Shallow Decompression Dives

[Kevrumbo]

Interesting & disconcerting . . .one of the case studies in my Ratio Deco Seminar two years ago was the typical Ginnie Springs Profile --on how cave divers were rushing to the log to breath their Oxygen deco bottles, and not spending any time at the deep stops, subsequently getting bent (type I DCS).

 

[Deefstes]

Not really.

You track how much pressure you use and relate that back to volume with the end goal of understanding how much gas you consume per unit time * depth.

Sure, you may be computing Liters/Bar as opposed to CF/PSI, but that's not a fundamental difference.

Right?

Actually it is a fundamental difference, at least to my mind it is. Gas planning involves not just calculating how much gas you would need and what your critical pressure would be but also what cylinder size would be sufficient to provide that. Cylinders measured in imperial units are even measured differently, their sizes are given as the volume of gas they contain when filled to working pressure. Metric cylinder sizes are given in their dry volume. Apart from that, calculating air consumption is slightly easier when doing so in metric units because translating from meters depth to ATA in bar is a fairly straight forward matter. With imperial units it is slightly more difficult.

For many of us this is not a big problem but there are people who have zero aptitude for maths and this certainly requires some mathematical skills.

My point is that you can't expect a new diver to know EVERYTHING. What is the age requirement for open water students, 15 years? I would think the math behind gas planning (and especially if you have to do it both ways, imperial and metric) is pretty advanced math for a 15 year old. Even 30 year olds, I know some who just don't have that sort of mathematical prowess, not because they're stupid but because their aptitudes in life differ from ours.

I would love to believe that even these people can become divers, if only Open Water divers then. Sure, if you don't have the faintest notion of math, perhaps advanced staged decompression diving on various gas mixes is not your thing but surely you can dive to 18m depths with charters where you know there's a boat waiting for you at the surface and simply follow the "I start heading for the surface when I hit 50 bar" rule.

Just my thoughts, take it for what its worth because I'm a fairly new diver myself.

 

[Thalassamania]

Gas planning, it's naught but arithmetic, 15 year olds are in their first or second year of high school and should be able to handle it.

 

[Kevrumbo]

Here Deefstes: a reference value using the most common tank out there, the AL80.

Double AL80's are rated in the metric system as 22 liters/bar.
Now use a fairly common & conservative SAC rate of 22 liters/min*ATA (which happens to be my nominal cold water SAC rate).

Divide 22 liters/min*ATA by 22 liters/bar which equals 1 bar/min*ATA

Now all you need to know is your depth in meters (translates to ATA very easily) and multiply it by the time at that depth (for example, a 10 minute interval), and you now have an idea how much your gas usage is in that interval before even looking at your SPG. . .

(If all you're using is a single AL80, then the reference value becomes 2 bar/min*ATA, so multiply depth in ATA by time interval at depth, and multiply again by 2 bar/min*ATA)

 

[vladimir]

I have never in my life encountered an AL80. It's an American concept and anywhere outside of the States you will be diving with cylinders in metric units. Moreover, not only are the cylinder sizes given in metric units but also as a physical size of the cylinder - as opposed to the volume of gas in a cylinder at working pressure.

Perhaps because many dive operators cater to traveling Americans, all of the cylinders I have used outside the United States--90%of my diving--have been AL80s. I agree that describing cylinder size using liters makes a lot more sense.

Even 30 year olds, I know some who just don't have that sort of mathematical prowess, not because they're stupid but because their aptitudes in life differ from ours.

As a math graduate I am clearly not objective, but I have a word for people with "different aptitudes": stupid. Just kidding , but we're really talking about arithmetic here, and it should be well within the grasp of a 10-year old.

Gas planning, it's naught but arithmetic, 15 year olds are in their first or second year of high school and should be able to handle it.

Is the new gold user name equivalent to an honorary degree, or does it come with actual responsibilities?

 

[Deefstes]

Exactly Kevrumbo, and that is all done in metric units. I know it very well because that's what I was taught. Ask me to do those calculations in imperial units and I'll probably struggle a bit more. Ask someone without any love or aptitude for maths to do it and you'lll be met with a blank expression. You started with a metric cylinder size, used metric pressure units and metric depth units. Yes, the math is then very easy. Now a diver used to all that finds himself diving in the States with rental gear. The charter tells hims, here's an AL80, filled to working pressure, have fun.

He now has to know how AL80 translates to metric units, how working pressure (a phrase he will likely never have heard) translates to metric pressure units and how the psi and fsw readings on his gauges translate to metric units. Should he have been taught all this in his OW course back in South Africa or is it acceptable that he knows nothing of it, until he starts planning a dive vacation to the States, at which time it will be his responsibility to familiarise himself with the way things will work in the States?

Where do you draw the line between what is necessary to know by the end of his OW course and what can wait for future? My opinion is that diving in imperial units (if you live in a metric country) doesn't have to be taught in OW just as gas planning for deco diving doesn't have to be taught either. Both of those can wait for when the diver will start needing it. Rather allow him now to first get to grips with Dalton's and Henry's laws. Many peeps struggle just getting their heads around even that.

 

[waterpirate]

Well if you run V-Planner at nominal conservatism and use O2 for deco you can get that profile.

