Swimming to Surface Question

[Teamcasa]

I learned the 60 feet/min limit in my open water certification but never learned specifically why it was necessary. From what I've read in the many great responses to the OP's questions, the ascent rate limit is there (1) to help prevent DCS and (2) to help prevent "embolism or barotrauma." I'm not clear exactly how the ascent rate limit helps this.

Could someone explain (1) why, if you've dived within the NDL, you need to limit your ascent rate, and (2) what are the "embolism or barotrauma" risks (does that include lung overexpansion injury?) of a fast ascent?

Thanks in advance.

I'll explain in non-scientific terms.
Ascending faster than 60fpm will/can allow small micro bubbles in your blood stream and other tissues to expand. It's like when you take a bag of chips up a mountain. Once you reach the top of the mountain, the bag has expanded to that ready to pop mode.

The same thing happens in your body. As you ascend that pressure wants to go somewhere fast. If you ascend to fast the growing bubbles can rupture tissue or get stuck in joints, your blood stream and other seriously bad places.

A slow assent allows those bubbles to be expelled in the lungs and out with each exhale.

 

[dougchartier]

I'll explain in non-scientific terms.
Ascending faster than 60fpm will/can allow small micro bubbles in your blood stream and other tissues to expand. It's like when you take a bag of chips up a mountain. Once you reach the top of the mountain, the bag has expanded to that ready to pop mode.

The same thing happens in your body. As you ascend that pressure wants to go somewhere fast. If you ascend to fast the growing bubbles can rupture tissue or get stuck in joints, your blood stream and other seriously bad places.

A slow assent allows those bubbles to be expelled in the lungs and out with each exhale.

Thanks to you and others for the helpful replies. What are these "micro bubbles" made of? Nitrogen (assuming you were breathing compressed air)?

 

[bfw]

I am wondering if I am misinterpreting your post though as this stuff is covered quite extensively in OW training so maybe I am missing what you are actually asking?

That was my reaction as well.

If you're currently undergoing OW training, then you should be asking your instructor to explain this, not a bunch of faceless text posts on the internet. Your instructor, in a face to face interactive context, can better determine if his explanation is making sense to you, and there is the immediate give and take of questioning.

If you have recently successfully completed OW training, you need to go to your instructor, and either A) ask him to fill in the blanks he left in your knowledge, or, B) if he DID cover this, ask him why he passed you without determining that you understood it.

This isn't some esoterica that awaits in a future tech course - it's basic conceptual understanding of decompression, which every certified diver should have. Your instructor shouldn't certify you without putting in the time and effort to make sure you understand it. If he did, you didn't get your money's worth.

 

[Nemrod]

Hi all,


I'm fairly new to diving and had a question I couldn't answer. As I understand, the general safe ascent rate is about 1 foot a sec (or 60 feet in a minute). Theoretically then, if you didn't have any nitrogen built up in your body, you could swim safely to the surface, exhaling, from 60 feet (right?)..

My question has to do with longer or multilevel diving when nitrogen has built up. After a certain depth or period of time, is it impossible to swim safely to the surface (from 60 feet or above) without a safety stop?

For example, if I went down to 90 feet for a few minutes, then spent the rest of the dive at, say, 45, does the nitrogen exit such that, at some point, I could swim directly to the surface w/o safety stop?

How can you calculate such a thing, minus a dive computer? Is that what that wheel is for??

Sorry if the post doesn't make sense..

Also: I have no desire to put this to the test. Just curious to know the math behind it...

Thanks!

A safety stop is not a required stop. You can ascend from 60 feet after 59 minutes directly to the surface at 60 FPM. I am old school, per the original Navy Tables. No deco limit for 60 feet was 60 minutes. No stops required.

N

 

[bleeb]

The ascent rate is an integral part of the model from which the table are calculated. Change the ascent rate, you change the M-values and you change the No-D limits. Slower is okay, as long as its not too slow, but faster breaks assumptions required by the model.

Out of curiosity, which agencies' tables specify which ascent rates? Or maybe more directly pertinent to the original posting, which agencies still teach 60 feet per minute for basic OW? The OP doesn't have hundreds of dives so I guessed the certification was relatively recent. Might be worth asking if you're buddying up with someone new and not diving on computers.

I'll start the list:
- NAUI 1989 table, NAUI 1997 Nitrox tables (32 & 36): 30 feet per minute
- DCIEM 1995 table: 50 +/- 10 feet per minute (i.e. <60)
- USN 2008 dive manual: 30 +/- 10 feet per minute

Pretty sure this topic has come up before, but couldn't come up with any decent combination of search terms.

 

[Dan Weidert]

"For example, if I went down to 90 feet for a few minutes, then spent the rest of the dive at, say, 45, does the nitrogen exit such that, at some point, I could swim directly to the surface w/o safety stop?

How can you calculate such a thing, minus a dive computer? Is that what that wheel is for??"

