Steel Tank?

[MikeFerrara]

Especially if I'm wearing much exposure protection I'd rather have a steel tank. You'll have more of the weight up where you need it and with a thick suit you'll still need to add weight but not as much.

Al tanks make great stages. If you don't need a suit they'd be great but our pools around here aren't even that warm.

 

[DA Aquamaster]

As an aside, when I play around with my double hose regs in warm water, I can wear the tank, the reg, a horse collar BC and a T-shirt (to keep the straps from chafing) and a steel 72 will give me near neutral bouyancy throughout the dive. An AL 80 requires me to use a weight belt with a couple of 2 lb weights to allow me to stay down toward the end of the dive.

But back to the question at hand, I agree that a large amount of ditchable weight presents more hazards than it prevents. In nearly 20 yrs of diving, I have seen several incidents caused by unintentional loss of weight pouches or belts but have only encountered one situation where a weight pouch or belt was intentionally dropped to resolve a problem.

This involved a dry suit diver getting himself inverted with too much air in the suit that then went to his feet. He dropped his integrated weights at the surface to be able to get bouyancy high on his body to get horizontal on the surface. Personally, I would not have done this. The same thing could have been accomplished in a couple more seconds just inflating the BC, and with proper technique a properly trained dry suit diver can recover from this position by tucking his legs and rolling upright. Dropping weights was AN answer but not really THE answer to the problem.

I think it could be argued that the only really legitimate need to ditch your weights with modern scuba equipment (assuming you are properly weighted etc) would be to quickly establish positive bouyancy on the surface.

 

[rrh]

I still think if you'll find it difficult to swim steels from depth if you are weighted properly i.e., find your weight with near empty tanks, then fill the tanks with air, drop down to 30ft and let all the air out of your drysuit and BC, then swim up. Like I said, I am not telling people that your hair will catch on fire if you dive steel tanks, I am just saying...believe it or not AL 80s are safer then steel. Again, the key is the minimal bouyancy swing compared to steel.

My buddy and I go round and round about this, so we are going to conduct a little control-test. My honest opinion is that many people dive over-weighted because civilian agencies beat it into their heads about to fear ascents. I have seen it enough times in 17 years. Yes, ascents are bad...but you screw up, you come home to the surface. If you dive weighted to stay down...you can have an accident that will keep you down if you can't swim your kit or get rid of some weight.

This will crack you up, but proves my point. When I first learnedd to dive in the Navy, I wore twin 72s (straps were part of the tank bands) and a weight belt...no BC just a UDT vest with enough CO2 to get me up 30-90 depending on the number of cartridges. Bouyancy was obtained by finning. Deeper down, it got very hard to swim up. (But we were in pretty good shape comparied to average diver.) As I said before, many people in my class were dropped because they could not tread water for 10minutes with a wetsuit weight and twin steels.

take care

 

[Genesis]

Again, the key is the minimal bouyancy swing compared to steel.

Horsefeathers.

19cf of air (or nitrox) weighs about a pound. It matters not what kind of tank it happens to be in.

Trimix weighs less, with 30/30 being the lightest of the "common" mixes. Again, the tank its in doesn't matter.

 

[rrh]

Wow...hit a nerve, you just got nasty and silly all at once.

Go back to scuba 101 and Archimedes principle. Buoyancy is about displacement, not just weight. (Even my brother Jar Heads know this.)

A tank’s buoyancy shift is not JUST about the gas, its about the container material, external size and internal volume. Remove or add, any equal amount of the same gas from or to dissimilar containers and there is a difference. For example, I have 38cf balloon and a 38cf cylinder full of He. I will vent 19cf of He from both… Seriously, reread Archimedes principle.

I am done with this thread.

Later...........

 

[MikeFerrara]

Archimedes principle...

An object is bouyed by a force = the weight of the water displaced by an object - the weight of the object

The only thing about a that changes during a dive is that it gets lighter due to having less gas in it. Therfore, it's buoyancy changes by a force = weight of the air used.
Seems simple to me.

 

[Genesis]

Go back to scuba 101 and Archimedes principle. Buoyancy is about displacement, not just weight. (Even my brother Jar Heads know this.)

Yes, it is indeed.

The tank volume is a constant. At least you hope it is; otherwise, its likely exploding on your back.

The only variable is the mass of the air inside, which you consume during the dive, thereby creating a buoyancy shift.

The material that the container is made of is irrelavent to the shift.

An AL80 goes from about -.5 to +4 during a dive.

A steel HP80 goes from about -5.5 to -1 during a dive.

A steel LP80 would typically (depending on who made it) go from about -7.5 to -3 during a dive.

It is the same shift. All of these tanks undergo about a 4.5lb shift in a buoyancy from full to empty. A larger (higher capacity) tank undergoes more shift, but that is solely because it contains more gas.

If you need to wear weight to overcome positive buoyancy (say, for example your exposure suit) you likely want some (or even most) of it to be non-ditchable. You probably also want most of it on or around your center of buoyancy, because anywhere else will play hell with your trim.

Your tank happens to be an excellent location for that weight, as it roughly correlates with the center of buoyancy.

Your fellow Jar Heads (and perhaps you!) should have remained awake during scuba (and physics) class.

 

[DA Aquamaster]

Oh my, it gets worse and worse...I gotta ditto the "horsefeathers" comment.

