Tank Choice - 5.7L Alu vs 7L steel

[Ashes]

Hi,

I am trying to choose a tank to carry for some extra air, and looking for some advice.

Background: my air consumption is on the high side, at about 24-25L/min. I have been diving with a 15L steel, and on some wreck dives to 25-30m I am always the first person to ascend, with the rest of the group being about 10mins later.

So I am looking at a way to get about another 5-10mins of air.

A lot of the local diving is in commercial boats, with two dives. I was thinking a 5.7L Alu tank or a 7L steel tank would have enough gas to give me that 5-10mins extra on both dives. (ie half the tank used per dive.)

I don't think a single set of twins would have enough gas for me to do two dives. And carrying two twinsets on the boat is not an option.


Late last year I did Deep and Intro to Tech courses and made use of 5.7L Alu tanks for extending bottom time on all my single tanks dives. But we refilled the tanks between each dive.

The 7L steel and 5.7Alu tanks are approx the same size, weight, and price.
The 7L steel tank has about 35% more air than the 5.7L Alu tank, which would be nice.
However, the steel tank is negatively buoyant.


Question for you more experienced divers.
Would the negative buoyancy of the 7L steel tank make it awkward or a pain to use as a slung stage tank?
Or should I just stick to the tried and tested 5.7L Alu tank?

 

[Diving Dubai]

Giving an opinion, purely on weighting:

You should consider the end buoyancy of the cylinders in your scenario. The Steel will be slightly negative, while the Al will be slightly positive. The consideration of which to choose would wholly depend on your current weighting and exposure protection.

But I think we're missing the point here.

Having just run some numbers to check, even with 25 l/pm SAC a 15l should be sufficient to run a 30m NDL on 32% and surface with + 50 bar. So if as you say you're running low on gas that early, then you really should address the reasons for an elevated consumption.

For sure, the number 1 reason for high consumption is worrying about consumption, then of course the old favourites of correct weighting and proper trim and propulsion techniques to be practised

After that you need to be looking at your comfort levels. It appears that you're anything other than relaxed underwater, and that needs some work. Consider trying to practice a more relaxed breathing technique, which you can do sitting at home. Slow inhale, pause, slow exhale, pause etc

Trying to mask a problem with more gas, rather than tryign to solve the actual problem is never the best way forward.

 

[Ashes]

The NDL is a good point, I haven't had a problem with this, as gas has always run out early.
A more realistic profile than a square, might be:
30m for 10mins, 25m for 10 mins, 20m for 20mins, + safety stop, gives a 50min dive, which does not exceed NDL.
My dive planning software puts me at -44bar at the end on a 15L 32% mix.

45-50mins is about the length these dives do go for. I am getting about 35-40mins in and accending early.
I have an AI computer and that SAC rate is reliable. (although sometimes I do get slightly better)

Weighting, trim, and propulsion are pretty good. I spent about 20 dives with an instructor late last year diving a wreck in 20-30m.
The biggest issue I think is that I am heavy, and a bit out of shape, thus use more air.

I don't see this as masking a problem, rather it is giving me options which allow me to keep diving and build up more experience and confidence.
In order to be more relaxed in the water, it is better to get in the water more.


Back on to weighting, I normally wear 2kg of lead. When I use a 5.7L Alu tank, I need to add lead (~ 1kg) otherwise I can't hold the safety stop.
If I was using a steel tank, I would still use the 2kg, as if I needed to offload the tank for whatever reason I need to still be able to hold the stop.

Does the steel tank handle differently while you are diving due to the additional negative buoyancy?

 

[stepfen]

Apart from asking professional help, the best you can do is ask somebody to take a video of you diving. Preferably at a moment you don't know he is recording you.

Once you have such a video you have 2 options:
a) study it yourself, compare your style and movement underwater with good dive videos you can find in youtube and hope that you can spot and correct the problem, or alternative
b) post it here so that people with experience (not me) can see what is wrong and advice you accordingly.

25lt/min is quite high (I have 15lt/min and I consider it high already), hence whatever needs to be done should be quite obvious by just looking at your diving style.

