S.A.C. and How to not suck so bad :/

[John_B]

I figured I'd get one of these responses. In my defense, I did search on "SAC" which yielded nothing and then "Surface Air Consumption" which resulted in a bunch of crap. I suppose "Consumption" would have been better.

FWIW, sometimes google searches turn up things that the SB search doesn't, esp. TLAs (three letter acronyms). Just go to google and type:

site:www.scubaboard.com [I]SAC[/I]

...and see if that doesn't give you better results. (And consumption is probably a better search term, but how would you know that if you didn't already know it? )

Edit: This thread turned up third in the google search results as of a minute ago.

 

[Blackwood]

But for calculating your air consumption after a dive, measuring it in PSI would only make sense if ALL of your dives were to the exact same depth.

Say what? If you know your SAC rate in units of pressure, you can use PSI as (or more*) effectively than ft³.

ft³ has no advantage over PSI from a depth perspective. Its advantage is that it isn't dependent on the tank from which you are breathing.




*I say "or more" since my gauge reads units of pressure, not volume. I know that I generally suck down about .55 cubic feet of gas per minute per atmosphere. That translates to about 10PSI/min/ATM in the tanks I generally dive.

So when I'm at 100 feet (4ATM), I can expect to see my gauge drop by 200PSI every 5 minutes (the interval at which I check). Can I do the math to convert from CF to PSI? Sure. But it's a whole lot easier to simply know PSI/min/ATM.

 

[TSandM]

First of all, I try to go in PSI, as it's easier to see how much of your tank your emptying. Your varying rates at SS and 95 make sense, as you're concentrating much more at the safety stop than when swimming around. A reasonable goal?? Where you're not getting hypoxia headaches, and breathing as little and as comfortably as possible.

First off, you don't get headaches from hypoxia. You CAN get them from retaining excessive CO2.

Second, keeping track of your SAC rate in psi makes excellent sense IF you are diving the same tanks all the time. Otherwise, you have to do something to normalize the value you are getting, which is what converting to cubic feet does. In either case, you have to correct for ATA.

 

[emttim]

First off, you don't get headaches from hypoxia. You CAN get them from retaining excessive CO2.

Second, keeping track of your SAC rate in psi makes excellent sense IF you are diving the same tanks all the time. Otherwise, you have to do something to normalize the value you are getting, which is what converting to cubic feet does. In either case, you have to correct for ATA.

Headaches from hypoxia? That's an interesting one. Yeah, you would actually begin to act like you're narced if you started to get truly hypoxic since your brain would not be getting enough oxygen and you'd develop an altered mental status. Only way I can think of someone retaining excessive CO2 from trying to improve his/her air consumption is by skip breathing, which isn't really a good idea.

I used to keep track of my SAC rate in psi, then I realized it doesn't take into account tank size and was extremely flawed, so I switched to cubic feet. I don't see how anyone could manage to use the same size tank every single time since each dive may require something slightly different, but hey if PSI works for you, power to ya!

 

[AbyssalPlains]

I second my predecessors' views. It's mostly about becoming ever more comfortable. I started as a terrible air hog, and now, 130 dives later, I'm still not happy with my rate, which is close to the OP's (0.4 when resting, 0.9 when exercising), but I find that with time, I get better and better. Also, at least in my case, switching from a jacket-style BC to a back plate and wing setup lowered my air consumption dramatically.

 

[ACR]

My average SAC rate for 14 dives is 0.65 SCFM.

I'd LOVE to have a sac of .65, so quit whining

My solution is to buy a bigger tank. More gas = more bottom time (within limits). Fact is, figuring out your ideal weighting, mastering trim and achieving a zen like state of relaxation while diving takes time. a lot of time. I'm not recommending that we stop working towards it but there's an interim fix out there

 

[BKP]

That's the main reason I use a HP100 and my wife uses a HP80. It works out very good for us.

I'm on an HP100, and my wife is on an AL63! I don't think she breathes at all... she does double duty as my buddy, and pony bottle...

tschmitt, .65 is really very good for new "big guy" diver. I wouldn't be concerned AT ALL...

I average *maybe* .55 and I've been diving a few years... (I once got under 4.0... fortunately, they were able to revive me... ).

 

[Gil57usa]

Search->advanced search, type in "consumption" for the keyword, limit the search to titles only and you'll find over 70 discussions from other folks that didn't use the search function either.

Roak

I think every subject and question imaginable has been brought up at one time or another on this website. Not everyone has multi-thousand posts either. This is the kind of answer/dig that will keep new Divers from participating here and thus ultimately deny them the vast source of knowledge and experience that is available through the members of scubaboard. I know that for the Old-timers it seems so easy to state what they see as obvious, but.......How about giving newbies a break and show some patience. Maybe even answer their questions so they can become better Divers themselves.

 

[Teamcasa]

I'm on an HP100, and my wife is on an AL63! I don't think she breathes at all... she does double duty as my buddy, and pony bottle...

tschmitt, .65 is really very good for new "big guy" diver. I wouldn't be concerned AT ALL...

I average *maybe* .55 and I've been diving a few years... (I once got under 4.0... fortunately, they were able to revive me... ).

I dived with woman in Hawaii last July, I had emptied my second AL90 when she finally finished off her first AL63! Both dives were 55min+.

Her lungs must have been the size of small potatoes, she was an amazing diver in any event.

 

[scubamickey]

I've never actually calculated my SAC rate, however, I know I can happily do a 30 meter dive profile and a 15-20 meter dive profile and end up with the same air left in the tank (assuming same max dive time and same water temp/conditions).

I've always put this down to a silent effect of narcosis, because I am more relaxed, which is good for dive time (within limits), but it's something to remember in terms of being more aware of my surroundings etc.

When I read something like this I scratch my head. How do you plan a dive if you don't know how much gas you need to safely make the dive and a proper ascent? Do you just wing it and come up when your computer beeps at you? This is an honest question on my part. I've always known my SAC rate and I just don't understand how I would dive if I couldn't calculate the gas I needed.

Thanks