When to use different EANx Mixes?

[Michael Kazma]

If you do not know the answer to this question then you need to do the course again with another instructor. Im sorry to be so harsh but this stuff is the most important to learn in the nitrox course. If you do not know what Dalton's diamond is then retake the course with another instructor and make your previous instructor refund your fees. If you do not know how to calculate your MOD, PO2 and OTU then diving nitrox is very dangerous!

 

[Divedoggie]

Not explained. They just said that I should just read the book and if I have questions the instructor will discuss them with me. I should just take the exam then off I go. Good thing I told them that I want to do the dives!!!

It looks like you have a lot of questions. If you have read the book and the Instructor is planning to answer your questions, then sit down with him and ask the questions. If you still don't feel that you understand the material, demand a refund and go take the course somewhere else.
Posing your questions here on SB and NOT to your Instructor will do neither of you any good.

 

[KarolinaKid]

If you do not know the answer to this question then you need to do the course again with another instructor. Im sorry to be so harsh but this stuff is the most important to learn in the nitrox course. If you do not know what Dalton's diamond is then retake the course with another instructor and make your previous instructor refund your fees. If you do not know how to calculate your MOD, PO2 and OTU then diving nitrox is very dangerous!

A bit harsh maybe, but very true and very close to what I was thinking when I read the original post.

 

[rjack321]

Well I have a rough idea from the book. PADI mentions that you should not exceed 100% O2 exposure per day, and that you should not exceed 1.4 or 1.6 max per dive. Using different mixes on each dive is not covered on the theory behind the calculations. However, you do have to do some calculations using different mixes. PADI suggests in their book that you don't have to even do the dives, but it is recommended that you do. After reading the book, this seems to be dangerous if you don't do the dives and understand how to do the calculations thoroughly.

It seems to me that the deeper you go the less EANx you want to use. This could be wrong, but it begs the question whether EANx would be better for deep dives, say theoretically 130ft? I am not sure if that's completely true. I did a mock plan for the following dives:

1. 100ft EANx32 20min
2. 60ft EANx36 50min
3. 50ft EANx40 80min

According to my calculations, this would be alright with S.I.s at about 1hr each. I don't think that I would ever do 3 dives with 3 different mixes like this, but they are just hypothetical. I would be interested in what you would have to say.

Best wishes.

Too complicated honestly. There's way too much likelihood that you couldn't do one or more of these dives because it was modestly deeper than anticipated. e.g the 60ft turns out to be 75ft.

I am not sure what you are thinking NDL-wise but you could basically do those depths/times on air anyway.

I would guess that in the real world 80% of nitrox dives are done on 32%. The other 20% are done on something less rich like 28 or 30%. 36% is not esp. useful to most people and more limiting on MOD and ppO2 when virtually the same dive could be done on 32%.

The "benefits" of constantly changing mixes are more hassle than they are worth. In particular getting a 40% fill for a 50ft dive is not worth it to most.

 

[halocline]

Not explained. They just said that I should just read the book and if I have questions the instructor will discuss them with me. I should just take the exam then off I go. Good thing I told them that I want to do the dives!!!

I am 100% certain that the concepts of maximum operating depth and equivalent air depths were somehow covered in your nitrox class. In fact, those are basically the main concepts for the class. Best mix, seems to me I remember that one from the class, but I could see where a bare-bones class might not cover it...it's not so much a safety issue like the other two are.

Anyhow, the example you gave about different mixes for different depths is pretty much the idea, if you have a specific depth you want to go to, and you have access to whatever percentage O2 nitrox you want, you can calculate the maximum safe O2 content for the dive and use it.

In reality, with recreational diving and nitrox, more often you get a blend that's available and stick to the MOD for that blend. 32% is a pretty versatile common blend because it's MOD (based on 1.6) is basically the same as a commonly accepted depth limit for recreational diving. I suspect that had something to do with 32% becoming a typical pre-mix. Now that 1.4 is a more commonly used PO2 for calculating MOD, I wonder if that will change.

 

[rjack321]

32% is a pretty versatile common blend because it's MOD (based on 1.6) is basically the same as a commonly accepted depth limit for recreational diving. I suspect that had something to do with 32% becoming a typical pre-mix. Now that 1.4 is a more commonly used PO2 for calculating MOD, I wonder if that will change.

The "accepted (basic) recreational limit" has for all practical purposes shifted upwards to 100ft with 130ft being the limit for "deep certifications". I don't see 32% falling off the standard/pre-mix horse anytime soon just to accomodate the relatively rare 110-130ft nitrox dive. The fact is that nitrox just isn't that useful for deep recreational (100-130ft) or shallow technical (130-150ft) diving so there's little market for e.g. 28% premix. And for the same reason there's little interest in 40% fills for 50ft dives.

