I know its very easy but I can't remember how to do it so can someone remind me how to do a best mix calculation. an example of a problem is as follows..."At a depth of 104 fsw, what is the best mix when you set your max p02 at 1.4 ata?". a link to the table that i believe should help with this is located at "http://www.rebreatherworld.com/photopost/data/508/Ron_-_Table_1.jpg". thanks.

3/31/2008 7:11:44 PM

Is this for PE226 or PE227?

3/31/2008 7:28:11 PM

The reason I ask is this:

I have no problems helping you, but I WILL NOT spoon-feed you the answer. If you are serious about diving, especially diving with nitrox, you NEED to know how to do this. Not memorizing it, rather, you need to know the concepts behind how the idea of best-mix comes about.

3/31/2008 7:38:32 PM

this is for PE 226 but I am also enrolled in the 229 open water class so i am quite serious about diving and want to know how to do the problems, not just get the webassign done. and i have many of the same problem to do with different numbers so giving me the answer to one and me not understanding it will not answer the 9 other problems i have to get done. i want to know how to do them. so please give me the details i need to answer them on my own.

3/31/2008 7:43:50 PM

Alright, here's the deal. There are a few things you need to understand in order to do the problems...

First: it is not just the PERCENT of oxygen that keeps you alive (or kills you) -- it is the PARTIAL PRESSURE. You want to keep the partial pressure of oxygen between 0.21ata and 1.4ata. Less than 0.21ata is considered hypoxic (lacking in sufficient oxygen, possibly leading to death of it's too low) and >0.21 is considered hyperoxic (more than normal oxygen). Each person is different, but if you go too far beyond 1.4ata pO2 you run the risk of oxygen toxicity, and possibly death.


A few other important things:
1: What is nitrox?
2: Partial pressures
3: pressure of water

--------------------------------------------------------------

Nitrox:
As a loose explanation, nitrox is any mixture of oxygen and nitrogen. Air can technically be defined as nitrox, since it's essentially a mix of 21% oxygen (O2) and 79% nitrogen (N2). However, for recreational diving purposes, "nitrox" is usually defined as a gas mixture containing between 21% and 40% oxygen, with the remainder being nitrogen.

--------------------------------------------------------------

Partial pressure:
Say you have a mixture of 3 different gasses. The total pressure exerted by the entire gas mixture is made of: (%gas) * (absolute pressure).
So for 32% nitrox (32% oxygen, 68% nitrogen), at 1 atmosphere absolute, the pO2 is 0.32 and the pN2 is 0.68. Since 32% is (0.32) in mathematical terms, that's (0.32 * 1ATA = 0.32 pO2). Still follow?


Let's say we were breathing 32% nitrox at 66fsw. What is the pO2 and the pN2 that you are breathing?
You need to know:
1: Absolute pressure at 66fsw.
2: % of gases in the mix.

Since there are 33fsw in every atmosphere of pressure, 66/33 = 2ATM, +1 (for the earth's atmosphere) and you get 3ATA (atmospheres absolute).
32% nitrox is 32% O2 and 68% N2.

pO2 = (0.32 * 3) = 0.96
pN2 = (0.68 * 3) = 2.04

Note that 0.96 + 2.04 = 3.

So how did we solve that? We needed to know how what partial pressure is and how to calculate it, how to convert feet of seawater into atmospheres of pressure, what percents of gas are in the mixture...and maybe some 3rd grade math.

--------------------------------------------------------------


If you need more help, or that confused you more, let me know.

[Edited on March 31, 2008 at 8:06 PM. Reason : ]

3/31/2008 8:06:18 PM

I'll give you one more hint.

Take the example I just gave and work backwards -- manipulate it with basic algebra. If you get an answer (right or wrong) post it here and I'll make sure you're understanding it conceptually.

3/31/2008 8:32:45 PM

thanks man I'll give those a shot tonight after work

4/1/2008 12:07:25 PM

nice, aced that shit on some web assign, thanks

4/9/2008 12:02:01 AM

but it's not about webassign . . . it's about knowing how to dive safely. Sure you could just buy a fancy dive computer to tell you all the things you do on webassign . . . but that computer is only as good as the programmer and could fail at anytime. I'd rather not leave my life solely in someone elses hands.

4/9/2008 11:02:10 AM

^ True.
I don't dive with a computer. They rot your brain.

<---goes the GUE/DIR route...

4/13/2008 12:30:55 AM