Tank Volume and Weight

I've been shopping for tanks recently, looking at the various options that are available (no, this isn't going to be another LP vs HP vs aluminum 'discussion' -- at least I hope not).

Somewhere (an on-line shop, but I don't remember where), I found a table of a significant number of tanks, their service pressure, capacity, dimensions, and buoyancy. What I find interesting is that, according to this table, many tanks of the same capacity (even different models from the same manufacturer) claim different weights for the air contained inside. In other words, if you take two 80cu-ft tanks and subtract the full buoyance from the empty buoyancy, you get the weight of the air in the tank (~6lbs in this case).

If you then divide by the capacity, you should get the density of air. In theory, that should be 0.07962lb/cu-ft, but most of the tanks on the list work out to 0.074-0.075lb/cu-ft. Some tanks go from 0.067 all the way to 0.108lb/cu-ft.

So what's the deal? Do all the tank manufacturers just over-state their capacity? Or maybe mis-state the buoyancy characteristics?

I'm just curious, because, although dry weight may change based on tank material and size, the weight of the air should be the same, whether you've got an Al80 or an HP120, so the difference in buoyancy between full and empty should be the same.

I think.

marky-d

Without seeing the numbers the only thing that comes to mind is that al 80's don't actually hold 80 cu ft. It's 77.8 or something like that. I would imagine that a steel 80 does actually hold 80 cu ft.

I can see small differences just in dimensions. For instance if a manufacturer is trying for a 95 cu ft tank and wants to market it in a standard size but perhaps his molds actually result in a volume of 94.5 he will still market it as a 95.

Maybe there's a better answer and I'm sure someone will provide it.

Many of those tables are riddled with mistakes.

I have never a found a table that was completely correct.

This was taking into account the Al80 not being an actual 80cu-ft, so that's not the entire issue. The table I used is on the Tech Diving Limited website (can't post the URL due to lack of posts).

So if I'm not crazy, and my reasoning is correct, then why all the complaints about Al80s being overly buoyant when empty? I mean, whether you dive with aluminum or steel, if the tank holds the same amount of air, then the change in buoyancy will be the same. So yeah, you'll have to carry more weight with the Al80, but assuming you are weighted correctly, there should be no difference in the characteristics during the dive.

Yes, I know, I'm ignoring the weight you have to carry around on shore, and issues of trim, but I think this main buoyancy point is lost on most divers (perhaps both new and experienced) that just read that aluminum tanks are somehow harder to dive because they are more buoyant at the end of the dive.

marky-d

The buoyancy characteristics have more to do with the properties of the metal than the air inside changing. At 15ft, 500psi, an aluminum tank is slightly positively buoyant, where a steel tank is slightly negatively buoyant.

As long as you check your weight at the beginning of your dives and are properly weighted for the full tank that you are wearing on that dive, there is no difference between tanks with regard to change of buoyancy.

markyd:

So if I'm not crazy, and my reasoning is correct, then why all the complaints about Al80s being overly buoyant when empty? I mean, whether you dive with aluminum or steel, if the tank holds the same amount of air, then the change in buoyancy will be the same.

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You are correct. The change in buoyancy between the beginning and end of a dive should be similar for tanks of equivalent size, although I agree with the comments of several posters who suggest that the data in the various tables may not always be precisely accurate. The comments (complaints) you refer to are generally based on the fact that AL tanks become positively buoyant when empty (+4.4 for an AL80), compared to steels which remain negative (e.g. -2.5 for an HP 80). But, as I suspect you are thinking, if you are weighted properly to begin with (i.e. you have previously done a weight check with a near empty tank, and therefore know what you need to be carrying), the change during the dive shouldn't be an issue. All it means is that a diver with an AL 80 is carrying more add-on weight, somewhere, compared to a diver using a steel tank. One other factor that appears consistent across tables - HP tanks are actually a little lighter when empty, compared to equivalent size AL tanks (80s and 100s), even though they are more negatively buoyant. So, at least on land the total weight of gear, including the weight necessary for the dive, may be less with HP steels.

here is one

Diver Discount Catalog-Scuba Equipment,diving gear,spear guns & scuba diving equipment

remember....google is your friend

markyd:

This was taking into account the Al80 not being an actual 80cu-ft, so that's not the entire issue. The table I used is on the Tech Diving Limited website (can't post the URL due to lack of posts).

So if I'm not crazy, and my reasoning is correct, then why all the complaints about Al80s being overly buoyant when empty? I mean, whether you dive with aluminum or steel, if the tank holds the same amount of air, then the change in buoyancy will be the same. So yeah, you'll have to carry more weight with the Al80, but assuming you are weighted correctly, there should be no difference in the characteristics during the dive.

Yes, I know, I'm ignoring the weight you have to carry around on shore, and issues of trim, but I think this main buoyancy point is lost on most divers (perhaps both new and experienced) that just read that aluminum tanks are somehow harder to dive because they are more buoyant at the end of the dive.

marky-d

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The difference between AL and Steel is great. The steel tank distributes the extra weight nicely over the length of the tank, instead of having the extra weight in one spot. Going from AL80 to HP100 I was able to remove 8 lbs of lead from my weight pockets. You get much better weight stability and trim when the weight is spread over your body. Also the steel tank is a little smaller.

Adam

gcbryan:

Without seeing the numbers the only thing that comes to mind is that al 80's don't actually hold 80 cu ft. It's 77.8 or something like that. I would imagine that a steel 80 does actually hold 80 cu ft.

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This is correct. An aluminum '80' is really 77cuft. Some steel 100's are actually 103cuft. As far as weight, some manufacturers include the weight of the valve in the tank weight, some do not. Some give the 'empty' weight at 0psig some at 500psig. Also, there is a lot of 'rounding' done in the formation of comparison tables.

So, you need to just consider any table as just 'ball park'.

I have an OMS 112 aka Faber 108, my wife has doubled OMS LP66s aka Faber LP65s.

Of course the real fun with cylinders that have a "+" rating is that they are at their rated volume only when filled to +10%.