, suspended off the bottom, when it gets too full, THEN go to the surface, haul it up and place it on your surface float. Or, just get a large capacity BC and clip a 5 gallon bucket to yourself and stay down, fill it up and add air to stay close to neutral.. then haul it up with a rope or send it up with a lift bag. I used to do this at a depth of 80 feet, collecting heavy shells.

 

[scagrotto]

at the end of typical inspiration you have about a 100% capacity buffer against over-expansion.

That's a wonderful theory that probably applies perfectly well to balloons and similar single chamber objects with no internal obstructions, which therefore have uniform pressure and density throughout their volume. Unlike balloons, your lungs have hundreds of millions of alveoli, all of which are very, very small. The fact that the total gas in your lungs may be only 50% of total lung capacity isn't a guarantee that none of those hundreds of million alveoli aren't at 80 or 90% of their capacity. As I understand it, it's highly probable that some (and with hundreds of millions of alveoli, "some" can be an awful lot of them) are above the average capacity.

There are things that can cause slower gas flow in some alveoli than others, and if the gas flow out of the alveoli is restricted there's a greater chance that an ascent of any particular distance can cause an overpressure injury. A restriction that doesn't result in damage during a slow ascent might result in overpressure during a more rapid ascent. Similarly, a restriction that doesn't result in damage during an ascent of 4 to 6' might result in overpressure during an ascent of 15 to 20'.

That's why asthma and other obstructive pulmonary diseases may be a contraindication to diving. Simply exhaling doesn't offer a 100% guarantee.

Rare anecdotes aside, what is the practical risk of lung over-expansion coming up from 10-20 ft?

As long as you're healthy and don't hold your breath the practical risk is very low. That's demonstrated by the very low number of divers who experience significant pulmonary barotrauma. OTOH, it definitely happens to a small number of people. An ascent from 20' or 10' to the surface at seal level will result in gas expansion of 60% and 30% respectively, as long as nothing restricts that expansion. If the gas can't expand the pressure will increase by 8.9 and 4.45psi respectively. The shallower you are the more rapidly the gas expands for any given ascent rate, increasing the chance that a restriction will result in some alveoli reaching 100% of their capacity. If that happens even the ascent from 10' has the potential to result in overpressure that's more than twice what your alveoli will tolerate. DCS isn't the only reason to make a slow ascent.

 

[jboneng]

As the others have said the biggest concern is lung expansion injury, the pressure at 10 feet is 33% greater than on the surface, meaning that the breathing gas you breath at that depth will expand by 33% on your way up, but as long as you have your airways open on ascent you should be fine.

An another problem, much more unlikely than the lung expansion injury, is DCS. According to the VBM family of decompression models, a dive profile like this will create bubble micronucleis, which can trigger bubble formation that can develop bubbles above critical diameter, it have been recorded instances of divers that have developed DCS after dives with max depth of 5m (15 feet), having a yo-yo profile like you describe. The chances for this to happen is small but not non-existing, if you want to take precautions against it, make sure you have a decent surface interval between each dive (like 10 min or so).

 

[mmerriman]

the dive tables were not developed for that type of dive profile.

 

[Diver0001]

Hi,

I have a question that was. Never fully answered in my diving days.

If one wanted to do a dive to 10 feet to find a shell or something and then swim to the surface can he do it safely according to the tables

how many of these can safely be done.

10 feet is 3 meters.

The pressure of the gas on your lungs (whether scuba diving or free diving) is 1.3 ata.

The current science suggests that at less than 1.6 ata no required decompression obligation can be gained. In 1908 John Scott Haldane pinned this number at 2 ata (about 30ft) which was later shown to be incorrect for certain circumstances (extremely long repetitive dives).

The obvious conclusion is that you could make an infinite number of such dives without issue.

R..

 

[phoenix31tt]

@ The OP risk is in the numbers...

I believe someone tried to represent it before... when ascending in scuba you are at a greater risk to lung over-expansion injury. Normally when diving you do one ascent and you take your time and do it properly... if you are doing multiple ascents in rapid succession you increase your risk, can become complacent etc.

Other than that, there are no other risks that i can see... you can do the dives safely as long as you ascend without closed airways everytime