Exaxctly, bingo.
This profile was planned for hp 100 doubles, with 40cu of 70% for clean up. With the nominal obligation is this truly a deco dive or just a dive requiring an ass load of gas that allows for an imediate ascent to the 3 minute safety stop at any point in the dive?
Will I dive this profile? Absolutely. Safety margin of 1 dive only 24 hour si minimum.

The sooner new divers are given alllll the information the sooner they can make their own decisions on risk management, gas management, gas choice, dive planning.
Eric

 

[Kevrumbo]

Exactly Kevrumbo, and that is all done in metric units. I know it very well because that's what I was taught. Ask me to do those calculations in imperial units and I'll probably struggle a bit more. Ask someone without any love or aptitude for maths to do it and you'lll be met with a blank expression. You started with a metric cylinder size, used metric pressure units and metric depth units. Yes, the math is then very easy. Now a diver used to all that finds himself diving in the States with rental gear. The charter tells hims, here's an AL80, filled to working pressure, have fun.

He now has to know how AL80 translates to metric units, how working pressure (a phrase he will likely never have heard) translates to metric pressure units and how the psi and fsw readings on his gauges translate to metric units. Should he have been taught all this in his OW course back in South Africa or is it acceptable that he knows nothing of it, until he starts planning a dive vacation to the States, at which time it will be his responsibility to familiarise himself with the way things will work in the States?

Where do you draw the line between what is necessary to know by the end of his OW course and what can wait for future? My opinion is that diving in imperial units (if you live in a metric country) doesn't have to be taught in OW just as gas planning for deco diving doesn't have to be taught either. Both of those can wait for when the diver will start needing it. Rather allow him now to first get to grips with Dalton's and Henry's laws. Many peeps struggle just getting their heads around even that.

The converse unfortunately is true as well --ask or show any basic OW diver trained & certified here in the States (or even some seasoned tech divers here on this & other boards) about the virtues & advantages of using the metric system for gas/deco planning and real time on-the-fly management, and they'll give you a blank stare as well. All the positive responses with comprehension to my bar SPG post --which I have posted many times over the years-- have come from divers trained in countries using the metric system (which is every country in the world except the United States . . .and Liberia?). . .

 

[Blackwood]

Actually it is a fundamental difference, at least to my mind it is. Gas planning involves not just calculating how much gas you would need and what your critical pressure would be but also what cylinder size would be sufficient to provide that. Cylinders measured in imperial units are even measured differently, their sizes are given as the volume of gas they contain when filled to working pressure. Metric cylinder sizes are given in their dry volume. Apart from that, calculating air consumption is slightly easier when doing so in metric units because translating from meters depth to ATA in bar is a fairly straight forward matter. With imperial units it is slightly more difficult.

I disagree. To my mind they're fundamentally the same. You could measure your volume in cubic miles and pressure in micro pascals as long as at the end of the day you have something in there representing volume/time*pressure, and something in there representing volume/pressure.


I believe that as long as someone understands the calculations, they can also do them in imperial or metric.

He now has to know how AL80 translates to metric units, how working pressure (a phrase he will likely never have heard) translates to metric pressure units and how the psi and fsw readings on his gauges translate to metric units.

No he doesn't. Unless he's diving imperial tanks and metric gauges, or metric tanks and imperial gauges, all he needs to know is how tank volume relates to gauge pressure, how much volume he consumes in a given time at a given ambient pressure, the relationship between depth and ambient pressure, and depth. Units don't matter.

On the fly, sure, it's easier to divide by 10 than by 33. But as a whole the idea is fundamentally the same.

 

[Deefstes]

The converse unfortunately is true as well --ask or show any basic OW diver trained & certified here in the States (or even some seasoned tech divers here on this & other boards) about the virtues & advantages of using the metric system for gas/deco planning and real time on-the-fly management, and they'll give you a blank stare as well.

But see, that's exactly my point. Teach a diver what he needs to know. There's no need to teach a diver in a metric country to do imperial math and understand how equipment in imperial countries are named. I used that as an example of something that doesn't make sense to teach someone who won't be using it. My real point was that gas planning for deco diving and the math that accompanies it, will probably not be necessary for Open Water divers, so why teach it to them? Sure, if they want to learn about it, give it to them, but don't make it a requirement for them to pass their OW certification. That's all I said.

I may still be wrong in this, I mean you guys certainly know a lot more than I do, but I just thought that some shortcuts are acceptable. Like this one: "When my gauge shows 50 Bar, I end the dive. That will leave me (and my buddy if necessary) with enough gas to do a safe ascent and a decent safety stop."

Do you guys not think that, for OW divers, the above shortcut is sufficient, rather than having to calculate:
1. How much gas will I need to go down to maximum depth and stay there for x minutes?
2. How much gas will I need to make an ascent from that depth at 10m/min (remembering to calculate, not the average depth, but the average ATA for that duration)?
3. How much gas will I need for each of my stops for their respective durations (knowing I will only ever do one stop in OW anyway)?
4. What is my critical pressure?

To some of you this is "basic" arithmetic. To some it isn't, if it was a requirement for them to pass OW, they wouldn't pass OW. Call that Deefstes' LOAF rule if you want but I just thought that the 50 Bar shortcut is good enough for ANY scenario that an OW diver would find himself in if he dives within the limits of OW. So why exclude the Deefstes' LOAFs if there is such a shortcut?