Tank bouyancy, help in comprehending

[Central Fla]

Please be patient, I know the answer is right in front of me but
just like algebra I am having a hard time comprehending it.

I am trying to understand what makes such a great difference in bouyancy
when the values are so close. My understanding is that I will be able to
remove about 12 pounds of weight going to steel, I just don't understand
why. My tanks are Catalina Alum. 80's vs. Faber steel 80.

Al 80 cap 77.4, press 3000, Diam 7.25, length 25.8, empty wght 31.6,
full bouy -1.7, empty bouy 4.1, Real weight 35.7 .

St 80 cap 80, press 3180+, Diam 7.25, length 20.1, empty wght 32.4,
full bouy -13.2, empty bouy -7.3 Real weight 25.1 .

The steel tank has a slightly smaller hollow cavity, and weighs about
the same when empty. I have no Idea what "real weight" means or why
it is so different. the difference between the two full bouyancies suggest
that I can remove appx that weight from my gear but why, they dont
appear that different.

I also would like to ask if being bouyant from al tanks are good from a safety
standpoint, If I become ill or try to bring someone else up I can dump that
extra weight and be bouyant without having to add additional air in my BC,
silly ?, can it be good to be bouyant?
Thanks.

 

[reo]

The reason the steel tank buoyancy is less is that the steel tank volume is roughly 80% of the AL tank and the empty weight is slightly more than the AL tank, making the steel tank more dense (less buoyant) than the AL tank.

The "real" weight takes tank buoyancy into account and provides a convenient way to compare tank buoyancy characteristics and weighting requirements when using different tanks.

Real weight = empty weight + empty buoyancy. For the steel tank, that works out to:
32.4 +(-7.3) = 25.1.

Since the AL tank has a real weight of 35.7, you'll need 10.6 lbs more weight (lead on your weight belt) than you would with the steel tank, everything else being equal.

If you usually dive with the AL tank and wanted to try the steel tank, you would be able to remove 10 lbs of lead from your weight belt.

Dumping weights is the last resort to increasing buoyancy. I would add more air to my BC to increase my buoyancy if I needed to lift something to the surface (if I didn't have a lift bag).

Ideally, dive with the least amount of weight necessary. Try to weight yourself where you are neutrally buoyant with 500 psi in your tank and no air in your BC. If you are too heavily weighted, you'll be adding/venting air from your BC every time you change depth to maintain neutral buoyancy.

I hope this adequately answers your question.

Ron

 

[Charlie99]

The "real" weight takes tank buoyancy into account and provides a convenient way to compare tank buoyancy characteristics and weighting requirements when using different tanks.

Real weight = empty weight + empty buoyancy. For the steel tank, that works out to:
32.4 +(-7.3) = 25.1.

Since the AL tank has a real weight of 35.7, you'll need 10.6 lbs more weight (lead on your weight belt) than you would with the steel tank, everything else being equal.

I hope this adequately answers your question.

It answered it, but incorrectly. Empty weight is not used in figuring the change in lead when changing tanks. The only thing "empty weight" affects is how much total dry land weight you have to haul down to the beach, or up the ladder of a boat.

For comparing how much lead one has to wear, the only spec that counts is "EMPTY BUOYANCY". The change in lead when going from one tank to another is simply the algebraic difference in EMPTY BUOYANCY.

Using the original poster's numbers of +4.1 for the AL80 and -7.3 for that particular steel 80, you can see that the difference is 11.4 pounds. So going one way, drop 11 pounds, going the other way add 12.

A simple way to visualize or comprehend this calculation is to realize that if you have 11.4 pounds of lead strapped onto an empty +4.1 pound buoyant AL80, the tank + the lead will have the same 7.3 pounds of negative buoyancy as the empty steel tank without any lead.

Charlie Allen

 

[spectrum]

Hi Central Fla,

First you have two things going on with cylinder properties. One is buoyancy (+ or -) which will effect how much lead you need to wear. The second property is weight which you will feel when walking and which is also applicable to Newtons laws while in the water. Less lead and less weight combine to make you 12 pounds lighter when going from a Luxfer AL-80 to a PST E7-100. Other combinations can have similar results.

You did your homework so lets see what you have here.....
Your buoyancies are 4.1 and -7.3 combined that is 11.4 pounds of lead you can remove from your perfect AL80 weighting. The aluminum cylinder will lift you as it empties, the steel will continue to help you stay down.

Your aluminum 80 has an empty weight of 31.6. One dirty little secret of Al cylinders is that empty weight excludes the valve so add another 2 pounds for 33.6. Valve weights will vary but consider this close enough.