Having a pony to extend dive times is a bad idea for several reasons. Some of them are: Adding extra gear/tank increases your SAC rate even more (due to extra lag, extra workload, extra stress etc). Also 7 lt are not as many as they look. At 200bars you have 1400lt of air inside it. You can't/shouldn't go below 30bars (~200lt - some would say 50bars which are 350lt) hence you are left with 1200liters for 2 dives or 600lt per dive. At 25lt/min that's 24 minutes on surface, 12 minutes at 10m, 6 minutes at 20m and just 3 minutes at 30m. Does it worth it? In reality it will be even less. Not to mention practicalities (extra cost to buy and maintain the tank and the second set of regulators, extra fills, extra weight/gear to carry around etc).

I would advice you to keep diving. Don't worry about sac rates, who comes up first etc - all these just add stress. If you want to dive longer stay shallower or reduce expected dive times. Seek extra training (hence learn more skills and become more comfortable underwater). Keep diving and SAC will naturally reduce.

Just my 2c.

 

[leadduck]

The biggest issue I think is that I am heavy, and a bit out of shape, thus use more air.

Not to omit the obvious, the most effective solution will be addressing the root cause ... a healthy diet, and endurance training like long distance running will do wonders to your SAC. And no it won't take forever, just do it.

 

[stepfen]

Not to omit the obvious, the most effective solution will be addressing the root cause ... a healthy diet, and endurance training like long distance running will do wonders to your SAC. And no it won't take forever, just do it.

I forgot this brilliant advice.

 

[Diving Dubai]

Weighting, trim, and propulsion are pretty good. I spent about 20 dives with an instructor late last year diving a wreck in 20-30m.
The biggest issue I think is that I am heavy, and a bit out of shape, thus use more air.

You would be surprised how little difference (apart from weighing) being unfit and overweight has.

I'm a slightly overweight unfit +50 yr old smoker. My consumption is at the 11-12l/pm

I enjoy teaching PPB its one of my fav course and help people with gas consumption problems. It always comes down to the basic
fundamentals of correct weighting, trim and propulsion, moving slowly and relaxed breathing.

Of course there is a limit and everyone is different. But once again you're not making difficult dives, so a 15L is plenty for a 1hr dive on that profile

Certainly worrying about air, all the time underwater will increase consumption, so you need to get past that. Accept for the moment you'll be the first up.

Double check your weighting at the beginning of the dive, and again at the end. A fun skill is making a stop with a visual reference (say anchor line) but not holding onto it (at most touch it with some fingers) Watch your computer and try to keep the minimum vertical movement by slow controlled breathing. Lets say +/- 1m max to start with. You shouldn't need to dump or add air in your BCD.

Again practise at home slowing your breathing down. new divers often breath like a train as soon as a reg goes in their mouth. Remember you're in a strange environment and your brain sometimes subconsciously "freaks out"

People often turn to extra weight or bigger cylinders to solve issues, and then it becomes harder to fix later down the line. Certainly in water time helps and it will get better.

I got where I am skill wise by practice and working on ways to improve. I used to be where you are now.

 

[Marie13]

Not to omit the obvious, the most effective solution will be addressing the root cause ... a healthy diet, and endurance training like long distance running will do wonders to your SAC. And no it won't take forever, just do it.

If I can get off my butt and get in better shape to dive, then you have no excuse. If your knees can’t take running, then highly recommend you bike. Doesn’t matter if it’s at the gym or outside.

 

[wKkaY]

Is your 15L pumped to 200 or 230 bars? that difference could get you the extra 5-10 minutes you are looking for.

Just to add some encouraging words, my big-sized buddy used to have 20L/min and after a year of regular diving managed to get it down to 15L/min over time. I don't think he paid much attention to his fitness or physique, on the other hand he has good technique in the water.

 

[Ashes]

Ok, This is all off-topic.
I can hold my decompression stops, buoyancy is good, weighting is good.

I know how to do gas planning, and my planning is telling me I need either more gas or less time.
When I was with the instructor, this is what we did, and I used a rented Alu 5.7L.

This topic is about the differences between Alu and Steel cylinders.

Has anyone dived with a 7L Steel slung on the left?
How does it carry compared to an Alu cylinder?