what is the ndl for eanx28 at 120' dept

[s7595]

I just finished my padi enriched diver course
the instructor was just great ,the course was fun but confuseing at times .the formulas were hard to figure out . now the reason for the ? on the enriched rdp eanx32 at 110' has an ndl of 25mins and a ata of 1.39 ,when you use the ead formula for eanx28 you get 3mins more time on the rdp
120' becomes 106 wich is rounded to 110 ' giveing you 16mins ndl .also eanx28 max ata of 1.4 is 132' so diveing to 120' should be safe so I need to know what the exact ndl is?
hope there is a formula I can use
thanks as always

 

[IndigoBlue]

Equivalent Air Depth is called EAD.

The formula is EAD = ( (1-FO2)/0.79 ) x (D+33) - 33

Therefore, for EAN28 at 120 fsw the EAD is:

( (1.00 - 0.28)/0.79) x (120 + 33) - 33 = ...

YOU do the math!

 

[divad]

the formulas were hard to figure out

I try to do this myself even though I know "why do you think they call the formulas."

 

[Tom R]

Two words to make your life easier. "Standard Mixes" this is a 32% dive

 

[Rick Murchison]

The short answer is:
If you can't calculate your own NDLs for the gas you're diving you have no business making the dive. Learn how before you do. Don't take someone else's word for it or trust someone else's math. Tom R's advice is germane.
For Tom R. - Major agencies now have a recommended limiting PO2 of 1.4 ATA; the MOD for 32% is 111' - 120' is not a "32% dive."
Rick

 

[Spectre]

*nod* I agree 100% with Rick. If you are having trouble after the class figuring this stuff out; then either the instructor was -not- great; or you need to go back to him and get some more help.

FWIW. 106 for an equivalent air depth is correct. You don't actually need to know any formulas other than how to calculate out partial pressure for a given depth. Calculate out the partial pressure of nitrogen with the given mix at the given depth; then figure out what depth that partial pressure of nitrogen is with air.

e.g. EAN28 @ 120. that's 28% o2, 72% nitrogen.

153/33 = 4.64 ATA = PpN2 of 3.34

Air is 21% o2, 79% nitrogen.
PpN2 of 3.34 / .79 = 4.23 ATA -1 * 33 = 107 fsw

Tom. Yea; standard mixes help; but standard mixes have depth ratings. EAN32 is standard to 100. 21/35 is standard for the depth in question.

 

[diverbrian]

Another option is that I have tables for EAN28 from my Advanced Nitrox course. I tend to use that mix for hard bottoms (on wreck dives, on wall dives the hard bottom can't be figured, LOL) of 135 ft. Things do happen.

The actual NDL in minutes depends upon the table that you are using when finally get your MOD. I know how to do it, but life is a lot easier with tables for the "common mixes" (in my case EAN 36,32, and 28). I have air tables that I can use to come up with the others the old fashioned way.

 

[jeadiver]

I have to admit that I haven't done "the formulas" in quite awhile, I have two nitrox programmable computers and dive both of them for redundancy. My suggestion is get a computer dude. Understanding the formulas is important. I remember many of my algebra and geometry formulas from school and can use them if I need to. But, like when trying to add a column of numbers on paper I am more likely to use a calculator, when diving nitrox I'm going to use a computer.

 

[Rick Inman]

I have to admit that I haven't done "the formulas" in quite awhile, I have two nitrox programmable computers and dive both of them for redundancy. My suggestion is get a computer dude. Understanding the formulas is important. I remember many of my algebra and geometry formulas from school and can use them if I need to. But, like when trying to add a column of numbers on paper I am more likely to use a calculator, when diving nitrox I'm going to use a computer.

You must know my PADI Nitrox Instructor, who said, "The only thing you need the formulas for is to pass the test. After that, just dive your computer." ,which, by the way, he will sell you.
But , then again, your signature line says, "The most important tool you will use while diving is the one between your ears"
Hummm....
Personally, I enjoy playing with the math and the level of understanding and control it gives me,

 

[GoBlue!]

I agree with Spectre. I don't remember the formulas, I just remember the concept. I haven't even taken Nitrox yet, but the "equivalent air depth" concept is not difficult...just figure out your ppN2 with Nitrox & determine what depth you'd be at with air (ppN2=.79 at surface) for equivalence. Spectre goes through this nicely a few posts back....no need to memorize formulas, you can essentially derive it every time just as quickly.

Jim