Numbers of diving

[lunar girl]

Hi everyone.
I have been hanging out on this board for about a year and have picked up so much and feel it that has helped me become a better diver. I only recently signed up on scubaboard because I have some questions that are important to me I would like answered.
I have logged 138 dives so far. I was AOW and Nitrox certified about five years ago (approx 75 dives ago). I definitely understand the dangers of nitrogen levels in your system. I believe I understand and have seen how easy someone can get narced…even in depths as little as 30 feet. I also I understand the hows and whys of oxygen toxicity.

But reading some of the posts, such as the ones in the recent Recreational Limits post (OP 2004 but recently re-opened Jan 08), I just feel dumb when reading it...and I don't consider myself dumb. I did my Nitrox certification. Out of 10 people, I was the one showing everyone the "common sense" of the formulas. But I admit, that was a while ago. I have my computer, which I will also admit I rely on too often. But I do know how to use my dive tables for diving with air and Nitrox and just to make sure I remember, use them in addition to my computer just to make sure I dive within my limits. But I was never able to spout off some of the things I read in that thread. Such as…

“At 130 feet, I guess you're breathing a partial pressure of oxygen equivalent to breathing 100% at the surface.. but even a ppO2 of 1 is way lower than even the most conservative limits of 1.4 and 1.2. Assuming an O2 percentage of 21, 130 feet give you a ppO2 of between 1.0 and 1.1. You'll need to reach 190 feet before you violate the 1.4 ppO2 limit.”

“The basic premise is based on Dalton's Law. At 132 feet you are close to breathing the equivalent of 100% O2 at the surface. One atmosphere is 14.7psi and at 132 feet you are breathing the equivalent of 1 atmosphere worth of O2...or ppO2 = 1 ata. Since 100% O2 is toxic...especially with increasing pressure...the suggested recreational depth limit was set at 130 feet. However...Oxygen toxicity is based on a dose-time relationship and not just dose. So it is very, very doubtful that one would suffer an ox tox hit at 132 feet within the NDL for a 132 ft dive. Nevertheless...since no one knows his or her tolerance to dosing of 100% O2, and since it is "possible" to get an ox tox hit at 132 feet...it was deemed a play it safe recommendation to keep the recreational dive limit at 130 feet.”

OK so assuming they are not looking at their tables, and recognizing the fact that I am a vacation diver, should I be able to spout off numbers like that? What am I missing or do a lot of people recognize that they don’t get the numbers the way other people do and just rely on the tables? Am I putting myself at a greater risk because I don’t see the depth of the numbers? Is this something that is just inherent in some people? Are these facts things you grasp with more experience?

Although I am a vacation diver, I want to be good at what I do. If I am missing something and need to pay more attention to other aspects of my dive, then let me know. I don’t want to get ripped personally by asking the question, but I am asking with an open mind because I want to be a better diver.

Thanks for your help and input!
ME

 

[mkutyna]

You may want to write a little cheat sheet about what the most common formulas and standard numbers that you'll encounter during your dive trips and then laminate it. It'll be small enough to take on trips with you and durable enough that you won't have to re-create it every time you go on a trip. Of course, there's something to be said for having a digital copy of the sheet and update it before a dive with any new info you've learned since your last trip.

Personally, I think as long as you understand the concepts and have access to what you need to calculate maximum operating depths, best mixes, etc then you don't really have to memorize these formulas or constants for typical recreational diving.

 

[seaducer]

I think alot of that is the product of advanced training and an insatiable curiousity with regards to diving. As long as you understand what type of diving you are doing and stay within the limits of your training, gear, experience and comfort level you will be fine, even if you can't do the math in your head or quote formulas in abstract.

 

[do it easy]

That's quite the question. I guess the short answer is maybe and the long answer is yes.

For everyday recreational diving, you need to know a few things very well- things like how to figure out your ppO2 and how that might limit or affect your dive in terms of depth, time, and gas.

