Dive planning/ Air consumption question

[Nailer99]

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

 

[H2Andy]

nailer, an excellent question.

first, you will have to figure out your surface air consumption, or SAC rate

once that is known, you can calculate your depth consumption rate (DCR) with the
following formula:

DCR = SAC Rate x (Depth + 33)/33

so, how to calculate your SAC?

boring, but the surest way is to breathe off a tank at the surface, and
see how many psi's you use up in 15 minutes (or 20, or 30, or
however long you can stand it). then divide the psi's per
minutes, and you get a psi/minute SAC.

(for example, if you use 500 psi in 15 minutes, your SAC is 33.3 psi/minute)


to calculate for depth, plug in the formula. say you want to figure out
your DCR for 80 feet:

DCR = 33.3 x (80 + 33)/33

DCR = 114.03 psi/minute


keep in mind this is an "at rest" SAC. it will be higher if you are swimming or
doing anything that requires effort.

------------------------------------------------------------------------

now for another part of your question. the VOLUME of a cylinder is dictated
by the amount of gas it will hold at a given pressure.

for example, a Luxfer aluminum 80 will hold 77.4 cubic feet of air at
3000 psi. that is, 38.8 psi per cubic foot of air.

if you have 2400 psi, that means you have 61.9 cubic feet of air.

if for some reason you end up with a fill at 3300 psi, that means you
have 85.1 cubic feet.

here's a good link with the various rated pressures of quite a large
number of cylinders:

http://www.huronscuba.com/equipment/scubaCylinderSpecification.html

 

[Kim]

boring, but the surest way is to breathe off a tank at the surface,

I have to repectfully disagree with this!

If you breath off a tank at the surface unless you strap it on your back and walk around while you do it then you'll get a too low reading. Even walking around will use a different amount of energy to swimming. I would suggest the following:

Dive to 10 meters (33ft). That is 2ATA as you'll know from your OW class. Now check what air you have in your tank and write it down on a slate. Swim at a normal dive speed staying at 10 meters for a fixed amount of time - 10 mins...20 mins it doesn't really matter although the longer the better. When the time is up check you air again and write that down.

Now you can divide the air you used by the time in mins that you were swimming. That's what you use in 1 minute at 2ATA. Half of that is your SAC. Using the rest of the equations that Andy supplied you can work out consumption for any depth.

 

[Nailer99]

Very good replies, thank you! I'm still kind of shocked this stuff isn't in the PADI texts, even the "advanced" one. I'm also disapointed in myself for just now realizing I knew nothing about it. All my dives (all 7 of them) have been with my instructor, who I trust very much. But I'm the kind of person who needs to KNOW all this kind of stuff for himself. So far, she's just told me "let me know if you get down to 700 pounds", and the like, which has been fine by me, so far; I've been busy learning to manage my buoyancy and gawking at stuff, but geeze, how come it's not a mandatory part of certification?

 

[TSandM]

I hear your pain . . . I was taught in OW, "Plan your dive and dive your plan." But I realized after OW that I had no idea HOW to plan a dive. And after AOW, I STILL didn't have any idea how to plan a dive, other than to say, "We're going to swim that way until we're at 500 psi."

Then SB's own NW Grateful Diver did a gas management seminar, and in one evening, I learned more about what was in my tank and how to figure out whether it was enough and how to manage it than I had learned in my classes to that point.

The information is out there. It isn't rocket science. It's VERY applicable, especially if you get to doing any deeper dives, or dives off boats, or dives where you really would rather not do a long surface swim to shore (been to Edmonds yet?) You just have to go somewhere where it's taught. I found Bob, and then I found the people Bob learned it from.

 

[bradshsi]

Take a look at these threads on calculating Rock Bottom. Along with the valuable advice given above, this will help you get a good handle on gas planning.

http://www.scubaboard.com/showthread.php?t=90264
http://www.scubaboard.com/showpost.php?p=735077&postcount=33

I can relate to your problem. My PADI OW was pretty light on gas planning. Using the Rock Bottom rules along with much useful advice on this board, got me sorted out

 

[IwakuniDiver]

There is definitely nothing wrong/wierd/odd/anal with wanting to learn this information for yourself. In fact, I'm also of the same mentality (I've been spending the majority of the day reading DAN's medical publications).

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. After I download the dive to my laptop I then enter in my beginning and ending pressures and the software figures out my SAC and DCR based on average depth. I then take both the values and enter them in an Excell Spreadsheet to track trends in my air consuption and use that data for planing future dives.

For example, my SAC rate is usually around .59 Surface Cubic Foot Per Minute (SCFM). So, if my upcoming dive is going to have a max depth of 80fsw, I can estimate how much air I will use at that depth. At .59 SCFM my DCR at 80fsw will be approximately 3.5 CFM by using the formula described above. Divide 80 (For an 80 Cubic Foot Tank) by 3.5, and that means I will completely exhaust my air supply in about 22 minutes. This of course, assumes that I am magically teleported to 80fsw and back again.

If you do the math, you will notice that I rounded my DCR up and my air consuption time down. This is simply becuase it is better to err on the conservative side. Obviously, I wouldn't plan on an overall bottom time of 22 minutes since I'll need air to get down and back again safely, but I will use this number to help plan the dive.

Note, this does not take into effect any factors which may increase your DCR like anxiety, distress, or anything of the like so these tools do not replace the need to monitor your air supply while on the dive.

 

[CD_in_Chitown]

I'm still kind of shocked this stuff isn't in the PADI texts, even the "advanced" one.

Welcome to the club, its not just a PADI phenomenon.

 

[Quero]

I have to repectfully disagree with this!

If you breath off a tank at the surface unless you strap it on your back and walk around while you do it then you'll get a too low reading. Even walking around will use a different amount of energy to swimming. I would suggest the following:

Dive to 10 meters (33ft). That is 2ATA as you'll know from your OW class. Now check what air you have in your tank and write it down on a slate. Swim at a normal dive speed staying at 10 meters for a fixed amount of time - 10 mins...20 mins it doesn't really matter although the longer the better. When the time is up check you air again and write that down.

Now you can divide the air you used by the time in mins that you were swimming. That's what you use in 1 minute at 2ATA. Half of that is your SAC. Using the rest of the equations that Andy supplied you can work out consumption for any depth.

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. First you need to find your average depth for an ordinary dive. Many dive computers furnish this information. The math is the same--for example, if you find that your average depth was 13.4 msw, the ATA number is is 2.34. All that's left is to do the division.

 

[alpaj]

"at what PSI are you dangerously low on air, anyway? That might be a helpful nugget of information to throw in, wouldn't it?"

Yea I hear you. I need to know how it works as well, I can't just take someones word for it. I think that is a very good and safe attitude to have about diving. Just wanted to throw in to the above, that pressure drop and volume drop are fairly linear. Ideal gases will be completly linear holding other variables like temperature the same. When breathing down a tank it is fairly close to ideal. That means that when there is half of your pressure "left" there is roughly half of your volume left. You can work out the volume by going back to grass-roots chemistry PV=nRT where P pressure in atmospheres in the tank V volume in liters n number of molecules of the gas in moles R just a constant 0.08205 and T temperature in Kelvins which is degrees C plus 273. You can use this formula and you will find out exactly how many molecules of gas are in your tank or you can just use some of the other "short-cut" methods that are out there in scuba world. It will give you the corect number but you have to convert all the units.