Weight belt equation

[ChrisA]

This doesn't make sense. if an Al80 and an ST80 (LP) are the same weight and dimensions, then their bouyancy would be the same, thus the steel LP will not take weight off your belt. If an HP80 is smaller and lighter, then it might (depends how much smaller in relation to how much lighter).
Yes steel is stronger (and important to tank life, more elastic), but is also much denser (2.9 times as dense).
Aluminum, 6061-T6 2.7g/cc
Soft Steel (0.06% C) 7.87g/cc.

First off, an AL80 is not the same dimension as an LP80. The LP80 is two inches shorter. Yes, my LP80 filled only to 2640psi is physically smaller then an AL80. The AL tank has very thick walls.
(The new PST e7-80 is _very_ small, about 4 inch shorter than an AL80)

The weight of a tank in water equals the weight of the tank itself minus the weight of the water displaced by the tank The steel tank, being smaller has less bouyant (upward) force and a little more weight

The reason the steel tank can "take weight off the belt" is because it is physically smaller and displaces less water.
The steel tank is a "win" two ways 1) because the total cubic volume of you and your tank is less and 2) because the steel tank weights more. #2 does not matter as much because if the tank were lighter yu would only need more wlead. It's #1 that reduces the total weight on your feet.

 

[eponym]

Whether you are using AL tanks or steel or anything else doesn't matter, as the weight of the tank stays the same, but you have to adjust for the air that you have used, as you will be carrying that much less weight at the end of the dive.
Peter

I agree with you, Peter (and with Don), and I applaud your reasoning. When I strap 80 (okay, 77.4 or so) cubic feet of air on my back, I get weighted neutrally in around 15 feet of water. Then I add six pounds (the weight of the air in the tank) so I can breathe the tank empty and still hover at my safety stop.

I breathed my tank down to 500psi at the end of my safety stop on my first deep dive (100 feet, completely dark, 39 degrees F) at which point I became buoyant . . . hadn't allowed for losing five pounds of weight in the form of air.

 

[eponym]

This would be one of those "It's on the internet, so it must be true" things. I don't care what this guy's PHD is in, water density/pressure is not going to vary significantly with a change in altitude. Once you leave the surface and start descending, you gain another ATM of pressure for every 34 feet (give or take). If you believe this guy, you'll be adding another ATM of depth every 11 feet or so at the top of Mt Everest.

Um, glbirch, I'm quite familiar with "Harris" Taylor's altitude correction paper, and I'd like to quietly point out that he does not subscribe to this fablulous compressibility of water effect (which is perhaps limited to areas of Texas?). Taylor's paper just shows how to correct depth readings from a gauge calibrated to display fsw below mean sea level - when the gauge is used at higher altitudes and in fresh water.

I used his examples to put together a correction table for diving some of our Oregon lakes, and then took my gauge down and compared it to a marked line and to the table.

So, no offense meant, but please understand that Taylor's no internet looney.

 

[Groundhog246]

The reason the steel tank can "take weight off the belt" is because it is physically smaller and displaces less water.
The steel tank is a "win" two ways 1) because the total cubic volume of you and your tank is less and 2) because the steel tank weights more. #2 does not matter as much because if the tank were lighter yu would only need more wlead. It's #1 that reduces the total weight on your feet.

I agree that the steel tank can take weight off your feet. I just don't see why it's so important to do so. If we were talking 20 or 30 pounds I could, but not the few pounds for a single tank. For a deco dive, with doubles (and large ones at that), stages, etc, sure the weight difference could be significant. But the difference for an AL80 to a Steel, especially when around here I can almost buy 3 AL80's for the price of one steel. And life wise, my oldest AL is 30 years and going strong (actually my ST72's are about the same age). And note, I own and dive both AL80's and ST72's, so I do know how much weighting change there is between them.

