Dive planning/ Air consumption question

[alpaj]

Forgot to mention that V is the internal volume of the tank. If the world worked on metric units life would be so much easier!

 

[nauidiver2004]

What you are asking is not a dumb question! Several great replies already to get you started so no need for me to repost all that info again. I was fortunate to have an AOW instructor who tought this information, I am surprized at how many divers really have no idea how to plan their gas usage.

Good for you for asking, and good luck

Ken

 

[Dibblerr]

Forgot to mention that V is the internal volume of the tank. If the world worked on metric units life would be so much easier!

Down Here we do!

SAC ~20 Litres / Min

12 Litre Tank Pumped to 230Bar = 2760 Litres

Keeping a ~50 Bar Reserve = 600 Litres Reserve

Useable 2160 Litres

at 30 Mtrs = 4ATA using 80 Litres / Min

2160/80 = 27 Minutes Gas Time

Substitute your own SAC & Reserve & Depths etc
Obviously this is over simplified as it does not allow for descents, ascents, stops and assumes a constant depth but you get the idea

Seems so easy, I see people argueing that cfm is the best way... Ho Hum

 

[Kim]

I admit that I'm a bit of a "cheater" when it comes to calculating my SAC and DCR rates since I use my Dive Software to figure it out for me after I download my dive information from my Dive Computer.

Although I was trained originally to use the method I have outlined earlier I also now do it like this. This has several added advantages. First off it accurately measures depth and time all the way through the dive from beginning to end so the SAC rate produced is very accurate. Secondly it produces a SAC rate for every dive that you do so it soon becomes clear what kind of dives use a higher air consumption, (try comparing a easy drift dive against that one you had to fight the current!!!) Thirdly it shows you what your potentially highest SAC rate is (that one rounded up is the one you want to use!) Lastly it shows you how your SAC rate gradually improves with experience. All you need to do is input your tank type/size and your begin and end pressures.
I use a Suunto computer - I know that their software does this. While I don't know about other computers I should imagine that most if not all do. For me the ability to download dives from a dive computer and be able to see this information would be essential in any one that I bought.

Of course - Uncle Pug would tell me that I'm rotting my brain - he's probably right as well but hell, I LIKE computers!

p.s. dibblerr is quite right - metric is SO much easier when calculating this stuff in your head!

 

[Scuba_Steve]

I wonder if the OW course designers of the rec agencies are reading this.

It completely boggles my mind just how many folks see this as required info from about day 1, and yet somehow it's never gotten into the cirruculum.

This is a perfect snap-shot of how hooped OW training is.

It would take all of about an hour to teach this inside-out and backwards to anyone with grade 6 math.

"Useful info"..........lol

I'd say so

 

[Thalassamania]

This is a perfect snap-shot of how hooped OW training is.

You mean ADVANCED open water, no?

 

[Scuba_Steve]

You mean ADVANCED open water, no?

Well, I guess I really meant the whole damn process up to Instructor really.....well CD even, now that I think of it.

 

[MXGratefulDiver]

You know, I feel dumb asking this, but:

I just got OW certified (PADI). I'm in the process of getting AOW certified- did the first half of the class, waiting for everyone's schedule to synch up so we can complete it. I've read the texts, bought some gear, and I love it, but it suddenly occurred to me: I don't have the slightest idea how much air is in the tanks I've been renting. I understand that they have, like, 2600 PSI when I get them filled, but.....so what? I can pressurize a very small container to 2600, theoretically, and have, like, enough air for one breath in it, right? so pressure doesn't mean much without knowing volume, sure....and I understand, of course, that as depth increases, so does pressure, so that at 33 feet, consumption doubles, etc, etc, yadda yadda, but.....there isn't anything about this in the textbooks. I found one table of multipliers for calculating air consumption rates at depth, in the AOW guide, under the "deep dive" chapter, which is a starting point, but, well, it doesn't have a starting point itself, because you're supposed to see what your surface rate is, and go from there...

Anyway, I was kind of surprised that there wasn't any substantial data about this in the textbooks. I mean, at what PSI are you dangerously low on air, anyway? That might be a helpful nugget of information to throw in, wouldn't it? I'm gonna sit down and scour both texts again, just to make sure I didn't miss anything, but I'm pretty sure I didn't. Any advice/ comments/ insults, and especially recommendations on better reading material would be greatly appreciated

I love new divers who ask good questions ...

You are quite correct that the info won't do you much good without a starting point. That starting point is called Surface Air Consumption ... and it's a way for you to find out how much air you consume when you breathe. The reason they define it as a unit of consumption on the surface is because your actual air consumption will depend on the depth at which you are diving ... and it will be proportional to the pressure that the water exerts on you at a given depth. Since the pressure at the surface is one "atmosphere"... and multiplication by one is simple ... they use that as your starting point.

Consider your air consumption like the MPG sticker you see on the window of a new car ... it's not a hard-n-fast number, but rather a measure of "average" conditions, and typically there are two numbers to define that range ... in a car those are "highway" and "city" MPG. Surface Air Consumption is like that ... you determine your "resting" and "working" air consumption rates ... and the amount of air you will actually use on a dive will fall somewhere in that range, depending on a lot of different variables which include not only depth, but diving conditions and even your own physical and mental condition on dive day.

Once you determine that range, you have something to work with ... but there's a lot of other info you need in order to know how to use it.

FWIW - the way I teach this to my AOW students is to dedicate one dive to taking consumption measurements. We drop down, establish our buoyancy at 33 fsw ... which is 2 atmospheres of pressure. Then we write down our tank pressure, swim hard at that depth for 5 minutes, and write down our tank pressure again. Then we relax and float for a couple of minutes ... to let our pulse rate get back to normal ... and do a slow, easy dive (maintaining 33 fsw) for 10 minutes ... noting our pressure before and after that portion of the dive.

On surface interval, we use those numbers to calculate our "working" and "resting" SAC, respectively ... amount of air consumed, divided by number of minutes in the timed interval (5 minutes and 10 minutes, respectively) ... divided by the pressure at which you were diving (2 ATA). This gives us a PSI value for the amount of air we consume in one minute under each of those two conditions.

Send me a PM with your e-mail address in it and I'll e-mail you a write-up I put together for my AOW class. It provides answers to your questions ... and more.

... Bob (Grateful Diver)

 

[H2Andy]

I learned to do the same type of calculation using the data from an ordinary dive, which should give a fairly realistic idea of a diver's typical SAC.

yes, me too... unfortunately,i kept screwing it up. i kept forgetting
to start and stop my time, starting pressure, then depths got changed,
you name it...


so... i just put on my tanks and walked around for a while (i figured hefting
the thigns would be akin to swimming)

 

[billtylerjr]

I am curious where everyone learned of this sort of information? I do not remember hearing anyhting about gas management in any of my PADI courses. I only remember coming accross this in the DSAT (still PADI) TecRec course. I am working on becoming an Instructor now so I have run the gamut with PADI courses, perhaps I have just jumbled up all of the courses at this point and its been years since OW and AOW. But DM was just last year.

So I guess my question is when/where did you learn gas management?