When to use different EANx Mixes?

[grammaticus]

I am almost finished with the PADI nitrox course, but I am wondering if people on this forum can give me some information about when to use different nitrox mixes. I don't think that the PADI course explains this issue very well.

Thanx.

 

[Jim Lapenta]

If you are still asking this question it doesn't have as much to do with the course as it does with who is teaching it. I have not seen the new nitrox course from PADI. In the course I teach we cover the appropriate mixes for a given dive. It is based on PO2 levels, planned depths, time under water, required surface intervals, etc. My first consideration is always max PO2 level and max depth for the dive. Also how many dives will you be doing. Is this not covered in the course? If anyone could shoot me a copy of the new PADI one I'd like to see it. The PADI course I took covered all of this but the instructor was very thorough and way exceeded what was in the book I had.

 

[MXGratefulDiver]

I am almost finished with the PADI nitrox course, but I am wondering if people on this forum can give me some information about when to use different nitrox mixes. I don't think that the PADI course explains this issue very well.

Thanx.

Did your instructor not explain maximum operating depth (MOD)?

Best mix?

Equivalent air depth?

These are all things to consider when planning the appropriate EAN mix for a particular dive. If they were not explained in a way that you comprehend why they're important, then the class was not taught properly.

... Bob (Grateful Diver)

 

[Wayne at DiveSeekers]

Typically, most travel destinations only have 32% which would be good to 111' (1.4pp) but usually they don't want you deeper than 100'. Locally here in New Jersey we have a flat sandy bottom (no wall dives) so being that we have a partial pressure blending station, divers can get whatever mix they want. I find that most divers are choosing their gas based on 3 different depth categories.

130' - 28%
100' - 32%
80' & shallower - 36%

Due to some boats changing wreck destinations, some divers are even keeping it even simpler by keeping their gas choices to just 32% (100'/less) & 28% (130').

 

[Hawkwood]

Did your instructor not explain maximum operating depth (MOD)?

Best mix?

Equivalent air depth?

These are all things to consider when planning the appropriate EAN mix for a particular dive. If they were not explained in a way that you comprehend why they're important, then the class was not taught properly.

... Bob (Grateful Diver)

I just noticed the OP said "almost finished". Those items are covered in the "second half" of the course. If they are not covered by the time the OP is finished then the course was not taught properly.

The practical exercise(s) or dive(s) are to demonstrate to the instructor that you can plan a dive with the appropriate mix and test the mix, or after testing determine an appropriate plan based on what you have.

 

[grammaticus]

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.

 

[grammaticus]

Did your instructor not explain maximum operating depth (MOD)?

Best mix?

Equivalent air depth?

These are all things to consider when planning the appropriate EAN mix for a particular dive. If they were not explained in a way that you comprehend why they're important, then the class was not taught properly.

... Bob (Grateful Diver)

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!!!

 

[h90]

The point is, you want to have as much O2 in your mix as possible without coming over max pO2 level at any time.
So you take the deepest point you'll go on that dive. And that should be serious. Your max pO2. Don't know what PADI recommends 1.4?? And calculate what mix is the max O2 percentage you can use.
Point is: As much O2 as possible without hitting the limit.

 

[D_B]

You'll also find that that listed max of 1.4 PO2 will change as well , it could/should be less than that depending on if there is current, cold water, even how your feeling that day

 

[Quero]

What h90 said in post #8. Ideally, you would use the richest blend (i.e., the highest O2 percentage) possible for the depth of the dive you will be executing. But this only works if you are able to order a specific blend to be custom made for you. You will find the formulae for calculating the best mix in the second half of the Nitrox textbook.

In practice though, one of two blends is most commonly available--32% for deeper dives and 36% for shallower dives. Blending stations typically keep lots of pre-filled tanks of these blends on hand and you can just walk in and pick one up. Since 32% will take you to the recommended recreational limit of 100 ft at a pO2 of 1.4 and all the way to the contingency depth limit of 130 feet at the contingency pO2 of 1.6, this is the most common blend as it works for all recreational diving depths. Here in Thailand, if you just ask for "Nitrox" you get 32%, especially since our dive sites tend to range from the shallows to rather deep, and we don't have to worry that divers will violate their MODs.

However, if you know you will be doing a bunch of somewhat shallower dives and want to cut down on the nitrogen loading with a richer blend, it's possible to ask for 36% and the blending station will probably have pre-filled tanks ready to pick up.

Alternatively, if you want the ideal blend for a dive you are planning, you will have to ask to have your tanks blended to order, which takes time and may cost more.