Do you have practice questions for the DM exam?

[MikeFerrara]

I would guess most DMCs haven't used Boyle's law in over a year before starting their DM program, maybe not since high school. This stuff could be easy, if we first are comfortable with the fundamentals.

I think there are a couple of issues. One is that some people just don't have the math and science background to make it easy for them.

Second, dive training has almost tried to skirt the issue. Some OW texts show the student a picture of a balloon shrinking as it goes deeper but they don't really explain boyles law. Then they might not get anything until a nitrox course or DM training.

Would brief additional physics and physiology training for all Adv-OW divers make DMCs more successful???

Most manage to get through the tests as is but I'm not sure that they come out of it with the "pro" level of understanding that's intended. From my own DM, instructor and technical training, I'd say that there are lots of instructors out there that aren't real comfortable with the material either.

Could they set a higher bar for dive and deco theory if they did?

Maybe but my first thought is doing a good job of hitting the bar where it is. Just looking at some of the questions we see on the board, we can see that divers (including some instyructors) don't have a good handle on the basic principles. Specifically some of the questions/comments we see concerning nitrox or altitude or whatever.

It's like anything else, if you don't get the little pieces down well it only gets more confusing when you try to put all the pieces together.

 

[MikeFerrara]

ok, here's a study question for one of you DM candidates.

Define EAD and show me how the EAD equation works. Don't just show me the equation or calculate an EAD but break it down, explain the pieces and show me how it works.

 

[TheTrickster]

EAD is the calculated depth that a diver would be at, breathing normal Air, to feel the same narcotic effect of the selected gas mix, at a given depth. Normally you are increasing the percentage of Oxygen when dealing with EADs, so decreasing your Nitrogen absorption allows you to increase your no deco time, by basing your dive time on the calculated EAD depth on your RDP.

The reason why you do this is, is due to the partial pressure of the nitrogen increasing as the diver goes deeper (and so to the other gases in the mix) and with the increase in partial pressure of the nitrogen the more elevated absorption of nitrogen in the lipid hydrocarbons in the nerves cells in the brain, causing irregular electronic signals to be sent from nerve to nerve - causing you to feel drunk/happy/dizzy/narked etc. With a higher percent of Oxygen in the mix, the lipid are absorbing less Nitrogen, causing less narcosis, so allowing you to base your dive on a shallower depth.

As to explaining the EAD formula in detail - can I refer to a book? As I can't quite remember it

 

[MikeFerrara]

EAD is the calculated depth that a diver would be at, breathing normal Air, to feel the same narcotic effect of the selected gas mix, at a given depth. Normally you are increasing the percentage of Oxygen when dealing with EADs, so decreasing your Nitrogen absorption allows you to increase your no deco time, by basing your dive time on the calculated EAD depth on your RDP.

The reason why you do this is, is due to the partial pressure of the nitrogen increasing as the diver goes deeper (and so to the other gases in the mix) and with the increase in partial pressure of the nitrogen the more elevated absorption of nitrogen in the lipid hydrocarbons in the nerves cells in the brain, causing irregular electronic signals to be sent from nerve to nerve - causing you to feel drunk/happy/dizzy/narked etc. With a higher percent of Oxygen in the mix, the lipid are absorbing less Nitrogen, causing less narcosis, so allowing you to base your dive on a shallower depth.

The definition is buried in there someplace but you can really do it in about a single sentance. Think nitrox...Most assume that O2 is about as narcotic as N2 so we really don't use nitrox to reduce narcosis.

As to explaining the EAD formula in detail - can I refer to a book? As I can't quite remember it

You can use whatever you want but all you really need to derive the equation is an understanding or how to calculate partial pressures. I think if you really get the definition clean and straight in your head, the equation will be pretty obvious. A nice clean definition, in this case, is pretty much the equation in verbal form.

 

[TheTrickster]

The definition is buried in there someplace but you can really do it in about a single sentance.

Morning Mike,

Perhaps my description was a little long winded, and incorporated some END reasoning too. Here is my second attempt to describe EAD:

"EAD is the depth that a diver would be at, when breathing normal Air, where the Partial Pressure of the Nitrogen in the current gas mix matched that of the Air at a required depth".

:blinking:

 

[MikeFerrara]

Morning Mike,

Perhaps my description was a little long winded, and incorporated some END reasoning too. Here is my second attempt to describe EAD:

"EAD is the depth that a diver would be at, when breathing normal Air, where the Partial Pressure of the Nitrogen in the current gas mix matched that of the Air at a required depth".

