Quiz - Physics - Volume/Pressure

[ToneNQ]

....snip... It is like a physics question that begins, "Assume a spherical pig....."

Assume a spherical pig, at standard temperature and pressure (if I recall).

....snip....The reality is that for both containers, both pressure AND volume increase.

Whilst absolutely correct, the true reality is that the training material is made by senior diving instructors for a junior diving instructor to deliver to open water students. I will guarantee that whilst the student argues the answer with the instructor, no other person in the room will be listening intently. I can think of hundreds of these types of questions in various training that I have done in my lifetime, I'll bet beers that not one of them has been re-written to reflect reality.

 

[Aviyes]

This depends also on the material of which the balloon is fabricated. If it is made with just a thin bladder of rubber, then the pressure inside it will increase modestly, and instead the volume will expand a lot.
If instead the balloon is, say, a basket balloon, or a water-polo balloon, these are made of a composite structure including high tensile resistance fibers (Dakron, fiberglass, Kevlar or even carbon fibers). When filled, they already contain air at 4-5 bars, hence their volume has already almost maximized.
When heated, they will behave exactly as the cylinder: a significant increment of pressure, a modest increment of volume...

Questions like this are all about scope. Don't let it explode outward until we need to consider unobtainium cylinders and the like. If it is simple to demonstrate a point, keep it simple. If we need to dictate all the specific variables we are discussing, the question quickly becomes a page+ long question. Use logic and stick to the tools that a scuba instructor would have/easily obtain for such a demonstration. So, an Aluminum or Steel tank, standard rental regulator setup, and your standard party balloon. Assuming use of visual observation and an SPG. Then answer is c.

 

[TMHeimer]

But this is NOT reality, it is Basic Scuba, and what the question is about is rigid containers and flexible containers. It is like a physics question that begins, "Assume a spherical pig....."

Yes. I immediatley thought about rigid and flexible. As they say on courtroom TV shows-- the intent or the spirit of the law..
Even Mr. Picky agrees with you.
I've never noticed a tank increasing in size. In my mind, for all practical purposes it doesn't. Now a tank (or anything) will implode if there is enough external (ambient) pressure around it.

 

[Storker]

I've never noticed a tank increasing in size. In my mind, for all practical purposes it doesn't.

One thing I learned rather early in college was the concept of "significance". It took a bit of effort, but it was rather valuable to learn the difference between that which makes a noticeable difference ("significant") and that which doesn't make a particularly noticeable difference ("insignificant").

In this case, the increase in tank volume and the increase in pressure inside the balloon would be regarded as thoroughly insignificant. And could thus easily be approximated with zero. And could thus be removed from the equation, especially towards those students who were still struggling to grasp the basic concepts.

 

[AfterDark]

Yes. I immediatley thought about rigid and flexible. As they say on courtroom TV shows-- the intent or the spirit of the law..
Even Mr. Picky agrees with you.
I've never noticed a tank increasing in size. In my mind, for all practical purposes it doesn't. Now a tank (or anything) will implode if there is enough external (ambient) pressure around it.

One of the things a hydro test measures on a tank is the expansion during pressurization and contraction when depressurized. You wouldn't see the expansion but it is measurable. You will of course see expansion quite readily on a balloon and with less temp. increase than it would take to raise tank pressure measurably.

Heating a tank will increase volume within a tank as the gas molecules get further apart with the temp and pressure increase.

 

[Steve_C]

the increase in pressure inside the balloon would be regarded as thoroughly insignificant. And could thus easily be approximated with zero.

Really. The balloon increase in both volume and pressure. There is a restraining force of the rubber balloon. If there was not an increase of pressure the balloon would not expand.

 

[Storker]

Really. The balloon increase in both volume and pressure. There is a restraining force of the rubber balloon. If there was not an increase of pressure the balloon would not expand.

How many millibars is the increase? Just pulling numbers out of my arse, I'd guess it'd be less than one tenth of a bar. IOW, not significant.


And just FTR, one ATA is 1.013 bar, IOW 1 bar = 1 ATA within significance when we're talking about diving physics.

 

[TMHeimer]

One of the things a hydro test measures on a tank is the expansion during pressurization and contraction when depressurized. You wouldn't see the expansion but it is measurable. You will of course see expansion quite readily on a balloon and with less temp. increase than it would take to raise tank pressure measurably.

Heating a tank will increase volume within a tank as the gas molecules get further apart with the temp and pressure increase.

Exactly. Again, wording. If you included the word "noticible" in the question, things become very clear.

 

[AfterDark]

Exactly. Again, wording. If you included the word "noticible" in the question, things become very clear.

It's physics it either happens or doesn't, whether it can be seen by the eye is irrelevant, it can be measured. The reason the pressure goes up is because the space between the molecules is increasing as a result of increasing heat, resulting in greater volume and thus pressure.

 

[Steve_C]

How many millibars is the increase? Just pulling numbers out of my arse, I'd guess it'd be less than one tenth of a bar. IOW, not significant.
And just FTR, one ATA is 1.013 bar, IOW 1 bar = 1 ATA within significance when we're talking about diving physics.

Having blown up bunny rabbits, balloons, pool toys, and a lot of thing I can tell you that it gets a lot harder as the object expands. My lungs feel that there is a significant increase in pressure.