Math question - am I an air hog?

[TexasKaren68]

Yesterday in the pool was the first time I took note of starting and ending psi, and dive time. All my dives for OW certification weren't really good for keeping track of this. Either we were up and down, at the surface discussing things, not to mention that little free flow skill, so air consumption would have been difficult to measure and probably not very accurate.

So anyway, I went to the pool yesterday with my new computer and stayed at or near the bottom for 33 minutes. The tank had 2800 psi in it when I started, and 1000 psi at the end of that dive. My average depth was 9ft, max depth was 11 feet.

I'm going to be doing my first deep dive in another couple weeks so I'd like to estimate how long I can stay at 60 feet.

If I calculated correctly, with the same rate of air consumption I'll run the tank dry in just under 25 minutes. If we go to 80 feet I could last about 20 minutes before running out of air. Knowing that I'll have to allow for the descent, ascent, safety stop, reserve, this is going to be a really really short dive

 

[RJP]

Am I an air hog?

No. You're a new diver.

Air consumption will improve dramatically the more comfortable you get diving. Plus, here's my 13 tips to improve air consumption:

1. Slow down
2. Get your weighting dialed in
3. Slow down
4. Get your buoyancy control dialed in
5. Slow down
6. Get your trim horizontal
7. Slow down some more
8. Keep your hands/arms still
9. Slow down even more
10. Work on efficient propulsion techniques
11. You're still going too fast...
12. Get more comfortable in the water
13. Did I mention slowing down?

Note, you may well burn through air even faster than you calculated for your deep dive, simply because it's your first deep dive. You may be a bit more anxious, a touch of narcosis, etc. It's normal. Keep an eye on your SPG, and please don't calculate how long until you "run the tank dry" but rather how long until you need to leave for the surface, including a safety stop and reserve.

 

[DiveNav]

Let me add something .... 14) SLOW DOWN

 

[TSandM]

Karen, since you don't tell us what size tank you were using, it's not possible to compute what your gas consumption is . . . but since you are a new diver, we all know we can sum it up easily: It's high. New divers go through gas quickly for a lot of reasons. They aren't comfortable and relaxed in the water, the equipment is challenging, buoyancy is challenging, and they move too much. Some of these things improve with more time in the water, and others will improve a lot with some focused effort, and particularly if you acquire and can optimize your own gear.

I wrote a little essay on improving gas consumption a while back, which you might get something out of.

BUT -- the absolute most important thing about your post, and the thing I am so glad to see, is that you have realized something important, which is that any deep dives you do right now are going to have to be very short to be at all safe. If you look up Bob Bailey's article on gas management and consider the concept of minimum gas/rock bottom, you'll be even more impressed with how important it is to be very conservative with bottom time. DCS is not often a major problem for new divers (unless they panic and hold their breath) but running out of gas occurs far too often, and THAT can be lethal. Good for you for thinking about it beforehand!

 

[Diver0001]

Karen, I think it's brilliant that you're thinking about these things now.

In Europe we're required to teach students something about planning air consumption so it's pretty standard to record starting and ending air pressures and average depths on training dives. It's too bad that this is not a universal concept.

In terms of your air consumption, I work it out like this (unfortunately I have to convert it to metric to make it make sense):

Your air consumption was about 120bar in 33 min at about 1.3ATA. That makes your air consumption about 2.8bar per min when adjusted for depth. I'm *assuming* you were using an AL80 tank, which is has an internal volume of about 11 litres, which puts your air consumption at about 30-31 litres per min. These are all rough calculations.

That level of air consumption is high. Normally an adult at rest (say, watching TV) will have an air consumption of anywhere between 12 and 20 litres per min. For women it's usually a bit lower on average than it is for men just because of phyiscal size.

Ordinarily if I have a student with an air consumption level this high then I would be looking for a reason and putting some focus on breathing and relaxation under water. I'm not going to repeat what RJP said, but there is a lot of truth in it. Slowing down is very important, but as much as slowing down and relaxing your body, is slowing down and relaxing your mind. Unfortunately, many instructors pay zero attention to this.

As for your deep dive, I would calculate it like this:

- an 11 litre tank has about 2200 litres of air in it when it's full
- you use about 30 litres of that at the surface, which means you'll drain the tank at the surface in about 70 min.
- 1/4 of that goes off because you want to be at your safety stop with a reserve in your tank, so that makes the available air-time about 55 min (say 50 once you take out the decent/ascent time from the calculation)
- and a dive of 18 metres (60 ft) will put you at nearly 3 ATA, which means your bottom time (time before starting to ascend) will be a maximum of about 16 min, give or take.

once again, these are all rough (very rough) calculations.

R..

 

[TexasKaren68]

Karen, since you don't tell us what size tank you were using, it's not possible to compute what your gas consumption is . . . but since you are a new diver, we all know we can sum it up easily: It's high...

BUT -- the absolute most important thing about your post, and the thing I am so glad to see, is that you have realized something important, which is that any deep dives you do right now are going to have to be very short to be at all safe. If you look up Bob Bailey's article on gas management and consider the concept of minimum gas/rock bottom, you'll be even more impressed with how important it is to be very conservative with bottom time. DCS is not often a major problem for new divers (unless they panic and hold their breath) but running out of gas occurs far too often, and THAT can be lethal. Good for you for thinking about it beforehand!