 

[muddiver]

If you do not know the answer to this question then you need to do the course again with another instructor. Im sorry to be so harsh but this stuff is the most important to learn in the nitrox course. If you do not know what Dalton's diamond is then retake the course with another instructor and make your previous instructor refund your fees. If you do not know how to calculate your MOD, PO2 and OTU then diving nitrox is very dangerous!

The PADI manual/class does not cover the Dalton's Diamond. Honstly it was not covered in my ANDI class either.

I am 100% certain that the concepts of maximum operating depth and equivalent air depths were somehow covered in your nitrox class. In fact, those are basically the main concepts for the class. Best mix, seems to me I remember that one from the class, but I could see where a bare-bones class might not cover it...it's not so much a safety issue like the other two are.

Anyhow, the example you gave about different mixes for different depths is pretty much the idea, if you have a specific depth you want to go to, and you have access to whatever percentage O2 nitrox you want, you can calculate the maximum safe O2 content for the dive and use it.

In reality, with recreational diving and nitrox, more often you get a blend that's available and stick to the MOD for that blend. 32% is a pretty versatile common blend because it's MOD (based on 1.6) is basically the same as a commonly accepted depth limit for recreational diving. I suspect that had something to do with 32% becoming a typical pre-mix. Now that 1.4 is a more commonly used PO2 for calculating MOD, I wonder if that will change.

32% is a standard NOAA mix that was the first used to create a table to dive EAN. 36% came later and is the only other authorized NOAA mix. The NOAA was the first organization to publish a Nitrox dive table and most of todays theory/tables/math came from the NOAA program.

grammaticus, the PADI course and manual came with a Equivilant Air Depth (EAD) table. Why are you not using that along with the basic equations to figure the Best Mix for the dive problems you are working?

Best Mix is based on the Maximum Operating Depth (MOD) equation and the Equivilant Narcosis Depth (END) equations. You have to work both to find your limit, or your best percentace of gas mox for the depth you plan to dive.

MOD = 10 meters X [(ppO2/fO2)-1] ppO2 should not exceed 1.4

END = 10 meters X [(ppN2/fN2)-1] ppN2 = 4.0 maximum

Most courses either ignore or forget that you have to take into account the narcotic effect of Nitrogen in the mix as well as the oxygen partial pressure.

 

[grammaticus]

Posing your questions here on SB and NOT to your Instructor will do neither of you any good.

My main question was geared towards choosing the best mix at specific depths. Why can't I ask this question on SB when many people here are instructors themselves. Of course, my instructor should tell me, but so what if I post the question here as well? There are a lot of people on SB that pose similar questions, even more basic than mine. I am just interested to hear about other opinions - isn't that the purpose of a forum?

If you do not know how to calculate your MOD, PO2 and OTU then diving nitrox is very dangerous!

I never said that I didn't know how to calculate these. I posted a theoretical plan only. I calculated everything out, but I didn't want to burden the board with calculations.

I think that rjack321 answered my question.

Thanks.

 

[grammaticus]

I am not aware of the narcotic equation, unless it is something different in the PADI book.

END = 10 meters X [(ppN2/fN2)-1] ppN2 = 4.0 maximum

What I did to calculate the depths is use the equations and use the DSAT table provided by PADI. I also used the RDP table for air and for the Nx32 and Nx36. Best mix, what I mean is that, for example, if you go 60ft for x number of minutes and are well within the NDL, how would you chose either x32 or x36. I know about the o2 levels, etc. to take into consideration, but at 60ft, for instance, if two people went to a depth of 60ft for the same amount of time (1 dive) why would one chose x32 over x36, and vice versa.

I'm sorry if I didn't explain this very well.

 

[Divedoggie]

My main question was geared towards choosing the best mix at specific depths. Why can't I ask this question on SB when many people here are instructors themselves. Of course, my instructor should tell me, but so what if I post the question here as well? There are a lot of people on SB that pose similar questions, even more basic than mine. I am just interested to hear about other opinions - isn't that the purpose of a forum?

Sorry to infer that you shouldn't pose any questions on SB.

Instead of random people giving you an abbreviated Nitrox course through this thread, I'm suggesting that you make a list of questions and ask your Instructor. You aren't getting the information that you deserve and paid for with quick answers on an online posting board.
It just seems that if you are paying for a course, you should be taught a course. Anyone can read a book. By asking the Instructor questions, it might convince him to actually teach the course. If he doesn't know the answers, it would be appropriate to find an Instructor who is motivated to teach the course and who knows the material.

It will be interesting to see an update to this thread after you meet with your Instructor again.