OWD Course Question Quibble

[eponym]

Hi, Watson.

I have a comment about one of the study guide questions in the current Open Water Diver Manual.

In Section 1, study guide question 9 (when answered) is:

Aluminum cylinders will affect your buoyancy more than steel because there is a greater weight difference between a full and empty cylinder.​

In my current class, a student took issue with this question, pointing out that it's the gas breathed out of a tank during a dive that makes one more buoyant. I said he was correct, adding that breathing out 12 cu.ft. of gas adds about one pound of positive buoyancy--all other things (depth, relaxation, respiration) being equal--regardless of cylinder characteristics.

I defended the question by pointing out that many steel cylinders are higher-capacity than an AL80, so breathing a full 100 cu.ft. steel cylinder down to nearly empty will change one's buoyancy by about two pounds more than will breathing down an AL80.

I think that might be SSI's goal with this question, but perhaps you're also aiming to make the student aware that changing to a new type of cylinder requires re-evaluating weighting. I wonder whether the question might benefit from a slight rewording in the next edition?

Best regards,
Bryan

 

[300bar]

In Section 1 of the manual, study guide question 9 (when answered) is:

Aluminum cylinders will affect your buoyancy more than steel because there is a greater weight difference between a full and empty cylinder.​

This has all to do with the Archemedis law.Displacement of water.
If AL and steel tanks have a equal internal volume, the exterial of a AL tank will be larger than a steel tank,Thinkness of the wall,needed to get the same strengh,therefore displace more water then a steel tank thus the AL will float while the steel tank will sink.Therefore the answer is correct.

I'll just tag along for the SSI answer on this.

 

[spectrum]

I like it because it opens the door for the student to trip up on several levels. It is relatively vague however and I can see a student misinterpreting the question and going down a rat hole. I think the key is answer the question and nothing more. If you go onto a tangent of preferred buoyancy properties then you have veered off the road.

The heart of it is that it poses steel/aluminum against empty/full. Common sense says that everything else including cylinder capacity and volume used is the same. With those things constant it comes down to the loss of air weight and nothing else.

While the real concern is buoyancy it speaks to weight difference. Since the given cylinders are constants the change of weight equates to a change in buoyancy and the student should be drawing that relationship.

While steel cylinders do offer the highest volumes aluminum is on a very level playing field in the world of of OW candidates.

Pete

 

[eponym]

Pete - can't imagine what happened. I will try to re-compose and re-post my question.

 

[don Francisco]

For some reason folks continue to conflate various considerations in the steel vs aluminum tank discussions. These are weight in air, weight/bouyancy in water, & difference in weight full vs empty. lets deal with the last because it's the simplest & seems to be where the original question was pointed.

Air on the surface at 1 atmosphere of pressure weighs approximately .08#s per cubic foot. 80 cubic feet if it weights 6.4#s. If we squeeze that 80 cubic feet of air into a tank will add 6.4#s to the weight of the tank. That same 80 cubic feet will still weigh 6.4#s regardless of the size, shape or material of the tank we put it in. (Do 5#s of potatoes weigh any less if you take them out of the sack & stuff them into your pockets? Does it matter if the sack was paper or plastic?)

Therefore adding or removing 80 cubic feet of air will change the weight of any rigid container by 6.4#s, in air, on the surface, or under water. The bouyancy/weight of the container itself is a constant, determined by it's own weight & dimensions, both of which remain unchanged whether it's full or empty.

 

[eponym]

Have re-created the original post and posted it at the top of the thread.

Thanks, Spectrum, for letting me know it was so badly mangled.

To anyone who posted a reply to the first fragment, note that my actual question is mostly about phrasing and teaching goals.

 

[don Francisco]

Bryan- the phrasing is as bad as the physics. As written the statement is patently false. Depending on the intent it might be rewritten.

Equal capacity tanks of any material effect on buoyancy equally, since they have equal weight loss when emptied.
or
Larger capacity tanks affect buoyancy more because there's a greater weight difference, full vs empty.
or
Aluminum tanks affect buoyancy differently than steel tanks of the same capacity, because they differ in both weight & displacement.

The last is more complicated since steel tanks weigh more in the water, both by virtue of higher weight in air & lower displacement. Therefore they represent more of a net transfer of total weight from the weight belt to the tank itself, and it might be said that steel has a greater effect on buoyancy.

We could go on forever, but regardless of the intent it was a bad answer to a flawed question.

 

[24940]

Below is a short exchange I had with Rick Murchison.

Originally Posted by 24940

Aluminum cylinders will affect your buoyancy more than steel because there is a greater weight difference between a full and empty cylinder.

So, if I take a AL80 and a steel 80, suck 40ft3 out of each, the AL tank will have lost more weight than the steel? ... No, air weighs the same, regardless of the container. Perhaps it's just the wording or I am just dense, but I can't seem to wrap my mind around what they are saying. I'm all for having someone explain this to me. Rick?

Here's a portion of my letter to SSI dated 18 November, 2002, after my review of this edition of the OW manual:
Quote:

Page 1-14 last paragraph line 5, replace “Aluminum” with “Larger”; delete “than steel” in line six. The statement as written is not true – cylinder weight changes between full and empty depend solely on the change in the amount of gas contained in the cylinder between full and empty and is totally independent of cylinder material. I don’t know where this misconception about steel & aluminum started, but it (and an incredible volume of misinformation on cylinder buoyancy in general) is widespread and false. Let’s not perpetuate the myth. J

Hopefully the next edition will fix it.
Rick

 

[eponym]

Dave-
my sentiments exactly.
-Bryan

 

[kouhoutek]

I had this exact same quibble when I took my OW course. Air is air, and change in total buoyancy is the same if the tank is made of steel, aluminum, or Styrofoam.

I wasn't happy with my instructor's hand waving explanation, but I was a newbie diver, so I figured I was missing something, and let it go.

Then I dove in both steel and aluminum close together, and yes, there is a difference. A near empty aluminum tank goes positive, and "pulls" away from...a near empty steel tank stays negative, and will sit flat on your back. The total change in buoyancy is the same in both cases, but they feel a lot different.

In trying to explain this, the book makes errors of fact, and confuses things worse.