Thanks for the encouragement. I used Bob's article to figure out my gas consumption, along with my PADI book. I haven't done the rock bottom calculations yet though. Math was my best subject in school, but that was a while ago. I'm going to assume I was using an AL80, but can't say for sure. When the tanks for class were full, they were at 3000 psi which seems to be match up with that size tank

In Europe we're required to teach students something about planning air consumption so it's pretty standard to record starting and ending air pressures and average depths on training dives. It's too bad that this is not a universal concept.

In terms of your air consumption, I work it out like this (unfortunately I have to convert it to metric to make it make sense):

As for your deep dive, I would calculate it like this:

...which means your bottom time (time before starting to ascend) will be a maximum of about 16 min, give or take.

R..

16 minutes? Boy, that'll be fun I'd better take sunscreen for all my tanning time on the boat.

I do plan to record my air consumption on my future dives, and that metric stuff is just confusing but as I said, my training dives wouldn't have given me good numbers anyway. My first two OW dives were in Cozumel, overweighted, fighting against current, and surfacing quite often to discuss things with the instructor. I also got stung by a sea urchin which I'm sure made me breathe a bit faster My OW dives back home, to finish up my cert, were all in a river, drift diving. I got two 20 min dives on one tank but I'm reasonably sure my average depth on those dives was somewhere around 6 feet

Funny thing is I don't really feel like I'm sucking down air I feel relaxed but then again I do tend to breathe harder when I'm trying to adjust my depth with my lungs (hovering, fin pivot, etc). That might explain a lot of it.

Some people might think I'm a bit nuts for doing five adventure dives (AOW) in Kaua'i, and spending the time with an instructor, learning, instead of just going diving and enjoying Hawaii. My reasoning is that I'll be there by myself and don't want to get stuck with an "insta-buddy" I don't know when going diving. I don't want to dive with anyone other than an instructor or DM until I feel like I pretty much know what I'm doing in all these different situations. Nobody should want me as a buddy right now either - I'm too new and want to learn a lot more before putting someone elses life in my hands

 

[RJP]

I'm going to assume I was using an AL80, but can't say for sure. When the tanks for class were full, they were at 3000 psi which seems to be match up with that size tank

You can put 3000PSI into virtually ANY tank - has nothing to do with size.

 

[Diver0001]

Funny thing is I don't really feel like I'm sucking down air I feel relaxed but then again I do tend to breathe harder when I'm trying to adjust my depth with my lungs (hovering, fin pivot, etc). That might explain a lot of it.

It may. Conditions will affect your air consumption to a great extent. If you're honestly feeling relaxed then I'd say you have the beast by the horns already. It *will* go down in time as you get more experience. One bit of advice is to do regular buoyancy-checks and work on buoyancy control. Being "balanced" and properly weighted will help it go down.

"insta-buddy" I don't know when going diving.

People on internet forums make "insta-buddies" sound worse than they are in reality. In fact, that's a general rule on the internet. People seem to have a need to make things sound worse (or make themselves sound better) than they really are.

I don't want to dive with anyone other than an instructor or DM until I feel like I pretty much know what I'm doing in all these different situations. Nobody should want me as a buddy right now either - I'm too new and want to learn a lot more before putting someone elses life in my hands

I'd dive with you Seriously. I enjoy diving with new divers for the simple reason that their excitement and positive energy is fun to be around. Don't worry about being inexperienced. Not everyone is a boob who will judge you for it.

R..

 

[al248005]

Karen, I would not worry about that too much. Just make sure you follow the rules for a safe ascent and leave yourself with enough to ascend. My first dives were like this too. Its not uncommon for a new diver to go through the air quickly. My first 2 dives I think were around 50-60 ft and I was only able to say down for 20 and 22 minutes. After that my air consumption improved greatly. Now I can stay down at an avg depth of 45 ft for an hour. Just be calm, relaxed and make sure to check your weight and not overweight yourself. The more dives you do, the better you will be with your air consumption.

 

[fire_diver]

Your Surface Air Consumption rate (SAC) was 1.1 cubic feet per minute for that dive. While that is high, it is only one dive. You are also a new diver and it is common for new divers to breath down thier tanks faster than "normal". Don't worry about it. First, worrying about it will only make it worse. Just go dive and have fun. Keep track of your numbers like you did, and you can figure out an average SAC.

A few tips...
1 - do a proper weight check. Carrying excessive lead makes you spend more time fighting it. It also makes you keep more air in your bc which forces you to make more adjustments.
2 - have a buddy help you distribute weights to make you close to horizontal under water. Swimming "flat" is easier than swimming "upright", and less air used.
3 - have your buddy (an experienced buddy) check your kicking technique. bicycle kicking is a very inefficient propulsion method.
4 - learn to relax and enjoy the dive. This comes from just spending time diving. A relaxed diver uses less air, and actually needs less lead.

In the end, your SAC is what it is. Learn it to plan your dives but don't get hung up if other people have a lower rate. Don't try to artificially change your breathing rate by skip-breathing.