Stainless Steel Tanks?

[agilis]

Almost all dive shops in this area do 3500 lb. refills, no problems. Actually, they fill them a bit higher, so you get 3500 after the tank cools.
The hp steel 80 I bought from Scubatoys three months ago is a delight: smaller and lighter than aluminum 80s, with a much more even weight curve from full to empty. It holds, I believe, about 72 c.f. @ 3000lbs. These hp steel tanks are filled to pressures considerably higher than 3500 lbs in some other countries.

 

[spectrum]

The hp steel 80 I bought from Scubatoys three months ago is a delight: smaller and lighter than aluminum 80s, with a much more even weight curve from full to empty.

Care to explain the curve part?

 

[agilis]

Forgive me for being unclear. By curve I meant the gradual buoyancy changes as air is consumed. These changes can, of course, be plotted or envisioned as a curve. A steel hp tank starts off with negative buoyancy, and remains slightly so even when down to only a few hundred pounds, unlike the aluminimum 80 I used for years, which becomes positively buoyant when nearly empty. The approximately -8 to -3 of a hp steel 80 is, at least for me, an easier curve to compensate for during a dive, and allows the use of less lead at the start. I very much appreciate the smaller more compact dimensions and the moderate but noticably lighter out of water weight. My subjective impressions after a number of dives with the hp80 steel tank have all been positive, at least so far, and there has been absolutely no problem with hp air fills.
Several shops seemed quite willing to go a little past 3500, perhaps because the tank is new.

On the original topic, one comment: stainless steel also rusts.

 

[rjack321]

All tanks change bouyancy based on how much gas is consumed. 80cf of air or nitrox weights about 6 lbs, regardless of what cylinder held it. They amount of change can only be altered by using a different gas. E.g. trimix isn't so heavy to start, but you end up with basically the same bouyancy at the end.

Granted, the ratio of ending weight / starting weight may vary more or less depending on the material of the tank.

I have 2 hp80s, but I find them too small for most dives. Search for threads on "rock bottom" and you'll understand.

 

[roakey]

Care to explain the curve part?

I've never heard it called a "curve" and actually that's not a very good word to use since the buoyancy change is linear as you breathe a cylinder down.

The amount of weight/buoyancy change of a cylinder is typically called "swing" or "swing weight". rjack's right, in that an AL80 swings about 6 pounds from full to empty. Air weighs about 1 pound for every 13 cubic feet (CF). So an AL80/13 = ~6lb swing. A steel 130/13 = ~10lb swing. Note that swing is INDEPENDENT of material, pressure, whatever – it only depends on how many cubic feet of gas you pack into the cylinder.

However, the swing is only half the equation. The inherent density of the cylinder determines the absolute buoyancy of a cylinder at any point in time. An HP80 is lighter, but smaller cylinder, and it turns out its inherent density is GREATER than that of an AL80.

Therefore (speaking only in rough terms), though both an AL80 and HP80 swing 6 pounds from full to empty, the AL80 may swing from -3lbs to +3lbs and the HP80 may swing from -7lbs to -1lb. This delta (4lbs) is weight that may be dropped from your weight belt when switching from an AL80 to an HP80.

Commonly people state that it’s the material (steel/AL) that determines the absolute density of the cylinder. In actuality it’s the design of the cylinder, though it’s steel that enables manufacturers to create lighter, smaller cylinders with greater density than their AL counterparts.

An example how this is really independent of material is the poorly named “Neutral” 80. This is an AL cylinder that has about the same external dimensions as the original AL80, but has thicker walls and can take greater pressure (to make up for its reduced internal volume). It still packs 80CF internally, so it still swings 6lbs, but it weighs more, so its density is greater and therefore it swings (again, roughly) from -6lbs to 0 over the course of the dive.

Hint: For $100 less, you can fasten a 3lb weight to a "normal" AL cylinder and get the exact same buoyancy and weight characteristics.

Roak

 

[agilis]

roakey, I used the word "curve" because I envisioned the decreasing air density and consequent increasing buoyancy as a series of points on a graph, in a simplistic trigonometric sense. Thus, the word's meaning reflects a line defined as an equation rather than a deviation from a straight line.
Obviously, though, I overreached myself. I was not attempting to use any standardized terminology from the technical world of compressed air tanks, a subject about which I know almost nothing beyond those things pragmatically connected with their use. My extremely gradual learning curve makes it unlikely that situation will change. Your explanation was detailed, clear and enlightening, and I am sincerely grateful. I still like my new steel hp80 better than the al80 I had been using, and you have helped me to better understand why.

 

[meesier42]

the tank weights are terrible for me. I tried one on and AL80 and it threw my balance off so much that I couldn't swim anything other than vertically. I stick with AL80's cause they are cheap and everywhere, I have bought a couple of used ones now and haven't paid over $100, and that's after the money for the Hydro and VIP. Basically my thought is, if I always dive the same basic cylinder then I have one less thing to screw with when I travel.

 

[GDCB]

All tanks change bouyancy based on how much gas is consumed. 80cf of air or nitrox weights about 6 lbs, regardless of what cylinder held it. They amount of change can only be altered by using a different gas. E.g. trimix isn't so heavy to start, but you end up with basically the same bouyancy at the end.

Granted, the ratio of ending weight / starting weight may vary more or less depending on the material of the tank.

I have 2 hp80s, but I find them too small for most dives. Search for threads on "rock bottom" and you'll understand.

Okay I bit but no luck. "rock bottom"?

 

[roakey]

Okay I bit but no luck. "rock bottom"?

http://www.scubaboard.com/showpost.php?p=401869&postcount=65
http://www.scubaboard.com/showthread.php?t=90264

Bottom line: If you really value your buddy, and you really follow gas management rules (versus "back on the boat with 500psi" which isn't gas management) you have far less gas to use on the dive than you think. With an AL80 at 130 feet, for example, you start your ascent with 1500PSI left in the cylinder. Yes, half.

Most diver's trained today respond to this with "I'm not going to waste all that gas!" which to me shows how truly horrible modern SCUBA training has become.

Roak

 

[spectrum]

Forgive me for being unclear. By curve I meant the gradual buoyancy changes as air is consumed. These changes can, of course, be plotted or envisioned as a curve. A steel hp tank starts off with negative buoyancy, and remains slightly so even when down to only a few hundred pounds, unlike the aluminimum 80 I used for years, which becomes positively buoyant when nearly empty. The approximately -8 to -3 of a hp steel 80 is, at least for me, an easier curve to compensate for during a dive, and allows the use of less lead at the start. I very much appreciate the smaller more compact dimensions and the moderate but noticably lighter out of water weight. My subjective impressions after a number of dives with the hp80 steel tank have all been positive, at least so far, and there has been absolutely no problem with hp air fills.
Several shops seemed quite willing to go a little past 3500, perhaps because the tank is new.

On the original topic, one comment: stainless steel also rusts.

Slope would be a better term. Curve got me wondering.

I love my HP80s as much as the next diver. I think what you are experiencing is more of a prefered overall trim condition. The delta during the dive will be identical for the came cubic feet of air used regardless of cylinder.

The stout cylinders such as the HP80 often trim best for less than tall divers like myself, I'm 5-9.

Pete