Nitrox Question

[Blackwood]

You need to use your thumb, like I said above.

Yah, I understand how to use it. I just don't see how it's helpful. Besides, I know of no language in which a vertical line signifies multiplication



If you can remember how to draw a circle, inscribe a T, and fill in the variables P, F and p, then you can remember P=Fp. And if you need little tricks like the covering up of the one you want to determine, you can do it with P=Fp.

 

[nereas]

Yah, I understand how to use it. I just don't see how it's helpful. ...

It helps to remember the formulae.

I always remember that the "Post Office" for PO2 or ppO2 is at the top.

And then the other two naturally end up on the bottom next to each other.

If you max depth on a dive will be XXX fsw, and you want to know what fraction of oxygen equates to a ppO2 of 1.2 at that depth, then you first compute total pressure as XXX/33 + 1.

Then you divide 1.2 by this total pressure, because that is what your thumb told you to do in the diagram.

Next you do the same thing for nitrogen, where your ppN2 is something acceptible to you, say 2.0.

Finally 100% - FO2 - FN2 equals your helium fraction.

It is primarily a trimix issue, however it is normally introduced in basic nitrox.

 

[Blackwood]

It is primarily a trimix issue, however it is normally introduced in basic nitrox.

I disagree. It's as much a nitrox issue as it is a trimix issue. While there may be no F_narcotic calculations, it's still used. Nitrox divers need to track oxygen uptake, the first step of which is computing PO₂.

Then you divide 1.2 by this total pressure, because that is what your thumb told you to do in the diagram.

Well there's a signature worthy post if I've ever read one.

 

[Charlie99]

It's a way of writing P_x=F_x*p.

It's used as a teaching aid of other algebraic relationships as well (such as Ohm's law), but I fail to see how it's helpful in any way.

I agree. Indeed, with many students it may reinforce their tendency to learn things by rote and mnemonic rather than understanding the base principles.

The base principal is pretty simple -- the Fraction of each gas is indeed the fraction of Partial Pressure/Total Pressure. After that, just as with Ohms law, all it takes is enough skill at algebra that one can convert A = B*C into B = A/C.

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In much the same sort of philosophy, if one simply remembers that the equivalent air depth calculation is just figuring out at what depth air would have the same ppN2, then the EAD equation is no longer something that has to be memorized. The PADI nitrox book has a nice equation for EAD, but I find it easier to use a slightly longer version in which the various sections of the equation correspond to figuring out the ppN2 of air at a depth, figuring out ppN2 of nitrox at a depth, and then setting them equal.

 

[spt29970]

I agree with Charlie99, mnemonics like this are counterproductive since they teach how to get "correct" answers without understanding the underlying principle. Besides, it isn't any simpler than the equation that it is "helping" you memorize.

 

[ronbeau]

It's a way of writing P_x=F_x*p.

It's used as a teaching aid of other algebraic relationships as well (such as Ohm's law), but I fail to see how it's helpful in any way.

It is apparently helpful for the mathematically challenged. NAUI has it in the training materials. My instructor said that a lot of people (including him) found it helpful. Many people find basic algebra difficult.

I found it was not worth memorizing the circle T. I was easier to just manipulate the algebraic equation.

Having said that if it helps someone understand the material I am all for it. There is usually more than one way to approach many problems. I know some excellent diver's that were helped by the "circle T".

 

[Blackwood]

In much the same sort of philosophy, if one simply remembers that the equivalent air depth calculation is just figuring out at what depth air would have the same ppN2, then the EAD equation is no longer something that has to be memorized. The PADI nitrox book has a nice equation for EAD, but I find it easier to use a slightly longer version in which the various sections of the equation correspond to figuring out the ppN2 of air at a depth, figuring out ppN2 of nitrox at a depth, and then setting them equal.

Yah, I write things the way that makes sense to me, which doesn't always correspond to what most texts present. EAD/END and SAC rate are good examples of formulas I consistently write differently than most texts.

Having said that if it helps someone understand the material I am all for it.

Fair enough.