That being said, yes, there are some people who are bigger s than others. In the quotes you picked out, they were using some of the "easy" numbers- 130' is roughly 5 ATAs, so air (21%) at 5 ATAs is roughly 1 ATA ppO2 (or 1.04 if you want to get your geek on).

In the perfect world, all nitrox certified divers should be able to do "back of the napkin" calculations to figure out their ppO2 for a given gas and depth. In the real world, there are a few basic things you need to know to dive safely.

The fact that your asking the question is a good sign.

BTW: welcome to :sblogo:

 

[dkatchalov]

It is the appetite to know everything there is to know about diving.

I too have read that thread with great interest and whilst I understand the logic and some of the numbers behind the calculations I am now trying to read up on what I don't understand. I just enjoy it and like doing it.

 

[dkatchalov]

130' is roughly 5 ATAs, so air (21%) at 5 ATAs is roughly 1 ATA ppO2 (or 1.04 if you want to get your geek on).

Bingo! That's the part I could not get. It make sense. If air is 21% oxygen at sea level, under pressure of 5 atm it becomes equivalent to 100% oxygen at sea level (or 5 x 21% = 105%).

So I get the maths now the why. I think I understand some of the why regarding O2 toxicity - part of is due to the fact that breathing pure O2 is toxic. But there are other "whys" I cannot put my finger. Need to keep thinking about this.

Great thread.

 

[TSandM]

Some of us play with this stuff all the time . . . And technical divers HAVE to play with it all the time, to figure out what mixes they can use for the various dives they do.

I've also been lucky enough to sit through a whole teaching session on "Scuba Math", or how to do rapid fire and good enough calculations of SAC rate, required gas volumes, ppO2s, OTUs, etc. Add a bunch of practice, and those of us who do this can rattle off those kinds of numbers.

Do you need to know this to be a good recreational diver? If you dive Nitrox, you should know how to calculate ppO2 and MOD. If you're doing deep dives, you really ought to know about the concept of rock bottom, or minimum gas, and how to calculate needed gas for the contemplated dive. That means you should have an idea what your normal range of gas consumption is.

If your diving is generally 60 foot reef dives with live boat pickup, you don't need to know very much of this math at all.

 

[vshearer]

Breathing pure O2 at the surface has issues not related to CNS hits. The original 130' was adopted from Navy NDL limits and is not related to PO2. That quote is misleading. The more relevant issue at 130 that made it acceptable was narcosis.

 

[Kern]

ppO2 = fraction of O2 X ambient pressure. (convert depth to get ambient pressure)

a little algebra to readjust the formula for best mix, divide both sides by ambient pressure,

ppO2 / ambient pressure = faction of O2.

another readjustment of the formula for MOD, divide both sides of the 1st formula by fraction of O2

ppO2/ frO2 = ambient pressure. convert ambient pressure to depth

So, remember 1 formula, a little algebraic manipulation & with any 2 of the 3 pieces of info required to plug in & you have your

ppO2 for any gas mix at any depth
best mix for any depth at any ppO2
MOD for any fraction of O2 at any ppO2

 

[dkatchalov]

ppO2 = fraction of O2 X ambient pressure. (convert depth to get ambient pressure)

a little algebra to readjust the formula for best mix, divide both sides by ambient pressure,

ppO2 / ambient pressure = faction of O2.

another readjustment of the formula for MOD, divide both sides of the 1st formula by fraction of O2

ppO2/ frO2 = ambient pressure. convert ambient pressure to depth

So, remember 1 formula, a little algebraic manipulation & with any 2 of the 3 pieces of info required to plug in & you have your

ppO2 for any gas mix at any depth
best mix for any depth at any ppO2
MOD for any fraction of O2 at any ppO2

That is a good, clear summary of the math (for a noob like me). I get the math, thats the easy part.

Why is it though that air @ 140fsw = 100 O2 @ sea level? What happens to the nitrogen in the air at 140fsw? That is the part I'm still struggling with. I know that the nitrogen is absorbed into your body, etc.

Or am I on the wrong track completely here? Is it just the physiological effect of air @ 140 fsw = O2 @ sea level?