Air consumption and other ramblings

[SCUBASailor]

I was reading a thread on SAC rate, and it made me wonder. Can very expreienced divers, knowing their "typical" SAC rate closely estimate their upcoming air cumsumption rate on a dive by adjusting the SAC rate for depth, expected current, expected activity, temperature, etc? How close do your estimates come to the actual air comsumption rates?

On a related note, it seems to me that the dives I've been on so far, some of the other divers don't seem to make a serious attempt at dive planning. They have computers and just dive until the computer tells them to surface.

I've even seen one "experienced" diver who couldn't remember how to use his dive tables. My dive partner and I tended the boat while he and his partner dove. When he surfaced, he asked me what his bottom time was!!! I didn't know exactly, so I told him I had no idea. I didn't want to contribute to his DCS episode. I did answer his time related questions, though, and he worked his way backward into a bottom time. I'm glad he's not my dive partner.

I wonder what some of these divers would do if (when?) their computers fail. Hang up their dive gear, I guess. It makes me wonder about PADI supposedly no longer teaching tables. Heck, why teach our kids addition and subtraction? They can just use calculators!

 

[BKP]

I can usually guesstimate my impending consumption on a dive, just based on similar dive profiles and conditions I've already done, however, while that's fine for a broad stroke analysis for purposes of determining if a planned profile will yield enough remaining air, I wouldn't dream of splashing without an SPG.

Dive tables, as far as I'm concerned are useful for planning (also, for broad stroke analysis of bottom time, SI's, etc.). However, as above, I wouldn't dive without a computer, and on deeper, more challenging dives, possibly repetitive dives, without redundant computers. I don't see any advantage of using a table over a (conservative algorithm) computer in determining actual bottom time, NDL's, SI's, etc.

Tables still have their place, and I think should still be taught. However, I have no issues replacing them from dive to dive with a computer. I've never had a computer fail, and if I did, I'd simply thumb the dive, and switch to a backup (at which time I *would* use a table to determine parameters for the next dive, based on previous dives for that day, if the data wasn't available on the computer).

 

[TSandM]

After four years, I have a pretty shrewd idea of what my consumption is under different conditions -- what it is in cold water, with single tanks or double tanks, while cave diving, while swimming hard. It's easy to adjust for depth -- it's just ratios. If I'm diving and my consumption isn't matching what I expect, it raises a little alarm to look at what's different: Do I have a leak? Is my left post rolled off? Am I more stressed than I think I am?

It is WAY too easy to jump off the boat and dive until your computer tells you to come up, which is fine if you are doing drift diving and nothing goes wrong. Putting a little more thought into dive planning will make sure you have enough gas to get back to the boat, or to deal with an emergency.

 

[ScubaInChicago]

I can usually guesstimate my impending consumption on a dive, just based on similar dive profiles and conditions I've already done, however, while that's fine for a broad stroke analysis for purposes of determining if a planned profile will yield enough remaining air, I wouldn't dream of splashing without an SPG.

SPG's are over rated...you should be able to get to a point where you can guestimate within a few hundred psi based on a good track record of previous dives under various conditions...

 

[Peter Guy]

Can an experienced diver "closely" guestimate his usage? As Pres. Clinton might have said, "It depends on your definition of 'closely' I suppose." Am I generally able to "guess" my psi within 100-200 psi at any time of my dive, yes. Is this a result of a calculation, no -- just experience.

Knowing your SAC rate is good for PLANNING your dive but perhaps not all that useful in executing the dive. Tracking your pressure, and having "that sense" of what your pressure SHOULD BE at any point is what is useful for the execution of the dive. My biggest use of underwater SAC rate calculation is figuring out how narced I am -- I know that if I CAN determine my SAC rate then I'm not very narced while if I get halfway through the problem and then forget what I'm doing, I know I'm about as deep as I should be on that mix!

