When to use different EANx Mixes?

[rjack321]

Actually, it's more a long-winded way of disagreeing with you

I do not agree with your earlier statement that 'O2 is for all practical purposes narcotic'. I believe it is at best unproven; and I think the thread I linked to previously
(How Narcotic Is Oxygen? - Rebreather World) gives a great summary of why.

What do I personally recommend? Avoiding definitive statements on the narcotic properties of oxygen.

So apparently you'd rather discount the likely narcotic potential of O2 and tell people diving EAN32 its "better" than air vis narcosis than be conservative and call it a wash between lipid solubility, narcosis, and metabolism. Yet for trimix you somehow decide we better be conservative and up the helium just in case your chosen ppO2 which is bounded within a narrow range anyway turns out to be narcotic. Weird logic.

While Dr. Mitchell's a smart guy (and on this board as well albit rarely) that thread is over 4 years old and <still> this apparently measureable difference hasn't been measured. Which is saying something about the magnitude of the difference and its relevance to actual diving (not much).

 

[ameneon]

I think the minimum requirements for a PADI nitrox course was dumbed down even more around the beginning of 2010. No calculations is required anymore as long as you have a dive computer... In May 2010 I took the Nitrox eLearning course which covers the entire new book. I am guessing alot of the dive centres use this book as well. So if the instructor teaches the bare minimum I am not surprised by the OP's posts.

The course goes through how to calculate the PO2 when you have depth and oxygen %. However how to calculate MOD is not mentioned at all. While this is easy for those that know basic math and can rearange the equation for unknown MOD, I am guessing the average person wouldn't know how. Best mix is not mentioned.

The way to find the MOD is taught like this:
Set PO2 to 1.4 on your computer.
Analyze your tank and set the % on your computer.
Scroll down and see what the computer says is the MOD.

 

[vondo]

The way to find the MOD is taught like this:
Set PO2 to 1.4 on your computer.
Analyze your tank and set the % on your computer.
Scroll down and see what the computer says is the MOD.

That's quite frightening considering that MOD is the single most important concept that should be taught in the class. I imagine if it's taught that way, very few people would even surmise at first that you *could* calculate the MOD in a straightforward way.

 

[grammaticus]

I think the minimum requirements for a PADI nitrox course was dumbed down even more around the beginning of 2010. No calculations is required anymore as long as you have a dive computer... In May 2010 I took the Nitrox eLearning course which covers the entire new book. I am guessing alot of the dive centres use this book as well. So if the instructor teaches the bare minimum I am not surprised by the OP's posts.

The course goes through how to calculate the PO2 when you have depth and oxygen %. However how to calculate MOD is not mentioned at all. While this is easy for those that know basic math and can rearange the equation for unknown MOD, I am guessing the average person wouldn't know how. Best mix is not mentioned.

The way to find the MOD is taught like this:
Set PO2 to 1.4 on your computer.
Analyze your tank and set the % on your computer.
Scroll down and see what the computer says is the MOD.

I am not sure about the online course, but how to calculate MOD is in the PADI book. It's on p. 86 (the formula). The other formulae for EAD and O2 levels are on pp. 84-5. However, 'best mix' is not discussed, at least in detail. I looked up Dalton's diamond, or Dalton's gas law, and it was very informative. PADI should have included it, at least the formula for calculating the 'best mix', in the book. However, I think the reason that they didn't include it is that many recreational divers that go through the course will rely on the DSAT tables and will rarely, if ever, go off of them. Even the introduction for being able to calculate MOD, contingency depths, o2%, etc. is introduced in a way that suggests that you would only need it if you go under x30 or over x40, because all the calculations are already on the DSAT table provided.

Using a computer is covered in the book as well (pp. 47-9), but it's not discussed in detail. Basically, if you use a computer, you should consult the manual.

After reading the posts, but actually doing a bit of research, the PADI course is good as an introduction to an introduction to EANx. More detail is needed. In addition, there are many errors in the book, which makes me wonder how much time was put into publishing it.

