Which short steel tank??? help!

[SubMariner]

I have 3 HP steel 80s: some older Genesis and a new PST. I love 'em. Especially the fact that they are about the same height as an AL 63. All that air in a nice short package.

 

[mempilot]

I used to own two of the PST E7 HP80's. The quality is great. The weight is lighter than an AL80 but more negative so you can shed lead.

I didn't like the trim, but that doesn't mean they wouldn't work for you. I suggest renting or borrowing one to try it out first since they are fairly expensive. I paid $225 a piece and sold them for $195 a piece.

The other thing was the short stature made them hard to bungee in on some boats and I couldn't sit down on a bench and slide into my bc because the bc was at bench level if you can picture this.

 

[mack50md]

You had asked about comparing steel tank sizes. DiveRite Express has a comparison chart and tank info at http://www.diveriteexpress.com/gas/steel.shtml

 

[DA Aquamaster]

A Faber 71.2 is a good short tank option if you want extra negative bouyancy. It is about the same height as an AL50, weighs about the same as an AL80 and is 11 lbs negative when filled which will allow you to take a substantial amount of weight off your belt.

 

[derwoodwithasherwood]

HP vs LP isn't what produces the buoyancy swing, it's the weight of gas. All 80s regardless of material will swing the same given the same volume used.

An HP80 and an LP80 contain the same quantity (mass) of air. But the HP80 is smaller in volume. If my high school physics teacher was right, squeezing the same mass into a smaller volume increases the density. So when full, the air inside an HP cylinder should be more dense. But when empty, the density should be about the same between the two.

Since bouyancy is dependent upon density, would this not affect the bouyancy swing?

 

[Dan Gibson]

The higher the pressure, the more dense the gas would be, but you would now have it occupying a smaller space. It’s still 80 cft of gas. It doesn’t magically gain weight. The starting and ending points for a given volume but different materials can vary, but the swing would remain the same.

An HP80 and an LP80 contain the same quantity (mass) of air. But the HP80 is smaller in volume. If my high school physics teacher was right, squeezing the same mass into a smaller volume increases the density. So when full, the air inside an HP cylinder should be more dense. But when empty, the density should be about the same between the two.

Since bouyancy is dependent upon density, would this not affect the bouyancy swing?

 

[The Kraken]

Just look at the two different tanks.
They each will contain the same volume of air, but at different pressures.
What is getting involved is the difference between the displacements of the two tanks and also the difference between the empty weights of the tanks.

 

[jonnythan]

Put another way, density is simply a function of mass and volume. Volume doesn't actually matter, it's the mass that determines buoyancy. When you take 80 cf of air out of a tank - any tank - you reduce its mass (and therefore weight) by the same amount.

 

[Sideband]

Right. It's the, "which weighs more, a ton of lead or a ton of feathers?" In this case it's, "which weighs more, 80cft of air or 80cft of air?"

Joe

 

[Scubakevdm]

HP vs LP isn't what produces the buoyancy swing, it's the weight of gas. All 80s regardless of material will swing the same given the same volume used.

Whatever produces the swing, Table 1, "Sample List of Avialable Scuba Cylinders" in the PST dealer book lists the bouyancy characteristics of a variety of tanks. This table lists an LP 80 (78 cu ft. @ 2640 psi) and an HP 80 (80 cu ft @ 3500 psi).

Saltwater bouyancy for the LP 80.... 0.7 lbs empty..... -4.7 lbs full. Swing = 5.4 lbs.

Saltwater buoyancy for the HP 80... -7.3 lbs empty.... -13.2 lbs full. Swing = 5.9 lbs.

That's a half pound less. Granted, the LP tank has has 2 cu ft. less volume, but it just doesn't seem like 2.5% of the volume is going to account for 10 percent of the swing. Anyway, like I said regardless the reason why the LP tank swings less, at least according to PST.