Yes a 38 cu ft tank and a 38 cu ft balloon will have different bouyancy "shifts" if 19 cu ft are released from each. There will be about 1.52 lbs of bouyancy increase in the tank, (19 cu ft X approx .08 lbs/cu ft) due to the loss of the weight of air in the constant volume container. Meanwhile the balloon will lose (roughly) half it's size and about 1216 lbs of bouyancy. (that enourmous change in bouyancy is why we dive with scuba tanks rather than balloons. )

However the difference in this example is due almost entirely to the change in volume of the container and has exactly nothing in commom with scuba diving. Personally I try not to dive with scuba tanks, AL or steel, that massively expand when filled.

The volume of the tank whether steel or Al will for all practical purposes remain constant and any bouyancy difference between a full or empty tank is due soley to the decrease in weight of the gas in the tank.

HE mixes are still lighter than air and a mix filled tank would be less negative than an air filled tank. The difference would not be huge but would be measurable. If the HE comprised 16% of the mix in an 80 cu ft tank, the weight difference would be equal to the lift produced by a 5 cu ft helium ballon as the lift of the balloon is essentially the weight difference between 5 cu ft of Helium and the 5 cu ft of air it displaces. (assuming the balloon weighs nothing)

It is incorrect to say that an Aluminum tank is safer than a steel tank, or vice versa, in and of itself. Bouyancy depends on the whole package and you cannot consider the effects of a tank without considering all the other factors involved.

The simple truth is that if you are properly weighted at 15 ft, you are going to experience the same wet suit compression and loss of bouyancy at 100 ft. regardless of what tank you wear. The only difference will be how much weight is in the tank and how much is on the belt or in the weight pockets.

Diving an AL 80 and ditching a large amount of weight from 100' is going to take you all the way to the surface whether you want to go there or not.

In contrast, diving a steel tank and ditching substantially less weight will still allow you to swim your tank up and leave you much less bouyant at the surface allowing for a slower more controlled ascent and may possibly allow you to swim yourself "down" during a saftey or deco stop. (in my case I require only 12 lbs of lead with steel tanks and ditching one weight pocket would leave me 6 lbs positive at the surface and perhaps 5 lbs positive at 15 ft. - a manageable amount if I stay in motion and swim down.)

The third alternative, doing it correctly and using a liftbag, will allow you to keep your weights and control your ascent by swimming up or by using the dumpable lift bag and actually stop for a deco or saftey stop. The liftbag is also quite open minded and nonpredjudicial and does not care what tanks you are wearing.

An Al tank may well allow more ditchable weight, but this in my opinion does not make it safer - it's just not as simple as that.

 

[roakey]

rrh once bubbled...
Go back to scuba 101 and Archimedes principle. Buoyancy is about displacement, not just weight. (Even my brother Jar Heads know this.)

A tank’s buoyancy shift is not JUST about the gas, its about the container material, external size and internal volume. Remove or add, any equal amount of the same gas from or to dissimilar containers and there is a difference. For example, I have 38cf balloon and a 38cf cylinder full of He. I will vent 19cf of He from both… Seriously, reread Archimedes principle.

Note in your two examples the displacements changed, that's the key.

rrh started off so well and then took a turn into left field. He's (she's?) correct, it's all about Archimedes principle. So it's about just two things, weight and displacement. Absolutely nothing else, not materials and not internal volume.

As a cylinder is filled, we can assume it's external volume (displacement) does not change. It changes a little, but you need sensitive equipment (read: a hydrostatic tester's burette) to measure the change. So it's all about weight. Air weighs about 1 pound for every 13 cf. So for every 13 cf of air you stuff into a cylinder, its buoyancy decreases by 1 pound. So as an AL80 goes from full to empty, it's buoyancy "swings" 80/13 = 6 pounds. An HP steel 80 swings by 80/13 = 6 pounds. A hypothetical wood cylinder that held 80 cf or air would swing 80/13 = 6 pounds. A zillion ton nuclear submarine, if you vent 80 cf of air will swing 80/13 = 6 pounds.

This is because the displacement of a cylinder does not change from full to empty. So only it's weight, which changes at 1 pound for every 13 cf of air in it, has any effect on buoyancy

Now, what DOES change from steel to AL is the ENDPOINTS of this swing, and this IS due to the material. Taking some round numbers, an AL80 swings approximately from -3 to +3 pounds. A HP80 goes from about -9 to -3, a "neutral" 80 (misnamed for marketing reasons) goes from about -6 to 0.

So this is why you can typically drop weight if you move from an AL80. If for example you dive with 10 pounds with an AL80 and you switched to an HP80, the AL80 was -3 when full, the HP80 is -9 when full, so the delta is 6 pounds, so you could reduce your carried weight by 6 pounds, in other words drop your weight belt to 4 pounds. If you went from a AL80 to a "Neutral" 80, you could drop three pounds.

THIS is why you can drop weight when you move from an AL80 to steels.

So just to make sure: NO CYLINDER REMAINS NEUTRAL THROUGHOUT THE DIVE. If anyone ever tells you a cylinder can, they should run, not walk to the Nobel prize committee because they've just discovered a violation of one of the basic laws of physics that govern our lives.

Roak

Ps. This is why Genesis was so hot in his reply, this myth crops up from time to time, perpetuated by dive shop monkeys and marketing types that named the "Neutral" 80 to sucker people into thinking that it's neutral throughout the dive. We wish it this misinformation would just go away.

 

[roakey]

Except for the weight of air. I got mine from my CRC Handbook of Chemisty and Physics, and trust me, it's about 13cf per pound.

This also coincides with the more accurate cylinder swings published.

Roak