Quiz question

[DiveNav]

...

Originally Posted by Jax
Figured it through . . . thought it might have been a lost decimal.

I'd say the instructor's typist skipped a question and put the next question's answer set here.


Try this: What is the speed of sound in water?

If the water is 70F, the speed of sound is about 4800 ft/sec.

It's about 1400 m/s . . .

that would make a lot more sense ...

... Bob (Grateful Diver)

So ... to summarize:

Q: How fast is the diver breathing out his/her tank?

A: At the speed of sound!

Sorry ... I could not resist

 

[Brendon]

She understands quite well ... so let me explain further. Without knowing the size and working pressure of the cylinder, you don't know the volume that each psi you read on your gauge represents. It's a critical piece of information ... much like you don't know how many gallons of gasoline you have in your car's gas tank by looking at the gauge unless you also know the size of the tank. The reading on the gauge is only a relative indication.

For example ... if you run the numbers in the OP using an AL80, you get a SAC rate, expressed in volume, of 0.62 CF/minute. Now, if you run the exact same numbers using an LP95, you get a rate of 0.86 CF/minute ... because the cylinder holds a greater volume of gas for each psi you read on your gauge.

For planning purposes, knowing the psi/minute you use isn't very useful ... unless you've worked out the numbers in advance and done the necessary arithmetic ... because you want to answer the fundamental question of how much gas do you need for the dive plan ... and the one thing you know is how many cubic feet your cylinder is designed to hold.

As for the answers provided in the OP, they're simply nonsense. SAC is never expressed in feet/second ... that's velocity, not volume ...

... Bob (Grateful Diver)

Bob, we can both agree the question was not formed well, and the answers provided were wrong. Regardless, the question was easy enough to answer given the info provided. Neither of us had any issue solving the problem. Sure the usefulness of the answer is debatable... I see it as merely testing math skill and knowledge of the basic formula. TS&M did not understand... Mostly because what was gained by this knowledge is mostly useless without other factors involved in the equation. Which is understandable. I don't often test a diver on every part of what I just taught all in the form of 1 question. Think about this, what's the point of knowing absolute pressure if not used in a bigger equation? So a test question may read, what's the ATA @ 99'? By itself the knowledge is worthless, yet when combined with our many formulas it makes sense. Now does that make sense to you?

 

[MXGratefulDiver]

Completely ...

... Bob (Grateful Diver)

 

[TSandM]

TS&M did not understand...

AAAAGGGH! You "solved" the problem based on information NOT GIVEN IN THE PROBLEM.

I tried to answer the question posed in the original post, which was, "Was there something wrong with this quiz question?" There IS.

I can do these danged calculations in my head, and have been able to do so for the last six years. I understand just fine. I answered the question posed by the original poster, which very few people in this thread have done.

 

[irishsquid]

I think the only way I could have answered that question would have been with one more choice:
E) None of the above, I am awake.

 

[Brendon]

AAAAGGGH! You "solved" the problem based on information NOT GIVEN IN THE PROBLEM.

I tried to answer the question posed in the original post, which was, "Was there something wrong with this quiz question?" There IS.

I can do these danged calculations in my head, and have been able to do so for the last six years. I understand just fine. I answered the question posed by the original poster, which very few people in this thread have done.

75'
Starting psi 3000
End psi 500, amount used 3000-500=2500
Time 32min
This is all the info I used...

2500psi divided by 32min= A depth consumption rate of 78.125
75' = 3.27 Atmospheres Absolute
Depth consumption rate 78.125 divided by 3.27 atmospheres absolute= surface consumption rate
Surface consumption rate= 23.89 PSI per Minute. As Bob said round up to 24.

Now what information did I use? Only what was given.

 

[MXGratefulDiver]

Even if the answers provided had been appropriate to the question asked, the scenario would still remind me of the guy who was driving around lost, and seeing someone standing on a street corner he rolled down his window and asked "Where am I?" The bystander looked at him kinda astounded and said "You're in a car!" The answer is factually correct, and completely useless. This is an issue I have with several commonly asked questions on scuba exams I've taken over the years ... they're more testing your ability to remember something you read than they are your ability to understand why it matters.

Lynne's answers attempted to put the question into context, rather than simply provide a factually correct answer ... which is to say that knowing your psi/minute air consumption is only useful if you know how to relate it to the volume of air you've consumed.

Let's take a look at the original scenario and ask the question ... how much gas did this person use on this dive? You can only answer this question if you know the size and working pressure of the cylinder the diver was using.