 

[glbirch]

Um, glbirch, I'm quite familiar with "Harris" Taylor's altitude correction paper, and I'd like to quietly point out that he does not subscribe to this fablulous compressibility of water effect (which is perhaps limited to areas of Texas?). Taylor's paper just shows how to correct depth readings from a gauge calibrated to display fsw below mean sea level - when the gauge is used at higher altitudes and in fresh water.

I used his examples to put together a correction table for diving some of our Oregon lakes, and then took my gauge down and compared it to a marked line and to the table.

So, no offense meant, but please understand that Taylor's no internet looney.

Well, I've never met the man, so I can only go from his written work. From his web page:

The barometer reads 24.61 inches (625 mm) Hg. Thus, at this altitude, 24.61 inches (625 mm) Hg (not 29.92 inches (760 mm) Hg) is the atmospheric pressure! ...

Thus, every 27.9 feet of fresh water (not 33 fsw) at this altitude corresponds to one atmosphere of pressure at this altitude.

Water pressure is a function of the weight (mass) of the water column over you. The weight of the water column depends on the density of water and the force of gravity. If we assume that gravity is not changing then density must be?

I'm not offended, but even if his answers are correct I question the methodology.

And why Texas?

 

[O2BBubbleFree]

I could ne wrong, having just read over the above posts quickly, but I think there is one factor missing in the discussions regarding aluminum versus steel tanks. Archimede's Law deals with density (weight per volume), not weight per se. And steel is much denser than AL.

Oh, and glbirch, I think the 'Texas' slam was aimed at me, the initiator of all this altitude discussion. Didn't work, though, as I just live in Tx, I'm not a Texan.

 

[Don Burke]

I could ne wrong, having just read over the above posts quickly, but I think there is one factor missing in the discussions regarding aluminum versus steel tanks. Archimede's Law deals with density (weight per volume), not weight per se. And steel is much denser than AL.

There is also considerably less steel in a steel tank than there is aluminum in an equivalent aluminum tank.

That is why the steel tank is often lighter in air.

 

[Don Burke]

I agree that the steel tank can take weight off your feet. I just don't see why it's so important to do so. If we were talking 20 or 30 pounds I could, but not the few pounds for a single tank. For a deco dive, with doubles (and large ones at that), stages, etc, sure the weight difference could be significant. But the difference for an AL80 to a Steel, especially when around here I can almost buy 3 AL80's for the price of one steel. And life wise, my oldest AL is 30 years and going strong (actually my ST72's are about the same age). And note, I own and dive both AL80's and ST72's, so I do know how much weighting change there is between them.

My steel 72s weigh a few pounds less in air than my AL80s. They also require six pounds less weight per tank.

That is around nine or ten pounds for singles and around eighteen to twenty pounds for doubles.

My old codger knees really appreciate that coming up a ladder.

Over the twenty years plus I expect to use them, the cost difference is truly minor.

 

[Groundhog246]

My steel 72s weigh a few pounds less in air than my AL80s. They also require six pounds less weight per tank.

That is around nine or ten pounds for singles and around eighteen to twenty pounds for doubles.

I use 4 lbs less lead with my ST72's than my AL80's. I've never weighed them in air.

If you use six pounds less for singles, where does the nine or 10 pounds come from?

 

[O2BBubbleFree]

My steel 72s weigh a few pounds less in air than my AL80s. They also require six pounds less weight per tank.

That is around nine or ten pounds for singles and around eighteen to twenty pounds for doubles.

If you use six pounds less for singles, where does the nine or 10 pounds come from?

Maybe I shouldn’t answer for someone else, but it seems clear to me. Don was talking about reducing total gear weight in air, right?

“My steel 72s way a few pounds less...,” I assume 2-3 pounds.

They are less buoyant than AL80s, so for neutral buoyancy "six pounds less" lead is required on the belt.

2-3 pounds less in tank weight and 6 pounds less in lead = 9-10 pounds less in total gear weight.

Right, Don?