The steel Faber you are considering weighs 32.4 pounds, with valve. Subtracting the 2 you see that the steel is 1.2 pounds lighter.

Adding 11.4 and 1.2 pounds you see that you are now 12.6 pounds lighter walking across the beach or climbing the ladder.

80 CF of air will weigh the same and on the steel you are not short sheeted with the 77.4 true capacity of the AL80.

You can read the same analysis of a different case here on my site. The bottom line is that the steel cylinder is a more efficient package for the job.

I have never bothered with real weight. It's just a derivative of the essential specifications. Most divers wear enough neoprene to make the negativity of a single steel cylinder a non issue. When diving doubles or without neoprene you can begin to put yourself in a less than optimal condition. The value of steel is really amplified as the water gets colder and the suits get thicker.

Pete

 

[Nemrod]

It don't take much to see that from your figures the why of it, heck man, one tank is nearly six inches longer!!! It displaces more water as a result. Yeah, that matters. How can you say it does not look much different when it is nearly half a foot longer? Just wondering.

N

 

[Central Fla]

Thanks all, that was the help I needed. It was just hard to believe that by going with a steel tank that I was lighter. What was really surprising was that it weighed about the same, so where is the advantage in aluminum, why even build aluminum tanks?

It don't take much to see that from your figures the why of it, heck man, one tank is nearly six inches longer!!! It displaces more water as a result. Yeah, that matters. How can you say it does not look much different when it is nearly half a foot longer? Just wondering.

I thought that somewhere the steel would offset for that. I looked at the length and it appeared to be the only sizeable difference, I just didn't think a tube 7.25 x 5.7 (the difference) would float 12 pounds.

 

[AggieDad]

I dive a PST E7-100 and absolutely love them. ONE PROBLEM...travel.
I dive fresh water mainly and when I go on the road I usually rent AL 80's and am in salt water. It is mind blowing to have to go from 4lbs of weight and really don't need that to around 18 to 26 lbs depending upon the wetsuit.
.

 

[spectrum]

What was really surprising was that it weighed about the same, so where is the advantage in aluminum, why even build aluminum tanks?

You will often read that the advantage of steel tanks is that they are heavier. That's misinformation. A low pressure steel tank can be heavier but when you go to high pressure the weight per CFM of capacity drops. Faber is know for making some very heavy and highly negative cylinders. Obviously a heavier tank is not an advantage. A less buoyant tank can be a big advantage (less lead). Buoyancy and weight are two very different properties. A lighter weight cylinder can also be advantageous when out of the water. HP steel blends these 2 the best.

Advantages of aluminum? They are cheaper but the gap is closing. For stage cylinders they can be nice since they pivot around neutral. When diving wet in double cylinders they prevent that excessive negative buoyancy you expressed concern about in your first post. I know a salvage diver that likes them because they will cork to the surface after using them to fill lift bags. You can get them in many more spiffy colors and finishes!

Given the lower cost, lesser risk of internal corrosion (aluminum is not immune) and misconceptions that aluminum must be lighter they will be around.

Pete

 

[The Kraken]

Spectrum puts forth a great explanation . . . !

For me, I'm not too smart, but what I look at is the published buoyancy characteristics of a particular tank.

The whole thing, once again, comes back to your neutral buoyancy at THE END of your dive. Regardless of what you weigh going into the water, you need to be able to execute a controlled safety stop. This is where you need to look at the +/- buoyancy of a tank when it is empty or near empty.

One can get into discussions about how heavy the tank is to carry to and from the dive site, so forth and so on, but one thing is sure, you won't sustain an embolism from carrying a heavy tank to the dive site.

Without meaning to wax into Zen or any of the other orientaly philosophies, diving is a state of balance, the Yin and the Yang, if you would. What you gain on one side of the balance, you lose on the other.

And, like everything else in diving, it gets back to YOU, YOUR dive profile and YOUR physical capabilities and limitations.

the K

 

[spectrum]

I dive a PST E7-100 and absolutely love them. ONE PROBLEM...travel.
I dive fresh water mainly and when I go on the road I usually rent AL 80's and am in salt water. It is mind blowing to have to go from 4lbs of weight and really don't need that to around 18 to 26 lbs depending upon the wetsuit.
.

Of course only about 5.5 pounds of that is attributable to the cylinder change. 6-8 is going to saltwater. The rest must be wetsuit.

Just clarifying for those who are trying to make sense of this equation.

Pete