Swimming to Surface Question

[jamerson]

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!

 

[Sas]

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!

I am confused by your post.

Firstly, I was taught 9m/min ascent rate, not 18m/min as per your post. I think PADI teaches 18m though. I prefer the more conservative rate of 9m/min and for the last 5m or so I ascend a lot slower than 9m/min as well.

In answer to your question, safety stops are not required, just recommended, if you are doing no-deco diving. If your post is referring to no-deco diving, then you can swim to the surface without a safety stop and per a safe ascent rate if you feel like it. Deco diving, that is a whole other story...

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?

 

[Teamcasa]

Ascending faster than 30 FPM is not advised under normal circumstances. A 60 FPM can be made safely as well and in fact, used to be the standard ascent rate. There is also the old rule of surface no faster than your smallest bubbles.

Nitrogen buildup is not the only reason to do a slow ascent. As we breathe pressurized air at depth the risk of an embolism or barotrauma is significantly increased with fast ascents.

Ascending without a safety stop is also not advised but not absolutely required if you stay well within the limits. But for your own safety, make a stop, it can not hurt and only should be skipped in an emergency.

 

[jamerson]

My apologies for making the post overly-confusing, although the info is appreciated! I think I confused myself, basically.. It took going back and looking at a simple dive table to realize that once you cross a certain depth / time line, a direct ascent is impossible, w/o serious injury.

Sometimes I need to sit on these thoughts before I ask them... :doh2:

 

[LeadTurn_SD]

My apologies for making the post overly-confusing, although the info is appreciated! I think I confused myself, basically.. It took going back and looking at a simple dive table to realize that once you cross a certain depth / time line, a direct ascent is impossible, w/o serious injury.

Sometimes I need to sit on these thoughts before I ask them... :doh2:

Your original question is a good one. No apologies needed!

60' per minute used to be the standard ascent rate that was taught (or no faster than the smallest bubbles as Teamcasa said). The "safety stop" came later, along with the slower 30' per minute ascent rate. Both changes are believed to increase safety, but do not mean that 60' per minute is "unsafe".

As to your original question: If you are within NDL limits, you can ascend directly without stops to the surface, at 60' per minute, and you'll remain within NDL limits. Should you do that? Not if you do not have to. Its safer to come up slowly, and perform a safety stop.

Safe Diving!

 

[Brian Sharpe]

As there are so many variables that can affect your susceptibility to DCS a slower ascent rate and safety stop (unless you're OOA) are good precautionary measures.

We all profess to enjoying being submerged - it's not like an extra 3 minutes underwater is a punishment!

 

[Walter]

Any time you are diving, there is the possibility of decompression sickness (DCS). There are two ways to guarantee never getting DCS.

1. Don't dive.
2. If you do dive, don't come up.

Since most of us on this board have rejected both of those choices, we need to make things reasonaly safe, realizing diving will never truly be safe.

As others have said, almost everyone has rejected the old 60 ft/min ascent rate in favor of the 30 ft/min rate. Almost everyone has adopted safety stops at 15 ft for three or more minutes. Additionally, more and more are now making deep stops. NAUI adopted the practice of making a stop at ½ the deepest depth. On deep dives, I stop at 50 ft for 1 min, 40 ft for 1 min, 30 ft for 3 minutes and 15 ft for at least 5 minutes. I believe all these changes make DCS less likely.

Having said that, many of us dived without DCS incidents prior to those changes making ascents at 60 ft/min without making a single safety stop. It was only about 20 years ago that the first changes were adopted. It is relatively safe to use the old practices, but safer to follow the new ones. In an emergency, I would ascend at 60 ft/min and not make any stops.

As for your statement:

It took going back and looking at a simple dive table to realize that once you cross a certain depth / time line, a direct ascent is impossible, w/o serious injury.

True, but not really. There's no exact point. Once you have required decompression stops, it's much more dangerous to skip stops, but it's possible to do so without getting DCS, but your chances of serious injury or death increase with the more decompression obligation you skip.

 

[DA Aquamaster]

The 60' ascent rate was a compromise developed by the navy. The people in operational control of scuba divers wanted 100' per minute while the helmet diver crowd felt that was too fast for practical use by a helmet diver and wanted a slow ascent in the 20-30 fpm range. So 60' became the compromise ascent rate used in the US Navy tables for about 40 years.

Note this bureaucratic decision had nothing to do with what was "safe". In fact, if you dive the US Navy tables to the limits (square profiles) or in deco you end up with a hit rate around 1% on a single dive and this rises to about 4% on repetetive dives. In response to those limitations and risk factors, the Navy seldom did repetetive dives and only did deco dives when on board recompression faciltities were available. Every one who dove then old navy tables dove them conservatively with one or more fudge factors built in.

If you are interested in diving long term with minimum risk of DCS, you will find yourself gravitating toward slower ascent rates, deep stops and "manadatory" safety stops and won't even pose the question you just posed.

 

[jamerson]

Thanks for all the responses!!

On deep dives, I stop at 50 ft for 1 min, 40 ft for 1 min, 30 ft for 3 minutes and 15 ft for at least 5 minutes. I believe all these changes make DCS less likely.

So, in a multi-level dive to deep depths, would maintaining these depths and times, for an example, while still swimming be considered a 'stop' - although you're not really stopped??

That said, every dive I've done we've literally stopped for the final 5 min. @ ~ 15 ft.

Have a nice day!!

 

[Reg Braithwaite]

I prefer the more conservative rate of 9m/min and for the last 5m or so I ascend a lot slower than 9m/min as well.

I explain this to myself by thinking in the change in pressure. So from 5m to the surface, you are decreasing from 1.5 ATA to 1 ATA, or 33%. The same change to 1.5 ATA is from 2.25 ATA to 1.5 ATA, or from 12.5m to 5m. The same change to 2.25 ATA is from 3.375 ATA to 2.25 ATA, or from 23.75m to 12.5m.

So if you were at 23.75m and you wanted to reduce the pressure by 50% every two minutes, for example, you would ascend at just over 5m/min to 12.5M, then slow down to 3.75 m/min to 5M, then slow down to 2.5m/min to the surface.

I'm not saying this is how anyone should ascend, just pointing out that thinking in terms of the relative change in pressure may help to explain why you need to slow down nearer the surface.

Is there any basis in deco theory for this model?