 

[MauiScubaSteve]

After reading the posts, but actually doing a bit of research, the PADI course is good as an introduction to an introduction to EANx. More detail is needed. In addition, there are many errors in the book, which makes me wonder how much time was put into publishing it.

I would be interested in hearing of the errors you have identified?

 

[ameneon]

I am not sure about the online course, but how to calculate MOD is in the PADI book. It's on p. 86 (the formula).

That is not the newest book. The course can now be taken as a course where it is taken for granted that you are always using a computer. Same as the new open water course minimum requirements. The book which the online course is based on is less than 40 pages...

 

[grammaticus]

I would be interested in hearing of the errors you have identified?

The errors I am talking about are grammatical and editing errors. Some of the words have a hyphen in the middle where there shouldn't be one, for instance. This is a formatting error. Others have large gaps between words where something was obviously cut off because the sentence really doesn't make all that much sense.

There is also a calculation which needs to be reviewed in the chapter after the one which discusses using the PADI EAN tables. For example, it says that the answer to a problem should be O2 20% and ending on letter W (that's with the RDP), but if you use the EAN RDP then you end up on V. The question doesn't tell you whether to use either table, but the answers you get using either one is different. If I were to use x32 then I would use the EAN RDP not the RDP for air. That's my opinion.

 

[muddiver]

You might want to check your formula for END again. Of the two common variants I'm aware of (depending on whether you consider O2 narcotic or not), neither should produce an END that is deeper than the actual depth.

Thanks, I do not claim to be an expert in mixed gas diving, so my definition of END is probably incorrect. Feel free to correct me.

I will say that I make some asumptions, like: I based END on an pN2 of 4.0, MOD is based on a pO2 of 1.6, but I plan the dive using a miximum pO2 of 1.4.

 

[vondo]

The errors I am talking about are grammatical and editing errors. Some of the words have a hyphen in the middle where there shouldn't be one, for instance. This is a formatting error. Others have large gaps between words where something was obviously cut off because the sentence really doesn't make all that much sense.

There is also a calculation which needs to be reviewed in the chapter after the one which discusses using the PADI EAN tables. For example, it says that the answer to a problem should be O2 20% and ending on letter W (that's with the RDP), but if you use the EAN RDP then you end up on V. The question doesn't tell you whether to use either table, but the answers you get using either one is different. If I were to use x32 then I would use the EAN RDP not the RDP for air. That's my opinion.

The different answers on the test for 32% has been there for years. I remember this problem in 2006 when I took the course when dives were still required. It was annoying.

 

[Hepcat62]

Thanks, I do not claim to be an expert in mixed gas diving, so my definition of END is probably incorrect. Feel free to correct me.

I will say that I make some asumptions, like: I based END on an pN2 of 4.0, MOD is based on a pO2 of 1.6, but I plan the dive using a miximum pO2 of 1.4.

I'm definitely not an expert in any of this stuff - I'm posting this as much for my own education as anything else...

Re-reading your original post, I believe that we're talking about two different things. I was interpreting END as "Equivalent Narcotic Depth", which takes as input a fractional portion of the mix which is treated as narcotic and an actual depth. The two variants for computing END that I'm aware of are:

Oxygen is Narcotic

END (imperial, salt water) = (((1 - F_He) * Depth_ata) - 1) * 33

Oxygen is NOT Narcotic (common for recreational diving)

END (imperial, salt water) = (((1 - (F_He + F_O2)/0.79) * Depth_ata) - 1) * 33

I believe that what you were calculating is actually the MND, or maximum narcotic depth. That is to say, the maximum depth at which the partial pressure of the narcotic portion of the mix does not exceed 4 ATA.

MND (imperial, salt water) = ((4 / F_narcotic) - 1) * 33

In that case - and assuming oxygen is not narcotic - your math seems right to me.