Deco dive plan sheet

[Cave Diver]

I just spoke to two instructor friends from two agencies while discussing business and asked them that question. Both chose 81 feet as their response. Both would have rounded up to 82 feet using calculators. The point of the question tests the student's knowledge of the guideline to advance to the next greater depth and time.

An example of which is a question on the PSAI advanced nitrox exam which firsts ask the student, "What is the EAD of Nitrox 32 at 100 feet?" The multiple choice answers are: 70 feet, 80 feet, 81 feet, and 92 feet.


Using a calculator for Nitrox 32 at 100 feet the values would be: [(.68*133)/.79] - 33 = [(90.44/.79] - 33 or 114.48 - 33 = 81.48 feet. I left the decimals in place in my post so that readers could round up or round down as they chose.

On paper, I'd calculate it as 81 feet.

Call it 81, 81.4, 81.48, 81.5 or 82 feet. Any way you slice it, the student will discover that he or she must now consider that 81 feet is past 80 feet for decompression purposes and must move to the next greater tabled depth of 90 feet.

If they must move to the next greater tabled depth of 90 feet, why isn't that (90') included as an answer on the test? In my mind 81 feet is incorrect, because 81.48 is beyond 81', so you must round up to 82. If you're going to be precise, then lets be precise.

This questions seems more like you want to pick nits. If you're truly interested in determining a students ability to calculate it correctly, why not just make it a fill-in-the-blank rather than multiple choice?

 

[Trace Malinowski]

If they must move to the next greater tabled depth of 90 feet, why isn't that (90') included as an answer on the test? In my mind 81 feet is incorrect, because 81.48 is beyond 81', so you must round up to 82. If you're going to be precise, then lets be precise.

This questions seems more like you want to pick nits. If you're truly interested in determining a students ability to calculate it correctly, why not just make it a fill-in-the-blank rather than multiple choice?

John, I agree with that because I would call it 82 with a calculator. By hand the math would be:

EAD = [fN2 x (Depth + 33)/.79] - 33 = [(.68 x 133 feet)/.79] - 33 = (90.4/.79) - 33 = 114 - 33 = 81 feet.

Whether a student calculates it by hand or by using a calculator the math comes out to be beyond a tabled number. The difference in deco time between an 80 foot dive and a 90 foot dive regarding decompression for the same bottom time can be significant for planning purposes.

This stuff is open water 101.

 

[Rich Courtney]

If you're truly interested in determining a students ability to calculate it correctly, why not just make it a fill-in-the-blank rather than multiple choice?

Hallelujah!! Best line in this thread and I still can't believe I read it all!
My Dad's a retired college prof and he hates multiple choice. If you want to find out someone's knowledge use subjective questions....

Safe diving,

Rich

 

[ScubaFeenD]

Hallelujah!! Best line in this thread and I still can't believe I read it all!
My Dad's a retired college prof and he hates multiple choice. If you want to find out someone's knowledge use subjective questions....

Safe diving,

Rich

Or better yet, make them calculate it and then explain every term in the equation and why it is where it is. Monkeys can plug and chug.

 

[Howdy Doody]

it's about time a reply comes in from ScubaFeen

 

[Dive-aholic]

Well, Trace, if your students are routinely failing the written exam, then are you making it clear to them during the class, how that class expects them to answer the question? I mean, if I took a class that used tables, I wouldn't expect to give RD answers on the written exam for that class.

And, given that depth gauges will often differ by several feet among a team, I do think that rounding 81 feet up to 90 -- and insisting that's important -- verges upon ridiculous. Yes, it's what the tables tell you you have to do; I doubt that anybody did validation dives to prove that you have to do it to remain within the probabilities upon which the tables are based. Somewhere in the interval, it becomes important, but I doubt one foot is where that line is drawn.

Between Jen and I, we own 6 dive computers and a bottom timer. Not one of them agrees with each other at depth! I've had as much as a 4' difference between 2, one on each arm.

 

[TSandM]

I'm not suggesting that it's okay to teach people not to follow the procedures required by the tables, if they are going to dive by tables. But if you are teaching a class that involves calculating decompression using tables, and at the end of the class, all your students are doing the problems the wrong way, something is wrong with the teaching. If you are teaching a class, and ask people how to develop a safe decompression plan, then if they use an accepted strategy and execute it correctly, their answer ought to be satisfactory. If you have not made the distinction clear, then something was missed during the classwork preparation for the exam.

 

[rongoodman]

After all this discussion, I decided to go back and redo the last question on my Normoxic exam using an IANTD trimix table and grinding out gas consumption, %CNS, and OTUs, one 10' level at a time. While I'm feeling lots of good karma, I really can't say that I've learned anything I didn't know before. Cutting custom tables with V-Planner and running them with a bottom timer and the X1 as a back-up makes sense--this little exercise I've been doing is a waste of time, prone to error, and doesn't even match the gases I'd use on the dive.

 

[waterpirate]

My part time day job is a 400 level instructor. The thing we must address as instructors is did we teach the material in an acceptable manner that allows the student to meet the standards of the test, without teaching the test. It is a measure of the instructor not the student.
That being said, I have no time for equations and math formula that I do not do well. To plan and execute a decompression dive one only need understand the theory and use software to eliminate the potential for math erors, by math challenged people such as myself.
Eric

 

[Trace Malinowski]

I'm not suggesting that it's okay to teach people not to follow the procedures required by the tables, if they are going to dive by tables. But if you are teaching a class that involves calculating decompression using tables, and at the end of the class, all your students are doing the problems the wrong way, something is wrong with the teaching.

Yeah, the problem with the teaching is that I assumed that they had learned how to use dive tables in open water class.

Which is why I said I need to bring back dive planning worksheets to teach these concepts in tech classes in my first post.