EAN versus "no-Fly" delay

[Frontpointer1000]

RMV is a more readily usable data point than SAC (I would argue they are technically different, though they reflect the same thing), and yes "ppm" is psi per min, reflecting my predilection toward the imperial.

 

[KenGordon]

when diving deeper is the risk of DCS reduced, increased or does it stay the same?

 

[doctormike]

RMV is a more readily usable data point than SAC (I would argue they are technically different, though they reflect the same thing), and yes "ppm" is psi per min, reflecting my predilection toward the imperial.

They aren't the same thing, they are dependent variables. SAC means surface air consumption. RMV means respiratory minute volume.

For some reason, it has become standard in scuba education to use these terms in a manner that seems incorrect to me. Maybe the dive pros here can tell me why that is, but I have found this usage pretty consistent in all sorts of published material (here's an example from DAN).

They tend to calculate SAC as PSI per minute, and then use a tank baseline to covert this number to an RMV. This is incorrect since PSI isn't a measure of volume (because it depends on what tank you are using), and RMV is how much gas you are actually breathing in and out (so it changes with depth). So it seems that they are using the term RMV when they should be using SAC, and then using SAC in a way that is meaningless if you don't know the baseline of the tank you are diving. I guess it's fine as long as the math comes out OK, but it always bothered me to read this stuff.

Here's how I see it:

RMV = the amount of gas you breathe in and out in a minute (in cubic feet per minute using imperial measurements). This changes with depth.

SAC = RMV divided by the depth in atmospheres (ATA) = the amount of gas you breathe in and out at sea level. It tends to be fairly stable for a given diver, and goes up with things like workload and stress. Basically, it is your gas consumption adjusted for depth. RMV=SAC x ATA

So if your SAC is 0.7 cubic feet per minute (CFM), then at 33 feet (2 atmospheres) in one minute you would go through 1.4 CUF of gas (RMV=1.4), and at 66 feet (3 atmospheres) you would go through 2.1 CUF of gas, etc...

 

[tbone1004]

@doctormike I don't get it, I learned the term as SAC and was always expressed in unit volume per minute. Either liters or cubic feet depending on where you were. We used DAC for depth air consumption and that was obviously measured in PSI/min or bar/min.

@Frontpointer1000 the confusion on ppm was the fact that is normally referring to parts per million... never seen air consumption expressed like that.

 

[doctormike]

@doctormike I don't get it, I learned the term as SAC and was always expressed in unit volume per minute. Either liters or cubic feet depending on where you were.

Right, that's correct. I was just mentioning that if you look at a lot of dive training materials (like the one I cited from DAN), you will find that they distinguish between RMV and SAC by implying that SAC is measured in PSI per minute, so you convert it into RMV by using the tank baseline number.

Thus, they are using RMV as a surface air consumption rate, which it isn't - it's an absolute air consumption rate - it increases with depth, when using the correct (medical) definition. I was just saying that a lot of dive pros apparently use this incorrect terminology, and that causes confusion in discussions like this. They use the formula RMV = SAC x baseline.

Here are some more examples of that convention:

http://scuba.about.com/od/Theory/ss/Air-Consumption-Rates-For-Scuba-Diving-Sac-Rates-Rmv-Rates-Easy-Calculations.htm#step4

Surface Air Consumption Calculator | DiveBuddy.com

DiveNerd - SAC Rate / RMV Calculator, Imperial Units

 

[g2]

From the OP's original question...

... So, my question is, could it shorten, or even eliminate the no-fly restriction?

Just to be a smart-ass (and because nobody else has pointed it out), if you wanted to dive right before a flight, you could calculate the depth at which your EAN's ppN2 = normal atmospheric ppN2... And then dive up to that depth as long as you like without increasing your nitrogen loading.

Example: EAN32 has ~67% N2, or 86% of normal atmospheric N2. At 1.16 ATA your ppN2 would be the same as "surface air". (1.16 ATA - 1 ATA) * 33 fsw = ~5 fsw (1.6 m), or a nice long 5ft dive. Ha!

If you bump the nitrox mix up to EAN40 you could enjoy a 10 ft dive until you suck the last molecules out of the tank, and then immediately jump on a plane with no additional risk of DCS.

 

[BoltSnap]

@BurhanMuntasser yes, that is actually what the OP said, it was a direct quote from the original post. He literally said reduces the risk of DCS, and asked if it could shorten or eliminate the FAD time. The answer to that is a resounding no. Nitrox does absolutely nothing for any of that. You can change your diving plans and nitrox will give you perks of lower N2 loading for equivalent depth and time vs. air, but changing your dive plans is what does what he is asking, not having a lower FN2 in the mix

He didn't say that "literally" or even imply it at all. You are dreaming things up because you are so absorbed into your own way of diving and everything you read or hear you interpret it your way not what other people meant it or want it or need it.

 

[tbone1004]

@BurhanMuntasser that is not what I was implying by literally. You said "that is not what he said", I replied saying it is literally what he said, as in direct quote of what was actually said. Did he mean it like that? Did I interpret it in a way that is more aligned with the way I teach and dive vs. the rest of the recreational world? That's a valid rebuttal, but he literally said what I quoted and I was responding to what the OP said using the information that I have available in the way I thought was being asked.

 

[victorzamora]

I think most in here agree with the general principles of diving EAN but are bickering/quibbling over semantics.

Diving Nitrox doesn't, in itself, reduce DCS potential. It does reduce DCS potential over air (or a leaner nitrox mix) at the same depth for the same dive time. It's easier to be more conservative (farther from NDLs) when diving nitrox because one will likely become air limited over much of the diving spectrum. You can reduce your SIs when compared to air, but then you lose some bottom time benefit (less SI for same bottom time or same SI for more bottom time).

Whether or not a nitrox dive is more conservative than an air dive is dependent upon a few other variables...so it's unfair to state that "Nitrox makes diving more conservative" without defining other variables (depth, bottom time, mix, etc). Doing a dive to your NDLs and riding the computer during ascent will be similarly aggressive no matter the mix. Doing an hour at 50ft will be much more conservative on EAN36. than it would be on air.

 

[BRT]

there is actually a way to do it....
*disclaimer, this is not endorsed or recommended by anyone, including myself and any agencies that I am an instructor for, including DAN. Please do NOT try this*

Typical cabin pressure will max out at ~8000 ft. This is roughly 3/4 of an atmosphere. If you treat your last dive as an altitude dive at 8000ft, then you can basically fly immediately after the dive if you plan your decompression or NDL's appropriately. Why? you're basically saying that instead of surfacing at sea level, you are conducting the dive as if you were at the cabin pressure of the plane. Your NDL's will be short or your deco will be long, but it is possible to basically "trick" your body into decompressing to a point that it will not supersaturate at altitude. This is no different than the guys that live in the mountains and dive at sea level.

@BurhanMuntasser yes, that is actually what the OP said, it was a direct quote from the original post. He literally said reduces the risk of DCS, and asked if it could shorten or eliminate the FAD time. The answer to that is a resounding no. Nitrox does absolutely nothing for any of that. You can change your diving plans and nitrox will give you perks of lower N2 loading for equivalent depth and time vs. air, but changing your dive plans is what does what he is asking, not having a lower FN2 in the mix

So you think that by staying underwater longer "decompressing" you can fly sooner? I'm not a fancy tech diver but even I know that is not true.