Quiz question

[leabre]

Greetings,

A friend of mine is taking a scuba class and is struggling on the math part so I offered to help her. She's taking a NAUI ow class and since I'm a NAUI MSD, I figured I could help. So I looked at her quiz and I had no idea that they taught OW classes with all these gas volume and PSI related trivia. There's question in particular that has me truly stumped. I was wondering if someone can explain to me how the answer (given the choices) can be derived. You don't need to provide the answer, just a clue how to derive it.

Q: A diver makes a dive to 75 ft. and stays there for 32 minutes. the tank pressure at the beginning of the dive was 3000 PSI and at the end of the dive there is only 500 PSI in the tank. What is the approximate surface air consumption rate for this diver:

A) 2800 ft/sec.
B) 2200 ft/sec.
C) 4800 ft/sec.
D) 1800 ft/sec.


This question (or provided answers) just doesn't make sense to me. What is it that I'm missing?

 

[TSandM]

Well, to begin with, you cannot convert from psi to volume without knowing the volume and working pressure for the tank.

Secondly, the units of the answer do not correspond to any way of reporting SAC rate, because they are in distance per unit time.

If they MEANT ft3/sec, the order of magnitude of the answers is nonsensical. Even if they meant psi/sec, the magnitude is absurd.

I think the question is incorrectly written.

 

[Mr Carcharodon]

Sean,

The units seem wrong, are they correct? Were there assumptions that you did not include in your first post? Anyway I did NAUI master diver too and cannot recall any glaring errors in their texts. Or at least there there were none that were not philosophical disagreements.

 

[leabre]

I read through the most of the questions on the exam (all 100 of them). I was surprised to see MSD level questions on an OW dive test. Most I was able to answer, some I had to look up that were specific to the NAUI tables since I haven't had to calculate residual nitrogen in years now. But this particular question seems very ill-written (or answered). I was certain there was no way to answer the question but in the slight chance that I was wrong is why I posted it. There were two other questions that didn't make sense, either. One involved questions about pnuemothorax but all the answers involved dive computations.

Lynn, most questions on the exam mentioned an AL80 or 3000 PSI but nothing else.

Thanks for helping me confirm my suspicions. My advice to her was to discuss that question with her instructor.

 

[Brendon]

Greetings,

A friend of mine is taking a scuba class and is struggling on the math part so I offered to help her. She's taking a NAUI ow class and since I'm a NAUI MSD, I figured I could help. So I looked at her quiz and I had no idea that they taught OW classes with all these gas volume and PSI related trivia. There's question in particular that has me truly stumped. I was wondering if someone can explain to me how the answer (given the choices) can be derived. You don't need to provide the answer, just a clue how to derive it.

Q: A diver makes a dive to 75 ft. and stays there for 32 minutes. the tank pressure at the beginning of the dive was 3000 PSI and at the end of the dive there is only 500 PSI in the tank. What is the approximate surface air consumption rate for this diver:

A) 2800 ft/sec.
B) 2200 ft/sec.
C) 4800 ft/sec.
D) 1800 ft/sec.


This question (or provided answers) just doesn't make sense to me. What is it that I'm missing?

Thats funny... Here give her the answer so she gets it right anyways. SCR=23.88

 

[Brendon]

Well, to begin with, you cannot convert from psi to volume without knowing the volume and working pressure for the tank.

Secondly, the units of the answer do not correspond to any way of reporting SAC rate, because they are in distance per unit time.

If they MEANT ft3/sec, the order of magnitude of the answers is nonsensical. Even if they meant psi/sec, the magnitude is absurd.

I think the question is incorrectly written.

DCR= SCR X ATA, Of course the question isnt asking for Depth Consumption Rate, its asking for Surface Consumption Rate so just change formula around... SCR= DCR / ATA that of course is 23.88. Regardless the answers are wrong... Oh and DCR in this case is the PSI used divide by time, just like it would be if we started with SCR...

 

[Rhone Man]

Greetings,

A friend of mine is taking a scuba class and is struggling on the math part so I offered to help her. She's taking a NAUI ow class and since I'm a NAUI MSD, I figured I could help. So I looked at her quiz and I had no idea that they taught OW classes with all these gas volume and PSI related trivia. There's question in particular that has me truly stumped. I was wondering if someone can explain to me how the answer (given the choices) can be derived. You don't need to provide the answer, just a clue how to derive it.

Q: A diver makes a dive to 75 ft. and stays there for 32 minutes. the tank pressure at the beginning of the dive was 3000 PSI and at the end of the dive there is only 500 PSI in the tank. What is the approximate surface air consumption rate for this diver:

A) 2800 ft/sec.
B) 2200 ft/sec.
C) 4800 ft/sec.
D) 1800 ft/sec.


This question (or provided answers) just doesn't make sense to me. What is it that I'm missing?

I am pretty sure I have dived with people who had a surface consumption rate of 4,800 cu ft/second. It certainly felt that way.

 

[DivemasterDennis]

Rhone man strikes again. It is unfortunate when instruction because more confusing than elucidating. Not only were the possible answers all wrong, the wording of the question was confusing and did not teach. Testing is part of teaching. The proper way to word the question would be much clearer and would teach the point to be made, that the same amount of air consumed would last about 3 and 1/4 time longer at the surface than at 75 feet of depth. We make better divers when we know the point of information to be learned , and convey that point not only accurately, but clearly.
DivemasterDennis

 

[tbone1004]

that is absolutely NOT master diver level material there. That is not trivia at all, it is practical knowledge that is important for EVERY diver to know. Everyone knows what their cars fuel economy is for a trip and about how often they have to refill the gas tank, why shouldn't every diver know what their fuel economy is and how long they can dive on a tank? Seems to be that knowing your sac and dive times is MORE important for basic divers anyway. That way they don't get so hell bent on "well the tables say I have 35 minutes at 80 feet", but i'm on an al80, so I really only have 27 minutes if i surface with 500 psi *assuming .7sac which is probably wishful thinking for a lot of new divers. They need to learn that there are two time limits and very often for new divers, the NDL is not the important one.

 

[TSandM]

Brendon, I don't understand your response to my answers at all. I am quite familiar with the formula for RMV/SAC (and the differing definitions of the two). But neither is reported in distance per unit time. And no measurement involving volume can be derived from information given in psi, without knowing the capacity of the tank and the working pressure of that tank. This is entirely the reason that having someone know their consumption in psi/min is only useful if they always use precisely the same kind of tank.

The fact remains that, whether you are talking about surface consumption or depth consumption rates for recreational divers, your values in cubic feet will be in the 0 - 5 range, and your values in psi will be in the 10 to 100 range. Numbers like those which are given in the offered answers are just wildly outside anything reasonably possible, and are expressed in the wrong units. Something is wrong with the question and the answers as presented.