Calculating air consumption SAC / SCR / RMV

[Shadow]

I like my air integrated computer. It's great for lazy mathematicians, and doesn't give me SAC but instead tells me how many minutes of air I have left.

It calculates how many minutes of air I should have left if my breathing rate remains the same.

So, at the beginning of the dive, right after descending, when I've used a lot of air coming down, it calculates a fairly short dive ahead. Then, as I relax and swim around, the time becomes longer as it recalculates.

If I am trying new equipment, am too heavy, or am floundering around trying to follow a light I can't see, then I use way more air! (UP can confirm this.)

I have noticed, like others here, that I use more air in cold water than I did in the tropics. But when I notice I'm getting low, I go to the surface, where there's lots of air... something everyone should do, regardless of how low their SAC rate is...

 

[dlwalke]

I've only made a few dives, but I was surprised that everyone in the OW class (and I believe the instructor as well) seemed to use up about the same amount of air despite a fairly wide range of body sizes, and in the case of the instructor, experience. I hear a lot of talk about how beginners typically use up air a good bit faster than more experienced divers. I'm just wondering if my air consumption seems like something that has a lot of room for improvement or not. I would love to be able to stay down longer than I was on those initial dives. The dives to which I refer had comparable profiles. Basically a slow but steady descent to 50 ft (estimating but probably about 5 min), an 8 min ascent (including stop at 15 ft), and 15 minutes on the bottom. Total air consumed = 2400 PSI. Water temp was about 78, wearing a two piece (3 mm plus 2 mm) suit. Age = 41, non-smoker, 150 lb. A second dive to the same depth lasted 23 min and used 2100 lbs. Does this sound abysmal or normal or normal for a beginner but expect improvment? I'd like to think that my dives will last longer in the future as I consume less air, but I wasn't really exerting myself or breathing heavy (at least that I was aware). I was breathing constantly and with long deep breaths as this is what we were told to do.

 

[dlwalke]

And BTW, the tanks were most likely AL80's (filled to 3000 psi using dive shop rentals) so the total air consumed in the above instances would have been 64 and 56 cubic ft.

Dave

 

[Rooster1]

I am assuming you are using an aluminum 80 and if so yes I would say you are using too much air.
why you are using to much air may be different variables that can and should change with experience.


*are you using to much weight on your weight belt?
*are you feeling anxious when you dive?
*is your breathing rapid?
*do you use your inflator hose constantly to get buoyancy and then have to dump air and then add air again??
*are you using a lot of arm movement to swim through the water?
*is your gear not streamlined and not tucked in?

All these questions have answers you can answer. When you can answer no to some and eventually no to all these questions then your breathing rate will get much better.

 

[Wima]

This is an interesting topic. To bad air consumptions are so difficult to compare since it's depending on so many factors...

I felt I did fairly well for a newbie at my last leisure dives. For example, one dive had maximum depth os 28m and total dive time 44 minutes. I don't know the profile since I don't own a computer yet. I was content following the dive master. Using 3mm full suit and 5kg(10lbs) of weights I used 110bars(1650psi) with al80 tanks. The water temperature was maybe around 27 celcius.

I am a 24 years old guy and in reasonably lean shape. My dive buddy was a two years younger girl. She used on average 5-10bars less on each dive. We did much better than somewhat more experienced but older divers on the same dives. Sadly I forgot to compare with the divemaster...

How much could you cut with better swimming technique, more streamlined gear and better breathing control? How much do people use on average on similar dives?


- Wima

 

[JimC]

While is normal to have abysmal SAC rates on your first few OW dives, I am surprised your instructor has simmilar gas consumption.

I know my SAC rates on my first 4 dives were over 1.2cf, they quickly droped off to under 0.5cf ~20 dives later. With better trim, weighting, etc your SAC rate will go down, and it will go down fast.

Don't be discuraged if its high at first, diveing takes practice.

 

[seabass]

Your air consumption seems a little high. I started around 60 ft for about 40 minutes and have gotten to about 60ft for 45-55 minutes using about that much air. Since I always use AL80's I haven't bothered to calculate cubic ft/minute and just leave it as psi/minute.

My friend who is a complete air pig will suck down a tank in 20-25 minutes at 60ft.

In my limited diving experience, my air consumption is on par with others.

 

[Spectre]

JimC mentioned SAC rates [that would be Surface Air Consumption rate], which is really the key to everything here. If you learn to calculate your SAC rate, you will have an easy number to compare to. You'll be able to see your improvement, and you'll be able to use it to plan dives.

So lets look at your dives:

A 5 minute decent to 50 fsw. That's an average of 25 fsw for 5 minutes. 25 fsw is 1.75 ATA * 5 = 8.79.

15 minutes @ 50 fsw. 15 * 2.51 = 37.73.
50 to 0 == 5 @ 25 fsw. 5 * 1.75 = 8.79
3 @ 15 fsw = 3 * 1.45 = 4.36

8.79 + 37.73 + 8.79 + 4.36 = 56.67.
----
3000 psi / 77.7 cuft = 38.61. 2400 psi / 38.61 psi/cuft = 62.16 cuft

62.16 / 56.67 = SAC rate of 1.10 cuft per minute.
---------------------------------------------
Your second dive was the same profile, but 10 minutes @ 50 fsw instead of 15.
8.79+25.15+8.79+4.36 = 47.09

2100 / 38.61 = 54.39 cuft / 47.09 = 1.16 SAC
-----------------------------------------------

Calculate out your SAC rate for every dive, and keep it in your logbook. You will find your 'normal' rate. When you have a dive that is out of the normal rate, note in your logbook why [which is something you will know at the time... stressed, fighting a lot of current, etc..].

You can than start to do some gas management planning. e.g. "On a bad day, I need X cu ft of air to ascent from 110 fsw @ 20 ft/min with a 5 minute stop at 20 fsw. That's Y psi. add my reserve, that means I have to start my ascent at Z psi"

[note all the 'buffers' in place. You don't plan with your best SAC rates, you might ascend at 30 ft/min. Your safety stop'll be 3 @ 15 fsw]

 

[Rick Murchison]

First, let me make this perfectly clear:
Air consumption is air consumption. There is no such thing as using "too much air."
You use air at whatever rate is comfortable - be aware of it, plan for it, change the way you do things to improve it, but do not think in terms of "too much" or "bad."
"Bad" things are skip breathing, breath holding and the like in an effort to make your air consumption "normal."
My point? Air consumption is a reflection of other factors, not a thing to be attacked itself. Take care of planning, weighting, planning, buoyancy, planning, relaxation, planning, enjoyment, planning. Air consumption will take care of itself.
Always breathe.
Rick
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Addendum: Planning - the more time and effort you put into planning, the more relaxed and enjoyable will be your dive. By removing as many unknowns as possible before ever hitting the water, you can concentrate on just enjoying the dive - and you'll most likely get a pleasant surprise... at the appointed ascent time you'll likely have more gas than you planned on, which you can now factor into your next plan.
"Prior Proper Planning Prevents Piss Poor Performance"
Rick

 

[Scubaroo]

What is a "reasonable" SAC for an advanced beginner diver? I just installed Suunto Dive Manager and downloaded my last nine dives - my SAC ranged between 0.48 and 0.59, average was about 0.53

Ben