another question

[TMHeimer]

I just joined, so I can finally post questions. In the PADI Diving Knowledge Workbook, I am curious about the questions that require written answers. The answers given in the answer key (some of which I basically knew, others I did not), can be really technical and long-winded, with important knowledge added to what the real answer is (typical of PADI?). I just wonder how much discretion your instructor has in grading such responses--ei., if I show an understanding of the question without too much ellaboration, is it "correct". Don't get me wrong-I read all the manuals backward and forward and want to be the best DM I can, but I doubt anyone whoever took these written tests rambled on like the answers the Workbook gives. There isn't even enough space given for you to fit in their answers. Any advice?

 

[Joe-Diver]

The written tests are multiple choice. As long as you understand the concepts and can identify why the other answers are wrong, you'll be fine.

The Knowledge Reviews in the DM Manual can be a bit more long winded.

 

[TMHeimer]

Thanks again Joe.

 

[Joe-Diver]

Sure...remember, in the Knowledge Workbook, the answers are explained to you so in case you missed it, you understand why. You're not expected to answer in that much verbosity on the tests. BUT, you need to be able to answer like that when you're asked by someone about the subject matter. You're developing your professional level knowledge of dive theory.

 

[TMHeimer]

Good. I'm sure I'll be able to explain stuff in detail, as I signed up for the course for the Fall and have all the material now. Just hope I can pass the swim tests at my advanced age of 55. Thanks again.

 

[TMHeimer]

Joe, I went through the Workbook and have 3 more questions:
1. If you know Boyle's, Charle's & Dalton's Laws, in what sort of question (if any) would you use the General Gas Law (PV=nRT) found in the Encyclopedia?
2. One Workbook problem cites Charle's Law as P1*V1/T1=P2*V2/T2. The Encyclopedia states it as P1*V1 with both of these over T1=P2*V2 with both of these over T2. These are two different equations which would yield two differet answers, no? One a misprint?
3. In general, what, and how much other info. found in the Encyclopedia but not covered in the Workbook questions would be found on the tests?

 

[ClayJar]

Boyle's law, Charles' law, and Gay-Lussac's law can all be considered special cases of the ideal gas law (i.e. what you gave as the "General Gas Law"),

PV = nRT​

R is the ideal gas constant, so PV/(nT) = R for any P, V, n, and T. That being the case, you can then write

P₁V₁/(n₁T₁) = P₂V₂/(n₂T₂)​

as both sides of that equation are equal to the constant R. As you certainly know, P, V, and T are absolute pressure, volume, and absolute temperature, respectively. As for n, that is the amount of gas (usually in moles if you're in science class). If you're dealing with a fixed amount of gas, n₁ = n₂, so you can cancel that out of the equation, giving you what is often called the "Combined Gas Law":

P₁V₁/T₁ = P₂V₂/T₂​

Boyle's, Charles', and Gay-Lussac's laws, then, are merely the results of assuming one of the remaining variables is constant. If you assume T₁ = T₂ and cancel those terms, you have Boyle's law. If you assume P₁ = P₂, you have Charles' law. If you assume V₁ = V₂, you have Gay-Lussac's law.

Boyle's law: P₁V₁ = P₂V₂ (constant T)
Charles' law: V₁/T₁ = V₂/T₂ (constant P)
Gay-Lussac's law: P₁/T₁ = P₂/T₂ (constant V)​

And that's basically how the whole thing works. It's *all* the ideal gas law, just with various things being constants. If you can remember the ideal gas law (PV = nRT) and you are comfortable with basic algebra, you don't need to remember what law is written how and named after which scientist.


(By the way, regarding your item two, they are algebraically identical, as "(P * V) / T" and "P * (V / T)" are the same thing, but that equation is *not* Charles' law. Charles' law assumes constant pressure, but that equation does not. It is the equation for the combined gas law.)

 

[Joe-Diver]

Thanks ClayJar...you did a helluva lot better job answering that than I would have!

 

[SpaceGeek]

ClayJar - it is nice to know that I am not the only geek around here (I mean that as a compliment).

TMHeimer - The physics test was a lot of number crunching. But, they are all variations of the ideal gas law described above. If you can remember PV=nRT, then figure out that PV/nRT=1, it is just a small leap to P1*V1/(n1*T1)=P2*V2/(n2*T2). Figure out what is constant in the problem and nug through it. Of course, your "pressures" will be given in depths and you have to get to absolute pressure. Other than that, it is math. Make sure the answer makes sense at the end (Common sense test: if it is supposed to be a larger or smaller volume, for instance ... is it?)

Good luck on your tests.

Bri

 

[TMHeimer]

I own a Mares M2 computer (does nitrox). The DM Manual describes the 3 types of computers- Spencer-60 min. washout, etc. How do I find out which one mine is (nothing I could find in the manual). And as a divemaster, where would I find, say, a list ofthe most common brands and what type they are? This would seem to be quite important when supervising cert. divers. Thanks.