Didn't notice much difference between steel & aluminum tanks

[Ready4Launch]

Went out to the Channel Islands again yesterday, and since I've still not accumulated my own gear, I rented from the same shop that I've been certified through. This time I went out on a different boat than the shop's affiliated charter, which I've gone out on the previous time. On that boat, they've always provided steel tanks, but since I had to take a tank with me on the boat yesterday, I had to rent an aluminum tank.

Now, the guy at the dive shop realized this difference, and advised me to take more weight with me than what I had been using previously. My certification dives and trip out to the islands prior to yesterday were all with 22lbs with steel tanks. The shop provided me with 26lbs, and by the time I was on the boat and preparing my gear, I only then realized that the weight configuration provided didn't allow me to place 24lbs onto my setup like I was going to try. So, I went with 22lbs, and I was always diving aware that I should have been more positively buoyant than previously.

Now, I did find that getting to the bottom was slightly more difficult than before, but not terribly. I had to push myself down a little bit during the first dive. Went head first and kicked from the surface on the second, and the third I don't remember as well.

During our safety stops, again being cautious as I came up in depth, I came up slowly. I don't remember it being or feeling that much different from a steel tank. So, now I'm left a little bit confused as to whether i'm really weighted right or not. In either case, I'm not struggling with trim or feeling like I'm labored to move through the water.

Thoughts? Suggestions?

 

[Aquavelvet]

Well, you might have been slightly overweighted to begin with.

Your description of how you felt with the Aluminum tank matches what I would describe as having just (barely) enough weight. Another two pounds would let you keep more air in your suit, if diving dry. If you were already warm enough, then you've found a new baseline for the weight you actually need.


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[scubaalblake]

The other thing to bear in mind is that you need to weight for an empty tank at the end of the dive in shallow water. That means in all normal circumstances you should be slightly overweight. There is nothing worse than fighting to stay down on shallow stops in mid water when you are low on gas. Always weight for the worse case scenario then dive to make sure you don't need it.


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[g1138]

Might have already been overweighted in your Steel tank configuration, so when you're diving with the Al 80 (I'm assuming) you were weighted just right.

BTW, it also can make a huge difference which St or Al tank volumes you're using.
For me w/ a 7mm 2 piece
Steels:
St 72 (old school version) - 16lbs
LP 80 - 14lbs
HP 80, HP 100, HP 120 - 12lbs (my buoyancy is slightly different with each tank and I just compensate w/ my BC)

Aluminum:
Al 50 - 16lbs
Al 63 - 20lbs
Al 80 - 22lbs
Al 80N - 20lbs

You'll notice the blue highlighted St & Al tank use the same weighting for me because of their buoyancy characteristics. An Al50 is about half the height of a St 72 old version.
Highlighted red is 2x Al tanks, one's a few inches shorter than the other but I use the same weighting for both.

Quite simply my point is I can't rely on only 2 weighting configurations for Steel VS Aluminum. I have about 5 configurations that I've used in my life depending on which tank Material & Volume I'm using.

So for your Dive Log it's important to not only note your weighting but also what tank Material & tank Volume you're using. Plus a note relating to how your buoyancy felt on that dive with some ocean conditions for good measure. Because sometimes a swelly ocean can ruin your perceived buoyancy, especially when you're starting out your diving career.

 

[AfterDark]

It depends on the steel tank. I use 22 lbs with 2 steel 72's cuft LP, but 8 lbs with one 94 cuft steel tank. My 2 steel 50cuft LP require 24lbs, This is with a 7mm Farmer john wet suit. I've only owned one AL80 that required almost 30 lbs of lead, haven't used it in decades. I agree with others that you were probably over weighted with the steel tank, sounds like another 2 lbs and you'll be just right with the AL80.

 

[Frosty]

Just a point that seems to have been missed.-if you were diveing uber conservative then you might not have emptied the tank enough to notice the difference.
Also as already mentioned you may well be overweighted with the steel tank.
The difference is definitely in the last 1/4 of the tank and if underweighted you would know about it on the safety stop.

 

[Phil_C]

Like you I'm usually diving steels, either in the UK or in the seas off Cyprus - but very occasionally if the dive centre I go to is short on tanks I get to take one of a couple of Ali tanks they have that they don't give to students. After many (hundreds) of dives in the same configuration with steels I have pretty much got my weight down to a minimum, and I find if I don't add weight I can't get down in shallow water with the Ali tanks, let alone easily hold a SS with a near empty tank.

So I think that there is no doubt that it is not just a case of physics saying the buoyancy is different, it can really makes itself known on a dive if you don't add weight.

However in the same way that if I have been away from diving for a bit I usually add a little weight until I get back into it and then drop down to absolute minimum again, I suspect that you have not really found your absolute minimum or ideal weight yet and may be diving a little overweighted all the time. This is very common, don't worry about it, it happened to almost every diver I have known and it will get better when you have a few tens of dives under your (weight) belt. This would mean you might not notice the difference so much.

Also g1138 has it spot on, there are so many variables. I dive in several different configurations of exposure suit from drysuit to shorty wetsuit, single tank, twinset, side mount, and single and pony, depending on the dive, and I always try to keep a very accurate log of weight, exposure suit, sea conditions and so on. It is even worth recording if you are carrying reels, smb's, torches and so on, as these all add weight and affect your buoyancy.

Dive safe and have fun - Phil.

 

[Yoyodyne-88]

Just a point that seems to have been missed.-if you were diveing uber conservative then you might not have emptied the tank enough to notice the difference.
Also as already mentioned you may well be overweighted with the steel tank.
The difference is definitely in the last 1/4 of the tank and if underweighted you would know about it on the safety stop.

This is the salient point.

AL80s start out heavy and end up light. I can tell when I hit 1000psi just by the tank starting to float off my back. So compared to steel, with an AL80 you need to carry enough "extra" lead to do a neutrally buoyant safety stop at the END of the dive.

Ideally, a weight check should be done with a near-empty tank. One option is to do a check at the very end of each dive, with an empty BCD, until you are "dialed in." Then make a note of the lead needed for next time.

That said, conditions make a difference too. Against a current or when doing u/w work I like to be weighted slightly heavy. Drifting a wall... not so much.