Studying for nitrox class- please help

[pacificgal]

"If your scuba cylinder lasts 90 minutes on the surface, how long will it last at 60 fsw?"
Sounds like a trick question to me. How hard am I working? Did I use it to help me get past the surf zone?
I don't recall reading anything about this in my Nitrox manual, can someone please explain what caluclation they are looking for? Is there a calculation that, when you toss in the depth (or ata/bar) you get the difference in what your tank will deliver at 1 ata?

 

[NateSmith]

It should be in your nitrox class, but the answer would be around 22 minutes I believe (but I'm probably wrong in that)

 

[pacificgal]

Funny thing about this Nitrox class, a) It's not PADI--therefore I'm not expecting it to be easy b) you do the work before the class and whatever you get wrong the instructor will explain and go over with you and c) I don't like going a week without knowing something I am supposed to know, it'll drive me nuts.

 

[parzdiver]

Assuming the same breathing rate, they are probably looking for an understanding of the affects the additional pressure on the gas consumption rate. Also assuming they consider 60 fsw to be a total of 3 ata, the answer would be 30 minutes.

90 min @ 1 ata = 30 min @ 3 ata

Questions like this for class tests usually don't require you to consume gas on the way to depth or require any gas to surface, they are typically just straight math problems.

 

[DNCkevin]

When they ask these questions you assume you are doing the same work load at 1 ata and at depth. So at 60ft you will be at ~3 ata, which will require 3x the volume to fill your lungs per breath. PV=nRT - taking out the costants and assuming temperature doesn't make much difference, PV=PV. So Your time would then be 30min or 90/3. For gas planning, you should know your sac rate, which you then can calculate how much time a given tank will last for a given dive profile.


Cheers

 

[TSandM]

pacificgal, this isn't a trick question. All they are trying to get at is whether you understand Boyle's law. I won't give you the answer, but how many ATAs are you at at 60 fsw?

 

[reefduffer]

Expect that they're asking a question that has reasonable answers, trying to evaluate your understanding of a dive physiology concept.

So assume the same level of work. This is what SAC rate is all about, normalizing air consumption at depth to air consumption at the surface. SAC is generally considered a meaningful and useful thing to talk about, not a trick concept.

Without doing the calculation for you, how many atmospheres are you at at 60 fsw?
How many atmospheres at the surface?
The expectation is that actual air consumption increases linearly with pressure, because the air density increases linearly with pressure, and our lungs are evolved to breath, at a given level of activity, the same volume of air, not the same mass of air.

And the time the cylinder lasts would of course vary inversely with consumption rate.

 

[pacificgal]

OK, I know that on land we are at 1 ata, and for every 33 fsw we go down, it's another ata.
That's the easy part.
The physics is where I have the problem.
I'd love a lecture that includes gestures, lol. "You have this much air" (hands spread wide) and when you descend, the air gets compacted (hands getting closer together) by x amount, the number of molecules doesn't change, but they get squished closer togther..."
I need visuals!

 

[pacificgal]

I guess I'm having trouble with the concepts, but I see I either have a ton of help here, or a bunch of people who are wondering why I'm not getting it.
it's OK though, I'll get it eventually. I think I'm learning the difference between mass and volume already.

 

[parzdiver]

OK, I know that on land we are at 1 ata, and for every 33 fsw we go down, it's another ata.
That's the easy part.
The physics is where I have the problem.
I'd love a lecture that includes gestures, lol. "You have this much air" (hands spread wide) and when you descend, the air gets compacted (hands getting closer together) by x amount, the number of molecules doesn't change, but they get squished closer togther..."
I need visuals!

I'll try a quick example that you might be able to visualize.

Imagine nine pints of your favorite flavor of ice cream (each represents 10 minutes of air).

At 33 fsw / 2 ata of pressure, you only have 4 1/2 pints, or half as much.

At 66 fsw / 3 ata of pressure, you have 3 pints.

etc.

Each time you take a scoop (a breath) the scoop size is the same, but the more of the compressed ice cream (air) is taken out each time.


I'm hoping this helps more than confuses....