Regarding the tables and DCS of recreational divers. My very personal, and admittedly not based on much data, is that the tables are pretty much a useless exercise in learning by most "divers." AND, in addition, DCS should be the least of worries for the vast majority of "divers." IF, as most people do, they dive an AL 80, they really don't have enough gas to easily get into DCS trouble -- their consumption rates just don't allow them to stay at depths long enough. They DO get into trouble by consuming too much and not paying attention -- but that is a whole 'nuther issue.

 

[Bonairetrip]

To answer ScubaSailor, yes you can get pretty good at estimating your air consumption. And I do that as part of my dive planning. Of course, I still have and use a SPG.

One approach to gas management is the rule of thirds. You use 1/3 out, 1/3 back and have 1/3 in reserve. That is a rule I use in most dives.

My only real exception to that is on Bonaire, with a buddy, where I might surface with about 20% of my gas. There are many exit points there, I am shore diving, the dive ends near shore, there are minimal objective hazards, I am not in deco, no current, I know the sites well, with a solid buddy, etc. Otherwise, it is the rule of thirds...

I was certified in the tables only era but do rely on a computer now. I have had one computer die on me and that meant: a) I ended that dive at that point; b) No diving for 24 hours; c) Fix or procure a replacement computer. A second computer is moving up on my wish list (I got a new reg two years ago and a BP/W this year so....)

 

[SCUBASailor]

My biggest use of underwater SAC rate calculation is figuring out how narced I am -- I know that if I CAN determine my SAC rate then I'm not very narced while if I get halfway through the problem and then forget what I'm doing, I know I'm about as deep as I should be on that mix!

This is very interesting. It seems like a great acid test for narcosis, provided you could do the math to begin with.

 

[Bonairetrip]

My very personal, and admittedly not based on much data, is that the tables are pretty much a useless exercise in learning by most "divers." AND, in addition, DCS should be the least of worries for the vast majority of "divers." IF, as most people do, they dive an AL 80, they really don't have enough gas to easily get into DCS trouble -- their consumption rates just don't allow them to stay at depths long enough. They DO get into trouble by consuming too much and not paying attention -- but that is a whole 'nuther issue.

While I think OOA is the bigger risk for newer divers with an Al 80, some still get bent. At 50 or 60, I agree with the poster but the risk goes up as one goes deeper...

 

[DCBC]

Can very expreienced divers, knowing their "typical" SAC rate closely estimate their upcoming air cumsumption rate on a dive by adjusting the SAC rate for depth, expected current, expected activity, temperature, etc?

Starting with the first open-water dive I have students project their air consumption. At first it's not accurate because they have nothing to compare it to. I recommend that after they're certified they continue to do this. They keep note of anything that requires them to use gas faster than normal. In time they can leaf through past dives and compare water temperature, type of suit used, effort or work, current conditions, etc. This allows them to project the amount of gas they will use per minute at any particular depth. If you are conscious of this, with experience it becomes second nature. Just like a professional golfer estimating distance and choosing the right club.

 

[Kevrumbo]

It turns out to be easy for my own and others with similar SAC rates: depth in meters is equivalent how much gas I'll consume in 10 minutes. For example, 30meters depth square profile equates to 30bar consumed in 10 minutes; 45meters depth is 45bar consumed in 10 minutes; 60meters depth is 60bar consumed in 10 minutes etc. Very convenient one-to-one correspondence. . .

Parameters:
Twin 11 Litre Tanks (double AL80's)
SCR (SAC rate): nominal 15 litres/min.

If I'm working hard finning against a current, or diving in cold water, I'll use the ATA (absolute atmosphere in pressure at depth) to figure the increased on-the-fly consumption: 30meters depth is 4ATA; mulitply 4ATA times 10 equals 40. So in 10 minutes, I would use 40bar of gas. Similarly, 45meters is 5.5ATA, and in 10 minutes I would consume 55bar, and so on. . .

(See: Metric System Math Trick. . . - DIR Explorers)