Why do we track depth instead of ATAs?

[Reg Braithwaite]

...neophyte training for GUE-F here...

In looking around at various resources, the subject of ascents, deco and other subjects often come up. Every DIR-ish article I've read seems to be driven by ATAs, be that ratio deco for mixed gas diving or calculating Rock Bottom for a simple OW dive on Nitrox or the ascent rate for a simple no-deco dive.

So the procedure seems to be this: Obtain depth from your bottom timer. Convert to ATAs. Do some reasonably straightforward math to get an answer in time and ATAs. Convert back to depth.

So... Maybe I am missing something... Why don't we simply use a bottom timer that reports depth in ATAs instead of distance from the surface? It seems to me this would greatly simplify things like rock bottom. Just look at the BT, read the ATAs, and work from that.

I know I'm jumping ahead of myself in some areas, but I also wonder if it wouldn't simplify certain things like ascent rates. Is it really so many feet per minute up to 20' and then so many after that? Or is it actually so many minutes per ATA and we simplify things by converting to feet?

Just wondering if really I should be "thinking in ATA" instead of "thinking in feet." And if so...

Does anybody manufacture a bottom timer that displays ATAs directly on the screen during the dive?

 

[JeffG]

Huh?

Ratio Deco is depth/time driven.

Calculating Rock bottom is before a dive even takes place.

 

[Reg Braithwaite]

Ratio Deco is depth/time driven.

True, of course, but is it really depth driven, or is it the case that the various set points and tables and other calculations are actually derived from ATAs? I need to read more, of course, but I was under the impression that the first thing you do is ascend at 9M/min. to 80% ATA, for example. And GI's writings discusses ascending extremely slowly from 20' to the surface in part because you are experiencing the greatest change in ATAs.

Calculating Rock bottom is before a dive even takes place.

Of course, but I was under the impression that you do so by calculating the ATA for the maximum depth first. And then while under water, you may recalculate the RB. For example, if doing an OW swim to a wall, you want your RB for the planned deepest part of the wall, but after you ascend back to the top of the wall, maybe you want to fool around for a while before swimming back to the boat. Wouldn't we calculate another RB for the top of the wall since we have the option of a direct ascent to the surface?

And again, isn't this an ATA calculation rather than a depth calculation?

 

[Walter]

Why don't we simply use a bottom timer that reports depth in ATAs instead of distance from the surface?

Unless you have a measured line, you are not recording distance from the surface. Fsw, ffw msw and mfw are measurements of pressure, not distance. One fsw is close to, but not equal to one linear foot from the surface.

Also, bottom timers measure time and nothing else. Computers and depth gauges measure pressure and report it as depth.

 

[JeffG]

Learning to dive on the internet is a bitch.

 

[Bismark]

Learning to dive on the internet is a bitch.

ROFLMAO..................made my day............so far

 

[Bismark]

True, of course, but is it really depth driven, or is it the case that the various set points and tables and other calculations are actually derived from ATAs? I need to read more, of course, but I was under the impression that the first thing you do is ascend at 9M/min. to 80% ATA, for example. And GI's writings discusses ascending extremely slowly from 20' to the surface in part because you are experiencing the greatest change in ATAs.



Of course, but I was under the impression that you do so by calculating the ATA for the maximum depth first. And then while under water, you may recalculate the RB. For example, if doing an OW swim to a wall, you want your RB for the planned deepest part of the wall, but after you ascend back to the top of the wall, maybe you want to fool around for a while before swimming back to the boat. Wouldn't we calculate another RB for the top of the wall since we have the option of a direct ascent to the surface?

And again, isn't this an ATA calculation rather than a depth calculation?

Reg, everyone honestly appreciates your enthusiasm and interest, so don't take the humour at your expense to heart too much. We laugh at ourselves all the time.

You are correct in that you should be able to do the calculations between ATA's and Ft and as you gain more experience doing so, it becomes easier. However, don't forget to read the fine print. For RD, it is 80% your ATA, OR....75% of your depth in FSW. If you do the math for several examples, you will see that these numbers are pretty close but they start to diverge at greater depths and ATA's. My guess is you are a long ways away from needing to worry about this yet........

Also, we use feet because it is something we can wrap our brains around and something we are familiar with. I see you are also from Canada, so if you want to see things get a lot easier, you could do this all in metric. The math becomes a lot easier and things and you don't have to divide by 33. Unfortunately, unless you are in Europe or the the Middle East or some parts of the South Pacific, you will most likely see Feet and PSI instead of Meters and Bars.

Finally, you are correct in that you can recalculate your RB (or Minimum Gas, depending on who trained you and when......) and change this as your profile changes. This will keep you pretty busy for a while, so I suggest instead that you simply calculate your RB's for a couple of depths and use those frequently enough that they become memorized. Write them in your wetnotes and you can refer to them in your dive if you wish. Pretty soon, you will not need to refer to them.

I applaud your decision to take a Fundamentals class. A lot of this will be covered in class, but no harm in writing down any questions you may have and bring them to the class for your instructor. If you are in Eastern Canada, you are a stones throw from Kingston where there are several Fundamentals classes scheduled each year. Good luck and remember there are no stupid questions, just watch out for stupid answers.

 

[Bismark]

Reg, BTW, you are going to have to lose a bunch of D-rings and get your mask off your forehead on your avatar............(just joshing ya')...........

Best,

G

 

[Randy43068]

Reg, BTW, you are going to have to lose a bunch of D-rings and get your mask off your forehead on your avatar............(just joshing ya')...........

Best,

G

No worries, Fundies instructors have bolt cutters handy.

 

[Reg Braithwaite]

Reg, everyone honestly appreciates your enthusiasm and interest, so don't take the humour at your expense to heart too much. We laugh at ourselves all the time.

Don't worry, anyone who looks into DIR training realizes that you cannot swing a cat on a DIR forum without hitting someone with a sense of humour.

Also, we use feet because it is something we can wrap our brains around and something we are familiar with. I see you are also from Canada, so if you want to see things get a lot easier, you could do this all in metric. The math becomes a lot easier and things and you don't have to divide by 33.

The math is easy so far, in fact DIR math is much easier than just about anything else except "Ooh shiny computer, trust me." (And yes, I bought a computer two seasons ago, but the first thing I did was look up the model it uses and research what it thinks it is telling me).

It is just a case of my wondering what diving would be like if I had a pressure gauge strapped to my wrist that reported ATA directly. FWIW, everyone in diving up here seems to use Imperial, so feet are no problem whatsoever.

I applaud your decision to take a Fundamentals class. A lot of this will be covered in class, but no harm in writing down any questions you may have and bring them to the class for your instructor. If you are in Eastern Canada, you are a stones throw from Kingston where there are several Fundamentals classes scheduled each year. Good luck and remember there are no stupid questions, just watch out for stupid answers.

Dan "Dressed for Success" Mackay is running GUE-F up here, and we are trying to work out a Toronto session next season. I don't mean to trivialize it, but the math side of GUE-F does not strike me as the hard part of getting a pass. I would say that the big issue is getting my skills down so that my body does what it needs to do without any attention on my part, leaving my conscious mind free to work on situational awareness and doing any calculations that are needed. This strikes me as a big challenge.

In fact, I am off to the pool tonight to work the basic five and one minute ascents. Worse than shore diving, but better than surfing the Internet.