Quiz - Physics - Minimum Amount of Water That Must Be Displaced

[Brett Hatch]

When you actually lift something, it's a guessing game. You guess what size bag(s) you will need, and you make sure you bring enough. Then you add air a little at a time until you can move it.

Say some sailors lost an anchor, and you were tasked with bringing the thing up. You ask the sailors how much it weighs, they scratch their head and estimate it at the 175kg given. Like @CT-Rich says, there's no sane way to get the actual volume, but luckily they know what it's made out of. So you Google the material, and find out that its density implies that it would displace the 115 liters of water (don't ask me why they're using an anchor made of Delrin, maybe they were drunk, who knows). OK, so now we have our practical setup in the given question.

My practical follow-up question is, how many lift bags would you bring, and of what type?

Surely you would want to bring a little extra, so a single 55kg bag would be out of the question. On the other hand, bringing two 55kg bags seems dangerous: you might fill the first bag close to capacity (say 75%), which brings the anchor close to neutral, then slap the second bag on, fill it to say 25%, and now the system is positively buoyant. At that point you start bringing it up, both bags will expand, which will continually accelerate the ascent rate, possibly creating a very dangerous situation as the 175kg reaches the surface at high speed, possibly bringing you along with it. If the bags have OPVs then you will guarantee a maximum total buoyancy (and therefore ascent rate), but how would you know that it's not dangerous?

Instead of a small number of large bags, it would seem safer to attach a bunch of smaller lift bags with OPVs, perhaps 8 bags of 8kg each. Then completely fill each bag, one at a time, until positive buoyancy is achieved. This way, the maximum buoyancy of the system will be roughly +8kg, which hopefully will guarantee a safer and more manageable ascent rate. On the other hand, having 8 little bags hanging off our anchor might pose an entanglement risk, so I'm torn.

What would you lovely folks do?

 

[Jcp2]

I would lift a rope tied to the anchor with a small lift bag to the surface where the Big winch on my Big recovery shop would be and have it pulled up after everybody is clear and out of the water.

 

[Bob DBF]

At the beginning of a dive Diver A and Diver B are both 9 pounds negatively buoyant. Diver A's steel cylinder weighs 5 pounds more than Diver B's aluminum cylinder. During the dive, both divers consume 5 pounds of air. What are their buoyancies at the end of the dive?

First thing I would clarify is whether the 5# is weight or buoyancy. If weight, you might need some more information.

 

[Bob DBF]

I would lift a rope tied to the anchor with a small lift bag to the surface where the Big winch on my Big recovery shop would be and have it pulled up after everybody is clear and out of the water.

Need any hands. Asking for a friend.

 

[boulderjohn]

First thing I would clarify is whether the 5# is weight or buoyancy. If weight, you might need some more information.

I wrote " Diver A's steel cylinder weighs 5 pounds more than Diver B's aluminum cylinder." What further information do you need?

 

[Bob DBF]

What further information do you need?

Why you even put it in there.

 

[boulderjohn]

Why you even put it in there.

Have you never seen problems that provide more information than you need? In my first draft of the question I provided comparative tank volumes, too. I decided that was a bridge too far.

In this incident, you know that two divers started the dive 9 pounds negative and lost 5 pounds of air. They did not change total volume, and that is all you need to know: 9-5 = 4. They are both 4 pounds negative at the end of the dive.

However, countless ScubaBoard threads will have you believe otherwise. They will tell you that while the aluminum tank loses the weight, becomes buoyant, and pulls you to the surface, the steel tank magically retains its negative buoyancy and maintains your initial buoyancy throughout the dive.

 

[Bob DBF]

They will tell you that while the aluminum tank loses the weight, becomes buoyant, and pulls you to the surface..

Although that may happen, it is not because of buoyancy if the tank. I found that the old stab jackets and to a lesser degree the newer poodle jackets, the tank being lighter at the base as it loses air, will pull up the back of the jacket making the air bubble shift back and up, which will have the gas expand and possibly drag you up if you didn't correct early. The BP/W minimises the effect by holding the tank and BC more securely although you can feel the change in trim.

This effect made the vertical ascent popular.

It’s the air in the BC shifting and expanding causing the buoyancy issue, which is indirectly due to the tank, but the tank gets the blame when divers don’t take the time to find what is actually going on.

 

[EFX]

You have a good point in that it's the system (tank, BCD, single tank strap, tank positioning, tank type, etc.) that effects the buoyancy of the diver. But, to boulderjohn's point, this is all caused by the primary effect of an AL80 tank becoming positively buoyant at the end of the dive. Some AL80's go from -6 to +4 lbs in contrast to some steel tanks going from -10 to 0 lbs buoyant force throughout the dive. Some BC's exasperate the buoyancy effect caused by the tank. It's only those divers that blame the tank who don't understand the entire system's effect on buoyancy; they need to read more or take a buoyancy class.

 

[-JD-]

You have a good point in that it's the system (tank, BCD, single tank strap, tank positioning, tank type, etc.) that effects the buoyancy of the diver. But, to boulderjohn's point, this is all caused by the primary effect of an AL80 tank becoming positively buoyant at the end of the dive. Some AL80's go from -6 to +4 lbs in contrast to some steel tanks going from -10 to 0 lbs buoyant force throughout the dive. Some BC's exasperate the buoyancy effect caused by the tank. It's only those divers that blame the tank who don't understand the entire system's effect on buoyancy; they need to read more or take a buoyancy class.

Dry air at STP is about 0.0807 lbs./cu.ft.

Typical Al80s hold 77-78 cu.ft. of air at rated pressure. Unless you are cave-filling your Al80s (not recommended), the maximum buoyancy swing you will see is a little over 6lbs (6.2) if you are sucking them dry. Al80s don't have a 10lb buoyancy delta.

An Al100 contains about 8lbs of air.

A steel "80" is more likely to hit that 80cu.ft. mark (there are a lot of variations) and contains about 6.5 lbs. of air.

My HP120 steel on the other hand will swing around 8.3 lbs if I start at the rated 3500 psi and come up with 500psi remaining - 9.6 if i were to suck it dry.

Buoyancy swing of the cylinder/air system is only dependent on the amount of air that is consumed not the material that the cylinder is made of.

That said ...

Trim CHANGE will be determined by
- the weight change and (fixed) center of mass of the AIR in the cylinder
AND
- the dynamically combined displacement (volume and location) of the air in the BCD system

Steel tanks are generally more negative or less positive than aluminum for cylinders of the same rated capacity when filled with the same amount of air.