Deep South Divers
Contributor
Funny... I've never seen it on a steel tank - only aluminum.
...And the DiveGearExpress website reported the "+" as a function of hydrostatic testing, and yet, I've always seen it as a function of the visual inspection.
With the steels, overfilling seems so common that I thought they didn't bother with the "+" certification.
Weird how everyone's reporting something different. Inconsistencies in the industry, perhaps?
For what it's worth, I pump all of my AL80's to 3300. When the tanks cool (I don't do "hot" fills, but the tank can still be skin-warm) it's usually at about 3150 or 3200. I've never had a problem.
Yup, true... I was just making the point that different tanks - both aluminum and steel - were different weights. My counterpart there had cited the heaviest aluminum 100 against the lightest HP steel 100.
...But he's right about his point anyway - yes, HP steel 100's are lighter out of the water than aluminum 100's, across the board. Less bouyant, too - although that's not necessarily a clear advantage like having a lighter tank (out of the water) is. Regarding bouyancy characteristics, the answer is, "it depends." It depends on how much weight you want high and far off of your back as compared to on your hips. You can be trimmed too high as much as you can be trimmed too low... Although admittedly, it seems more common to have people trimmed low than high.
Aluminum tanks have the interesting bouyancy characteristic that they're neutral when half empty. This makes them great stages, since a diver doing staged decompression would otherwise have to swim around the entire dive with heavy tanks. If he uses aluminum tanks, they're a couple of pounds negative at the worst, then neutral and then slightly positive at the very end of his dive.
This bouyancy characteristic makes them interesting to use as back gas - if a diver turns sideways during the dive (say, to look at a vertical wall), they tend not to "turtle" the diver - especially when the tank(s) are at the halfway point. Steels - always negative - have the tendency to roll the diver onto his back, an issue brought about by having more weight up high and far off of the diver's back. The fuller the tank, the more weight, and the more weight, the bigger the tendency.
The "roll" action is a function of trim - and a skilled diver can tune most of this action out... But again, it's a matter of where the weight is going on the diver, and in some cases a steel tank can be too much and affect trim poorly.
Given that a lot of divers are already overweighted on their belts (and underweighted on their backs), usually a steel tank is an improvement in trim... But not all the time.
...Which is why I said that it's not a matter of one tank being great and another being terrible... It's a matter of matching the diver's needs with their tanks.
Personally, I wouldn't dive steels in salt water, period. I prefer other methods to trim properly so that I can take advantage of aluminum's unique corrosion properties. But that's just me - I know a lot of people who dive steels in salt water to take advantage of it's slightly lighter weight, slightly smaller size, or slightly more negative tendency.
And I don't see an aluminum 106?
Yep - on the link I provided a post ago... Found it in a couple of other places, too... I've never heard of it either, so it may be something new - or old - I don't know.