For an AL80, the person consuming 2500 psi from their cylinder would have used 64.5 cubic feet of air.

For an LP95, the person consuming 2500 psi from their cylinder would have used 90 cubic feet of air.

That's a significant difference! And it matters, because for dive planning purposes you are trying to answer the fundamental question ... do I have enough air for the dive I'm planning to do? Determining your SAC rate in psi/minute doesn't provide enough information to answer that question.

Knowing your SAC rate is very much like knowing the MPG in your car ... you can only determine how far you can go by also knowing how big your tank is ...

... Bob (Grateful Diver)

 

[Brendon]

... The formula is not quite as simple as you stated Bob. Your comparison to the question "Where am I?", "Your in a car." Seems to be attempt to discourage the reason for question in the first place...By the way in most teaching materials that reference SCR, you will find that the answer 24PSI is also the average for most divers on an AL80. Which is also included in the text. The question was designed to test math skill, not debate its usefullness. Personally I love the question because it brings the topic up and its another chance for review if the person gets it wrong. Its not ment to be easy for the one taking the test, or for them to completely understand why the question is formed the way its is. Its merely a gauge to test knowledge, not to philophosize over.

 

[MXGratefulDiver]

... The formula is not quite as simple as you stated Bob. Your comparsion to the question "Where am I?", "Your in a car." Seems to be attempt to discourage the reason for question in the first place... Had it been simple then TS&M and the OP's friend would have got the question right anyways regardless of the answers provided... By the way in most teaching materials that reference SCR, you will find that the answer 24PSI is also the average for most divers on an AL80. Which is also included in the text. My comments to TS&M may not show her in the light she would prefer since she "Knows these formulas" and been doing them for a whopping 6yrs of and of course she is doctor. Doesnt change the facts... Your attempt to save her honor is dually noted. The question was designed to test math skill, not debate its usefullness. Personally I love the question because it brings the topic up and its another chance for review if the person gets it wrong. Its not ment to be easy for the one taking the test, or for them to completely understand why the question is formed the way its is. Its merely a gauge to test knowledge, not to philophosize over.
I know you know this Bob, so dont act like you dont and form your responses to put TS&M in the best light possible just because your friends.

I'm not interested in testing my student's math ability ... as a scuba diver, it's not relevent.

I AM interested in testing their comprehension of concepts and skills. If I cannot explain cogently to them WHY they need to know something, it's either because that knowledge is irrelevent or because I failed to do my job helping them understand why it matters.

I wasn't attempting to put TS&M in the best light ... she doesn't need my help. I was attempting to explain why simply giving someone a numerical answer to the question isn't adequate, and is only one piece of data that they need to know in order to plan the dive properly ...

... Bob (Grateful Diver)

 

[fjpatrum]

... The formula is not quite as simple as you stated Bob. Your comparsion to the question "Where am I?", "Your in a car." Seems to be attempt to discourage the reason for question in the first place... Had it been simple then TS&M and the OP's friend would have got the question right anyways regardless of the answers provided... By the way in most teaching materials that reference SCR, you will find that the answer 24PSI is also the average for most divers on an AL80. Which is also included in the text. My comments to TS&M may not show her in the light she would prefer since she "Knows these formulas" and been doing them for a whopping 6yrs of and of course she is doctor. Doesnt change the facts... Your attempt to save her honor is dually noted. The question was designed to test math skill, not debate its usefullness. Personally I love the question because it brings the topic up and its another chance for review if the person gets it wrong. Its not ment to be easy for the one taking the test, or for them to completely understand why the question is formed the way its is. Its merely a gauge to test knowledge, not to philophosize over.
I know you know this Bob, so dont act like you dont and form your responses to put TS&M in the best light possible just because your friends.

The formula is trivially simple. As has been indicated by several posters, including yourself. The issue isn't whether it was difficult or not, it's whether or not it has any meaning to the student. Without further information, it doesn't have meaning. The question didn't even have the right answers associated for it to make sense, much less have significant meaning to the student, so whether it was simple or not is even less relevant.

As someone still new to this and just beginning to do the math and truly work toward an instinctive understanding of gas planning, Lynne's response was far more educational than any of yours so far. Putting the numbers in context and giving further understanding is much better than "23.88" or "24". That may be the "average of most divers using an AL80" but it still doesn't mean anything without more information as to what those numbers represent (other than average).