EAN versus "no-Fly" delay

[matt_w]

but no more conservative than doing that same dive with the same conservatism settings on air. Conservatism settings don't care what mix you are diving. The only way to make EANx more conservative than air, is to tell your computer you are diving air, and dive nitrox. Conservatism is then increased by the difference in FO2 of your mix and air.

Of course then nothing is correctly tracking your oxygen exposure and your computer can't warn you if you reach a PPO2 limit.

Conservatism effects vary by computer and algorythm. Diving for some amount of time under the NDL always works.

 

[redacted]

No fly rules of 24/18/12 are course but apparently effective. I wanted to stop wasting diving oportunities.

I do use EAN to shorten my no fly time. We dive older Oceanic computers that provide a graphic tissue loading bar graph with each of the 15 NDL ticks worth about 6% loading of the lead compartment. I noticed that after some agressive NDL diving in Cozumel that it was quite common to have one or even two ticks after 24 hours. When we dive Bonaire using EAN 32, I have found that we can dive to within an 18 hour window and still be clear on the TLBG. Heck, we could go down to 12 hours but that does not leave enough time to dy gear before packing. DiveNav offers computer simulations where you can examine the specific conditions you might be interested in.

Some computers provide a desat time that might be helpful once you understand how that will behave and how it relates to loading.

 

[redacted]

Of course then nothing is correctly tracking your oxygen exposure and your computer can't warn you if you reach a PPO2 limit.
.

O2 loadng is easily tracked manually. Although I never really bought in to this idea of diving EAN as air unless your computer did not handle nitrox.

 

[tbone1004]

if you limit your PO2 to 1.4, then per noaa you get 180 minutes/24 hour window. Odds of you getting up to a PO2 of 1.4 for that 60 minutes each day is pretty small and I don't believe it is possible with any tables/algorithms/conservative settings and any recreational nitrox mixes to hit that. I.e. on EAN40 you have to maintain a depth of 85ft for an hour, do that 3x/day and I don't think you can do that with NDL's so CNS clock isn't really relevant

 

[boulderjohn]

People in this thread and in many other such discussions seem to think you have only two choices--1) breathe nitrox on an air setting and get some level of additional safety or 2) breathe nitrox to the full limit of the nitrox tables. You therefore have a choice--increased safety or increased bottom time, but not both! There is nothing in between, dammit!

Let's compare two divers doing a typical 2-tank dive such as you might do in south Florida.

Diver I: This diver is following PADI air tables. He does the first dive to 80 feet, staying for the full PADI limit of 30 minutes. He finishes in pressure group R. After a one hour surface interval, he is in pressure group F. He does a second dive to 50 feet for the maximum allowed time of 56 minutes, finishing in pressure group X, indicating pretty severe decompression stress if on air.

Diver 2: Diver 2 is using EANx 36, and he is following the PADI EANx 36 tables. He does the first dive to 80 feet, but he stays for 40 minutes instead of the 30 for diver #1. He finishes in pressure group L. After a one hour surface interval, he is in pressure group C. He does a second dive to 50 feet for 83 minutes, finishing in pressure group R .

Analysis: Diver #2 did two dives to the same depth as diver #1, but he got 37 more minutes of bottom time, an increase of nearly 50%. He exited the water in pressure group R, with far less decompression stress than diver #1, who exited in pressure group X. Of course, if Diver #1 was actually using EANx 36 but using air tables, then he is exiting at an even greater level of safety, at the expense of 37 minutes of bottom time. Congratulations!

Diver #2 could have stayed for 190 minutes on the second dive, but, believe it or not, no one was there forcing him to do that. Amazingly enough, you do not have to dive to the full limit of your nitrox tables. No one will shoot you if you come up early.

 

[C P]

If gas is the limiting factor instead of NDLs than nitrox may (could/should) do what the OP is wondering about. Also if they are planning a dive for an definite amount of time instead of just staying until they run out of time or air, then nitrox will also cause less nitrogen loading which should also shorten the wait to fly time.
BUT
If they dive nitrox to its limits though then the longer dive means they get the same amount of nitrogen loading. Doesn't matter if you fill the glass slow or fast, if you fill it up you have same amount of tea to drink.

 

[boulderjohn]

As some have suggested in terms of the no flying rule, there simply is nothing to go on here. Logically, it makes sense that breathing nitrox will make a difference. Logically, it makes sense that breathing nitrox AFTER the dive will make a difference. We don't have any quantitative research to support it, though.

I have, however, seen a table that tells how many pressure groups (using the NOAA tables) you can drop by breathing pure oxygen on the surface after diving, but I don't have them with me right now.

