Safety stop on Oxygen or Nitrox

[Rhone Man]

Because?

Personal preference.

You were trained in the differences between 80% and 100% no?

Yes; thanks for trying not to sound too patronising.

 

[Den7]

Hello there!

I have one question for experts on ascent rates.

Let me start form a little foreword.

Common regular dive computer manuals mention two different ascent rates as follows: At depths greater than 60 feet (18 meters), ascent rates should not exceed 60 feet per minute (18 meters per minute). At depths of 60 feet (18 meters) and shallower, ascent rates should not exceed 30 feet per minute (9 meters per minute). I went through a number of various PADI class books on purpose but did not find a single word about the second ascent rate (30 feet / 9 meters) mentioned anywhere. -that would be another story-

The Oceanic computer I own (VT3) has a VARI (Variable Ascent Rate Indicator) that consists of 5 segments. The computer automatically switches its calculations in between two different basic ascent speeds of 30ft / 9m and 60ft / 18m per minute. Each segment represents your speed increase pretty much the same way like a speedometer does (it's also quite easy to calculate each segments' exact speed knowing the maximum allowed rate or simply by refering to your dive computer manuals' VARI charts).

Now my question is if the shallower ascent rate (30ft/9m) is violated (PADI classbooks mention only 60ft/18m) then I assume it can be disregarded and do not cause any decompression issues provided we do not go into deco and do not dive deeper than 30ft/9m? Does it not work as a conservative factor which in this case reduces the risk as twice as much (60:30=2 or 18:9=2) does it?

Would appreciate any advices. Thanks.

 

[DevonDiver]

The PADI ascent rates are based upon using their tables. The textbook does mention that if using a computer, you should follow the manufacturers recommendations (inc ascent rates) in all respects.

 

[Den7]

OK. Thanks. I always follow the manufacturers' recommendations but I was also wondering what others may think as I thought there was some logic in my reasonings... If I do not get exceeded nitrogen accumulations in my tissues then I could not possibly get any decompression problems is that not right?

 

[Crowley]

OK. Thanks. I always follow the manufacturers' recommendations but I was also wondering what others may think as I thought there was some logic in my reasonings... If I do not get exceeded nitrogen accumulations in my tissues then I could not possibly get any decompression problems is that not right?

No it's not right...
firstly - how do you know what the saturation limits of your tissues are? The models are purely theoretical and are accurate insofar as they have been tested. Maybe one person is a bit of an exception to the norm. There are all sorts of variables that can contribute to decompression problems - physical conditioning, body fat, illness, scar tissue, PFO, vascular/circulatory problems, heat, cold, level of activity underwater... That's why conservatism is essential.

As for the ascent rate - different studies arrive at different conclusions - in some cases an 18m per minute ascent seems to be beneficial over 9m per minute (I lost this paper, will try to dig it out again). The actual limit is somewhat of a compromise between older models and what navy divers were doing at the time - basically "we want 30ft/min - no, we want 100ft/min, okay, you can have 60ft/min" is what it boils down to - have a look at a brief history at this article:
DAN Divers Alert Network

Different computers adopted different ascent rates based on the mathematical model they were using.

Practically speaking, for recreational diving, 18m(60ft) per minute is fine - if it wasn't, lots and lots and lots of people would get bent, and they don't. It can't be a matter of pure luck that millions of divers worldwide abide by this and fail to get bent. For the same reason, back to the original post - the concept of doing a safety stop on an enriched air mixture or pure O2 is, for the vast majority of recreational dives, not relevant.

Your dive computer will calculate gas loss based on what you do. If it is programmed for different ascent rates at different depths and you violate that, it will maybe have you make a short "deco" stop or give you a "ceiling" at which you must maintain yourself for a period of time. This is purely down to the mathematics and the research behind the computer. Practically speaking, you're unwise to violate anything, even if one computer tells you different from another.

Cheers

C.

 

[LenC]

If you are not doing decompression diving, then the risks of using high levels of o2 outweigh any minimal benefits you might gain.

 

[String]

If I do not get exceeded nitrogen accumulations in my tissues then I could not possibly get any decompression problems is that not right?

Exactly wrong. Every year a certain percentage of bends are from people diving conservatively within the recommendations of their tables and/or computer.
The only way to not get decompression problems is to not dive.

 

[Den7]

Exactly wrong. Every year a certain percentage of bends are from people diving conservatively within the recommendations of their tables and/or computer.
The only way to not get decompression problems is to not dive.

Hmmm... I do not quite get it... Yes, statistics say that this "certain percentage" you mention is actually 85%...

Are you trying to say that almost every single diver will sonner or later get bends without any exceptions? The only ones who avoid it are just very very very lucky?

 

[Den7]

Thank you so much for your insights! I really do appreciate! My comments are highlighted below in red. Yes, my question was in pure mathematics where I gave you the counting out / reading point and hoped you could advise from the practical point of view. It is of course unwise to violate anything at all but doing pure mathematics and theory I thought that my simple calculations had some logic... (take a look at my first initial post)...

No it's not right...
firstly - how do you know what the saturation limits of your tissues are? I will use my computer's readings and will base on its implemented algorithm (Pelagic DSAT, Pelagic Z +, etc.). I might as well reduce the readings (conservative factor) to be on a safe side. The models are purely theoretical and are accurate insofar as they have been tested. Maybe one person is a bit of an exception to the norm. There are all sorts of variables that can contribute to decompression problems - physical conditioning, body fat, illness, scar tissue, PFO, vascular/circulatory problems, heat, cold, level of activity underwater... Agreed, it makes a big difference and adds to deco issues. That's why conservatism is essential.

As for the ascent rate - different studies arrive at different conclusions - in some cases an 18m per minute ascent seems to be beneficial over 9m per minute (I lost this paper, will try to dig it out again). The actual limit is somewhat of a compromise between older models and what navy divers were doing at the time - basically "we want 30ft/min - no, we want 100ft/min, okay, you can have 60ft/min" is what it boils down to Here you go. Despite various schools, systems, algorithms, tables, etc. it looks like everybody agreed to have one general basic rate - have a look at a brief history at this article:
DAN Divers Alert Network

Different computers adopted different ascent rates based on the mathematical model they were using.

Practically speaking, for recreational diving, 18m(60ft) per minute is fine - if it wasn't, lots and lots and lots of people would get bent, and they don't. It can't be a matter of pure luck that millions of divers worldwide abide by this and fail to get bent. For the same reason, back to the original post - the concept of doing a safety stop on an enriched air mixture or pure O2 is, for the vast majority of recreational dives, not relevant.

Your dive computer will calculate gas loss based on what you do. If it is programmed for different ascent rates at different depths and you violate that, it will maybe have you make a short "deco" stop or give you a "ceiling" at which you must maintain yourself for a period of time. This is purely down to the mathematics and the research behind the computer. Practically speaking, you're unwise to violate anything, even if one computer tells you different from another.

Cheers

C.

 

[LenC]

Hmmm... I do not quite get it... Yes, statistics say that this "certain percentage" you mention is actually 85%...

Are you trying to say that almost every single diver will sonner or later get bends without any exceptions? The only ones who avoid it are just very very very lucky?

I don't think that is what he was saying at all. If you are looking for a guarantee that you won't get bent, then don't dive. The chances of getting bent on any given dive range from unlikely to certainly, but it is never impossible. The way to keep it in the unlikely range is to maintain a conservative dive profile. Do that and you have an excellent chance of never being bent.