Jamerson,

It might be easier to start with a more generic view of what is happening. The first step is to remember that things tend to want to go from higher pressures/ concentrations/ energy levels to lower ones until things are about equal. We are more or less nitrogen saturated all the time and it doesn't really matter because we are roughly in equilibrium with the air we are breathing. If we were to dramatically reduce the air pressure, say to 4 PSI, we would run the risk of DCS. Interestingly, this is exactly the scenario for astronauts who have to desat nitrogen before spacewalks in a 4 PSI spacesuit.

When we dive, we change the balance in the other direction. Because the air pressure increases as we descend, the air we are breathing has a higher concentration (partial pressure) of nitrogen than our bodies and we begin to absorb more nitrogen. This will continue until the nitrogen level in our body (partial pressure) is equal to or greater than the nitrogen partial pressure in the air we are breathing.

So in the scenario you mentioned above, you could dive to 90 feet and be on gassing nitrogen, then spend the rest of your dive at 45 feet and continue absorbing nitrogen but at a slower rate. This would continue until you were saturated or you started to ascend (the more likely scenario).

At some point in your ascent, the partial pressure of nitrogen in your body will be greater than the partial pressure of the nitrogen you are breathing and you will start off gassing nitrogen. If the difference becomes too great, bubbles can form and your whole day is ruined.

Percentage wise, the greatest change in pressure happens in the last 30 feet of ascent. This means that the greatest difference in pressure between nitrogen in your body and in the air you are breathing will probably happen in the last 30 feet as well (think of uncapping a soda bottle).

To make things even more complicated, different tissues in the body absorb nitrogen at different rates. They also release nitrogen at different rates. Cold, hydration, conditioning, and CO2 concentrations are other factors that may impact nitrogen loading and DCS.

The various formulas used to calculate bottom times (dive table, dive wheel, computer) are really mathematical guidelines based on experience and some testing. They are not hard measures of when DCS will or will not hit.

Moral of the story is "Plan your dive and dive your plan."

Hope this helps,
Dan

 

[Thalassamania]

PADI tables are still at 60 FPM.

 

[Tortuga68]

This may be of interest: http://www.scubaboard.com/forums/archive/index.php/t-4109.html

I'd also highlight this point: "As far as the US Navy tables are concerned, there is a few percent of divers that will get DCS. Much of the risk is very diver-activity dependent, and remember that these tables are for decompression diving . US Navy divers are WORKING DIVERS"

 

[Web Monkey]

Although you probably already got more information than you ever wanted, I'm not sure if you ever got your questions answered.

I'm fairly new to diving and had a question I couldn't answer. As I understand, the general safe ascent rate is about 1 foot a sec (or 60 feet in a minute). Theoretically then, if you didn't have any nitrogen built up in your body, you could swim safely to the surface, exhaling, from 60 feet (right?)

...

My question has to do with longer or multilevel diving when nitrogen has built up. After a certain depth or period of time, is it impossible to swim safely to the surface (from 60 feet or above) without a safety stop?

The maximum recommended ascent rate is currently 30'/minute. It had been 60'/min, but was reduced to a safer limit.

You seem to be thinking about an Emergency Out of Air Ascent. As long as you're well within Recreational No-Decompression Table Limits, you should be able to make a direct ascent to the surface without getting killed . This doesn't mean that it's always safe, only that if you run out of air, a direct ascent to the surface should be safer than staying where you are and drowning.

Nitrogen off-gassing in humans isn't precisely predictable. If you do a 5 minute dive to 20', you can almost certainly surface at any time with little risk. If you do a 40 minute dive to 60', you can still ascend directly to the surface, however the risk of DCS is greater. Even though there is more risk, again, it's safer than drowning.

The actual limits depend on the individual divers' physiology as well as the dive conditions and aren't predictable, so a person may bolt to the surface on a particular dive with no ill effects, then do it again a week later and require medical attention.

There are depth/time combinations where a direct ascent to the surface is not possible because the chances of getting DCS are much greater. These are called "Decompression Dives" (as apposed to "No Decompression" as is listed on your dive tables).

Multi-level diving has the same effects as diving a square profile, except that it's more difficult to calculate in your head. The problem, however remains the same. "More time and more depth" = "more risk of DCS" if you bolt to the surface.

The question you didn't ask implies an unsafe practice that you would want to avoid: You need to either dive within the table limits, which means treating a multi-level dive as if it were a dive to the deepest depth you went to (as was taught in your OW class), or use a computer. In either case, as long as you're within the No Decompression Limits, a direct ascent to the surface should be possible.

Whether you'll have any damage depends on the time and depth and your personal physiology.

Terry

 

[Nemrod]

Using the standard, original Navy Tables adjusted for a 10% safety factor/fudge factor and a 30 FPM ascent rate above 30 feet coupled with a safety stop at 10 to 15 feet is my general non complex dive profile. I would rather do my own adjusting to suit my dive and me.

There was a thread some weeks ago about people getting bent in five feet of water if y'all recall, I would say anyone who got bent in 5 feet should not dive, standard tables should fit the majority if applied reasonably with prudence. N