 

[BoltSnap]

OP specifically asked about EANx being more conservative than air

Where did he say that?

 

[tbone1004]

Where did he say that?

... I find it interesting that a major reason for EAN is actually to displace nitrogen in the breathing mix. That in turn reduces the risk for DCS and, under the right conditions, 'could' even make the safety stop unnecessary.

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

Casey

the original post in the thread....

So to answer that back portion. The only reason for EANx is to displace nitrogen in the breathing mix because that is all it does, but it simply displacing the N2 with O2 does nothing to reduce the risk for dcs, doesn't make the safety stop unnecessary, and doesn't shorten or eliminate the no-fly restriction. Reason for that is unless you are diving the exact same profile as you would with air *i.e. my comment about telling your computer you are diving air and then you dive nitrox*, if you dive up to the NDL on your computer, you come out with the same theoretical tissue saturation on any nitrox mix as you would on air because the NDL time is ONLY calculated by your N2.

Simply put *and no the ratio isn't linear like this, it is a very simple explanation*. Your tissues are like a 5 gallon bucket. Air *21% O2* fills the bucket at a rate of 1 gallon/minute so you get to dive for 5 minutes until your bucket is full. EANx 42 *42% O2* fills the bucket at 0.5 gallons/minute. You get to dive for 10 minutes until the bucket is full. Your safety stop, NDL, DCS risk, No-Fly time is determined by how full the bucket is and at the end of both dives, the bucket is full. Full bucket means 0 change in any of the safety of that dive, you just got to enjoy a dive that was 2x as long.

Conversely if you dove EANx 42 and told your computer that you were diving EANx21, then it would think you were filling the bucket at 1gpm but really you were filling it at 0.5gpm, so at the end of the dive you are only half full. Only way to do that is to plan your dive to half the NDL, but you don't really know that until you are on the dive, or you plan the dive using air tables. Using air tables to plan the dive but diving EANx is no different than telling your computer you are diving air or some reduced mix of nitrox and with the exception of CNS tracking *which isn't a problem in recreational NDL diving*, then you are going to be conducting a "Safer" profile, but you don't get to enjoy the extended NDL's associated with EANx. Personal choices, but the original statement was by diving nitrox, you are conducting safer profiles, which is not true.

@boulderjohn we're going to have to disagree on that logic. I don't know the RDP tables, don't care to, but I'm going to assume what you said is true. Here's why I have to disagree with you. Your examples I'm sure are accurate, but what you are suggesting is simply that the diver didn't dive up to his NDL's. This has nothing to do with diving nitrox or not, only making a conscious decision to end your dive before you hit NDL. The bigger problem with this, is who in PADI is actually still teaching tables? Even if you learn tables, the important part here is that the diver is choosing to make his dive more conservative, and the nitrox mix does not make the dive inherently more conservative. Yes, my point is correct. You get to use nitrox to do one of two things, extend your NDL's, or increase your safety margins by treating it as air. You proved that in your example because the diver dove past the NDL's of air diving, not up to the NDL of the nitrox mix, but he used nitrox to extend the dive beyond the NDL's of air.

 

[RyanT]

@boulderjohn we're going to have to disagree on that logic. I don't know the RDP tables, don't care to, but I'm going to assume what you said is true. Here's why I have to disagree with you. Your examples I'm sure are accurate, but what you are suggesting is simply that the diver didn't dive up to his NDL's. This has nothing to do with diving nitrox or not, only making a conscious decision to end your dive before you hit NDL. The bigger problem with this, is who in PADI is actually still teaching tables? Even if you learn tables, the important part here is that the diver is choosing to make his dive more conservative, and the nitrox mix does not make the dive inherently more conservative. Yes, my point is correct. You get to use nitrox to do one of two things, extend your NDL's, or increase your safety margins by treating it as air. You proved that in your example because the diver dove past the NDL's of air diving, not up to the NDL of the nitrox mix, but he used nitrox to extend the dive beyond the NDL's of air

tbone: whether anyone cares about diving tables or not, boulder john was using them simply to illustrate the different pressure groups and hence, post-dive decompression stress. The table pressure groups are correct on that matter. Let's think about this for a moment. If you dive 36% EAN and overstay the air NDL by a few minutes, you get a longer bottom time than if you were diving air AND you are farther away from your NDL saturation limits because of the nitrox. In addition, you will offgas more efficiently on ascent using 36% EAN, particularly if you do a safety stop. I agree with you that if you push your nitrox NDL to the edge, you are going to have a similar DCS risk as diving air to the limits. But, if you dive a relatively conservative nitrox profile, you can extend your bottom time a bit and still have an increased margin of safety on that particular dive.