Quiz question

[leabre]

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.

I'm in full agreement, that every diver should know how to calculate their time at a given depth given their SAC and available volume. No dispute there. Some agencies even teach it that way from the OW level as basic requirement as well as how to set aside precise reserves for your buddy, too. My OW was NAUI and it was not taught that way beyond the tables to me. It wasn't until the NAUI MSD (part of LA County ADP program) that it was taught me. So my comment wasn't that divers should or shouldn't learn it, or that it is only to be reserved for MSD levels, but that in the NAUI space that's how it IS. Given that, I was suprised to see it taught in the OW level in a good way.

 

[tbone1004]

no, that's when naui REQUIRES it. we teach all that stuff at the basic level as well as advanced navigation courses and a slew of other stuff. beauty of naui. padi is not allowed to teach it at the open water level, and i'm not sure when it actually shows up.
I'm with Lynne though, that question is wrong on so many levels, and you can only calculate that in psi/min which is useless. Same as "would you rather have a tank with 3000 psi or 300 psi?". If it's my lp112's with 300psi vs my AL13 with 3000. Irritatingly that's actually the same amount of gas, but you can see that psi means very little.
TECHNICALLY! assuming an AL80, her sac in PSI/min is actually 24.03 not 23.88, due to the 77.4 which is largely ignored. Using an assumed 80cf, the cf/sec is .0106 which is stupid.

Check the test, something is wrong. That being said, NAUI does ask for sac expressed in PSI/min on it's tests. Which while largely irrelevant for the tech divers, for recreational divers it saves them a lot of steps and since they use al80's most of the time it makes sense to teach it. Plus, it's a good stepping stone for the cf/min since you have to get psi/min then multiply by tank factor. So the DAC is 78.13 psi/min, which makes SAC the 23.87 which is still nowhere close to any of the answers.
Sorry if that made no sense, i had surgery yesterday morning, so can't think quite as clear as normal.

 

[Peter Guy]

padi is not allowed to teach it [SAC/RMV] at the open water level

Fortunately, not a true statement. There is nothing that prohibits teaching ABOUT air consumption rates, and how to calculate them, in a PADI OW class. In fact, teaching this as part of Knowledge Review 1, Question 5, is entirely consistent with that question. Likewise, teaching this as a "remediation" of Final Exam, Q. 8 would be appropriate and perhaps necessary.

Whether this is commonly done in a PADI OW course is not something I can say, however.

 

[MXGratefulDiver]

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?

You're not missing anything ... the answers provided are nonsense in the context of the question being asked.

Here is the correct answer, and how you arrive at it.

Diver goes to 75 feet and stays there for 32 minutes ... consuming 2500 psi.

How many psi per minute (on average) does the diver consume?

Answer: 2500/32 = 78.125 psi per minute at 75 feet

What is the equivalent consumption at the surface?

Pressure at 75 feet (assuming salt water) is 75/33 + 1 = 3.272 ATA

Surface consumption would be 78.124/3.272 = 23.9 ... keep it simple and say 24 psi per minute

To determine volume used per minute, you'd have to identify the volume of the cylinder at its proper working pressure (e.g. an AL80 would be 77.4 CF at 3000 psi, which would give you 0.258 cubic feet per psi, so at 24 psi per minute that would be roughly 0.62 CF/minute

The answers provided ... described in feet/second ... are nonsensical ... you're calculating pressure, or volume ... not velocity

FWIW - this question is not on the standard NAUI OW exam. It sounds to me like something the instructor added, and just screwed it up ...

... Bob (Grateful Diver)

 

[MXGratefulDiver]

NAUI does ask for sac expressed in PSI/min on it's tests. Which while largely irrelevant for the tech divers, for recreational divers it saves them a lot of steps and since they use al80's most of the time it makes sense to teach it.

NAUI doesn't even ask for SAC calculations at the OW level. Insructors may add such questions to the exam, but it's not on the Standardized A or B exams.

I like to make the distinction between PSI/minute and CF/minute as follows ...

SAC expressed as volume (CF/minute) is useful for dive planning, since you know the volume of your cylinder. Expressing it in this manner can help you determine whether or not you have enough gas for your dive plan.

SAC expressed as pressure (PSI/minute) is useful for dive execution, since what you have to go by during the dive is the pressure display on your SPG. Expressing it in this manner is useful for helping you determine your rock bottom or turn pressure.

Most folks on ScubaBoard are aware of the resource, but for anyone who wants to learn more, please read about it here ...

... Bob (Grateful Diver)

 

[Jim Lapenta]

Thanks Bob for a couple things. Number one the question is worded poorly and you seem to have hit the nail on the head with the reference to the instructor adding it. Like NAUI we are also permitted to test on added material if it reinforces the lesson and benefits the student. Unless it;s been updated I have the standard NAUI exams. Mine are with the materials I got from 2006.

In fact SEI provides a list of an additional 168 questions we may sub for any on the standard test. Or we can add our own. Hopefully though when done they make sense.

Instructors are also encouraged to add materials and knowledge based on local conditions. And this can be tested and used to grant or deny certification.

And thanks for the link to your article and site.

Just as an FYI to anyone reading this thread. The knowledge should indeed be part of every OW divers toolbox. I actually use Bob's material in my OW class to reinforce the dive planning and gas management lessons and make sure they have his website as a reference as it has some of the best material in the business on it.

If you have not had this instruction in your OW class you owe it to yourself to get it. It's part of being a safe, competent, and responsible diver.

 

[Brendon]

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.

I'm sorry u don't understand. Maybe NWgreatfuldivers explanation of how u get to my answer would better suit u... Regardless my answer is correct and the info provided on the question was sufficient. I think your just over thinking it...

 

[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 . . .

 

[MXGratefulDiver]

I'm sorry u don't understand. Maybe NWgreatfuldivers explanation of how u get to my answer would better suit u... Regardless my answer is correct and the info provided on the question was sufficient. I think your just over thinking it...

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)

 

[MXGratefulDiver]

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)