:blinking:

Yes...it's simply the depth where air yields the same PPN2 as the planned mix at the planned depth.

I'm going to jump ahead here and look at the math and show why you don't need to look up an equation.

We're interested in partial pressures right? If we calculate the PPN2 of our mix at our planned depth...(D/33 + 1) X (FN2) = PPN2 which is just pressure in ATA multiplied by the fraction of gas. I left it in ATA because it's probably most familiar, but we could express it in FSW or any other units we like. You'll see why I pointed that out later. Now, if we take that calculated PPN2 and simply put it into another PP equation but this time solve for depth...where PPN2 is what we previously calculated for our mix at our depth and FN2 is that of air.
(PPN2)/FN2 - 1 (33) = D = EAD I'm assuming that everyone can see that I just rearranged it to get depth alone on one side of the equal sign.

Lets put some numbers to it and do an example...EAN32 at 100 ft.
(100/33 + 1) X 0.68 = 2.74 ATA

What depth does air yeild a PPN2 of 2.74? (D/33 + 1) X 0.79 = 2.74 Just solve for depth...(2.74/0.79 - 1) X 33 = EAD = 81 ft

I did all that to show that we're only dealing with two partial pressure calculations. In the first we solve for PPN2 and in the second we solve for depth. No one who knows Daltons Law and really knows what EAD is, needs to look in a book for an equation.

It's easy enough to write the whole thing as a single equation.
(D/33 +1) X FN2 = (EAD/33 + 1) X 0.79 Just a PP equation on either side of the equal sign. Rearange to solve for EAD and we get...
((D/33 + 1) FN2/0.79 - 1) X 33 = EAD

ok that doesn't look like what you have in your book so lets make it look more familiar. We'll first carry out that multiplication by 33 and we get
(D + 33) FN2/0.79 - 33 = EAD

What you can clearly see right off the bat is that EAD is just the pressure in FSW adjusted (multiplied) by the ratio of our FN2 to the FN2 of air. We then subtract the surface pressure in FSW (33) to get depth. That's all it is.

Does that look more familiar? If you have a PADI book, it still looks a little different because where I write FN2, they write (1 - O2%). Pardon me but what kind of complete azzhole came up with that? In the above example using EAN32, our O2% is 32. Where I went to school, 1 - 32 = -31. If they would write (1 - FO2) they would be correct but we're not dealing with FO2 so why not just write FN2? Why stick percentages in there at all when you just have to convert them to decimal? It's almost like they don't want you to be able to see what it really is.

 

[TheTrickster]

:thumbs_up:

Great explanation Mike! - Can you rewrite the PADI Encyclopedia?

it's simply the depth where air yields the same PPN2 as the planned mix at the planned depth.

And I thought I had explained it in simple terms! - the above explanation is even clearer!

 

[MikeFerrara]

I hope that was clear. It would no doubt be smoother if we were face to face and not reliant on my poor writing skills.

The point is that if you really understand the principles and the little pieces, everything else falls into place. The texts skips all the pieces and jumps to a poorly or even incorrectly written homogenous equation that makes it difficult to see what's going on.

The situation is far worse in some of the gas blending texts. That too, is simple but they skip all the important suff and make it look really complicated.

 

[MikeFerrara]

:thumbs_up:

Great explanation Mike! - Can you rewrite the PADI Encyclopedia?

I could if PADI would pay me. In the mean time, while we all sit around holding our breath waiting for PADI to make me an offer...If there's any diving related math/physics issue that someone would like to try looking at it in a little different way, I'll give it a shot.

If you wait until the movie comes out you'll have to buy a ticket like anyone else.

 

[rawls]

Ok...A few more questions for dm candidates.

#1) You are going on a dive to...let's make it easy...121fsw and you want to know the maximum oxygen mix you can use to maintain a PO2 of 1.4 ata. What is it?

#2) You are diving with EAN30 to a depth of 121fsw. What is your PO2?

#3) You are diving with EAN30 and your PO2 is 1.4ata. What is your total absolute pressure? What is your depth?

#4) What gas law are you applying and what formula do you use to determine the above answers?

10 point bonus question: This one is EXTREMELY DIFFICULT What is the MOD of EAN 50 and EAN100? What is the formula?

These should be easy questions for those of you in a dm course and each of the answers is in the other questions, except for the extremely difficult bonus. If you find them difficult then study, study, study.

Mike and Trickster do not qualify for the Bonus question