EAN versus "no-Fly" delay

[BRT]

@BRT, I was not talking about offgassing faster. You don't have to have your tissues at "surface" levels in order to safely fly. It is recommended, hence the 12-16-24 hour intervals depending on the tables you subscribe to, but as long as the tissues when exposed to 0.75 ata are not exceeding their theoretical limits, i.e. pretending to conduct your dive as if you were at an 8000ft altitude and stick to NDL's then you should be OK. I'm not talking about getting your tissues to the same point or lower as your surface tissue loading. That requires pure O2, and a LOT of it. All I'm talking about is basically diving at sea level, as if you were conducting an 8,000ft altitude dive. Your NDL's will be very short, but it is possible.

Again, not recommending it, I do it, I have lots of friends that do it, and if you have O2 that you can accelerate your deco it just means you can dive longer or fly sooner.

I agree with you entirely if you dive your NDL's as though you were at 8000 feet. However if you do what would be a decompression dive at 8000 feet and decompress as if you were at 8000 feet you are staying in the water under pressure, continuing to take in nitrogen in some tissues and offgassing less nitrogen from others than if you just got out of the water when you really could based on the altitude you are actually at. Spending extra time unnecessarily under pressure does not make for less nitrogen in the body unless you are spending that time on a mix and at a depth that gives you a PPN less than .79. 32% at 10 feet gives you more nitrogen than being on the surface. I'm not trying to be smart but there is a point here that you are missing.

EDIT I should mention that your tissues are not going to be at the same nitrogen level diving NDL's as though the altitude was 8000 feet because you still have the N loading from being at sea level plus whatever N you added during the dive.

 

[KenGordon]

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

Well, YES, of course.

So if it is agreed that diving shallower is safer (in DCS terms) than deeper then surely it follows that using nitrox to reduce ppN2 exposure is safer too?

I did the numbers. For the random selection of depths I tried the pattern was that using nitrox to the extended NDL resulted in lower saturation of fast tissues and higher of the slower tissues. Those slower tissues were pretty far away from their limits though. For flying it will be the slower tissues that count (assuming at least some SI before taking off) and so the answer would be 'do not bet on it' if diving up to the NDL.

If I get bored again (I was taking my son to a music lesson) I might try calculating the maximum dive times for nitrox where all tissues have less nitrogen than the NDL air dive, so definitely safer but also longer.[/QUOTE]

 

[BoltSnap]

@BRT

All I'm talking about is basically diving at sea level, as if you were conducting an 8,000ft altitude dive. Your NDL's will be very short, but it is possible.

Again, not recommending it, I do it, I have lots of friends that do it, and if you have O2 that you can accelerate your deco it just means you can dive longer or fly sooner.

This is absolutely rubbish!! I can't believe what I am reading here. You are playing games and making extrapolations, assumptions where you are qualified for none at all! As if all planes do always have the exact cabin pressure and that they never have emergency issues and lose pressure inside the cabin.

You folks are attempting to play games with your spinal cord and your health as if your becoming crippled and pissing and crapping on yourself for the rest of your lives is nothing more serious than the common cold.

 

[tbone1004]

@BurhanMuntasser the FAA regulates the cabin pressure to a maximum altitude of 8000ft. Most aircraft are lower than this.
I have stated multiple times that while it is possible, I do not recommend it. What I don't understand is why we allow and teach for altitude diving, have no restrictions against people diving at sea level and then driving to whatever altitude they live in, but don't allow it to happen when flying... 8,000ft is the max altitude, but many if not most planes are kept in the 5000-6000ft range because of all of the problems that most people have when exposed to altitude. Plan for the worst, of 8000ft, hope for the best of 5000ft. What I don't understand is why you don't think there is a difference in altitude diving, and getting in a plane that are pressurized...

@BRT what I am saying is only to help understand that if you have to/want to dive within that 12/16/24 hour no fly period, in my opinion/experience, the safest way to do that is to treat that final dive as a dive at 8000ft altitude. That way when your body is exposed to the max altitude cabin pressure, you are no more likely to get bent than if you actually conducted that dive to max ndl's using the correct altitude.

 

[KenGordon]

[
@BRT what I am saying is only to help understand that if you have to/want to dive within that 12/16/24 hour no fly period, in my opinion/experience, the safest way to do that is to treat that final dive as a dive at 8000ft altitude. That way when your body is exposed to the max altitude cabin pressure, you are no more likely to get bent than if you actually conducted that dive to max ndl's using the correct altitude.

But your entire dive would have been 0.25 bar higher pressure than it would have been if done at 8000ft. So your tissue ppN2 will be higher than if the dive actually been conducted at 8000ft.

 

[tbone1004]

@KenGordon, I probably worded that poorly.

Altitude diving at 8000ft says 33ft is basically 43ft. I.e. you are going to dive to 33ft, but use tables for 43ft, 66 at 87ft, etc. If you do plan and execute that dive, and stay within your NDL's, you are exposing your tissues to .25bar less N2 on the dive than you planned for. When you surface, you are basically staying at a "safety stop" depth of ~10ft, for the entire duration of your SIT. Your ascent in the plane is then equivalent to your ascent to the surface. On scuba, we say 30fpm ascent rate and the plane certainly isn't getting to altitude in 20 seconds.

You can stay at 10ft for an infinite amount of time and surface with no problems, and I believe DAN even says that pool dives don't count against your no-fly time. Yes some of your slow tissues may still be ongassing, but not enough to where you are going to supersaturate them by going to 8000ft. If there is a loss of cabin pressure, yeah you may have problems, but how often does that happen? If it does, they immediately head down to at least 8000ft so no one gets bent and whoever didn't get their oxygen mask on in time doesn't die of hypoxia....

again, this is something that I do, have no problem doing, and have many friends that do the same, but I do NOT advocate that anyone actually do it. All I ask is that before you make idiotic comments like some of the ones presented, you actually stop and think about what flying after diving really is and lay out why it could be a problem. The issue on commercial airliners is exposure to at most 8,000ft of altitude or 0.75ata. If you plan your ending decompression or NDL or whatever around immediately surfacing to 8,000ft, you can quite literally hop out of the water, get picked up by a jet, and go straight on up. For most people they are going to have at least 6 hours of SIT before you get on a commercial airliner by the time you get out of the water, get back to the shop, rinse your gear, pack everything, shower, get to the airport, get through security, sit at the terminal, get on the plane, taxi to the runway, and then finally take off. All of that time is just you offgassing to help make it less of a risk.

 

[KenGordon]

I'd need maths to believe this claim.

An altitude dive has a lower ambient pressure at surfacing. So in order to surface the tissue ppN2 must be lower than it would at sea level. This is the same thing as limits an ascent in the water and causes stops. There will be a limiting tissue ppN2 at a given altitude.

If we compare the same dive at sea level and altitude the ppN2 of tissues will be lower after the dive at altitude because the ambient pressure throughout was lower.(Ignoring any acclimatization which would lead the tissue ppN2 to be lower still).

So if the dive at altitude resulted in a tissue ppN2 the same as the limiting ppN2 then the same dive at sea level must result in tissue ppN2 which is higher and so exceeds the limit.

All the above assumes simple dissolved gas models, but if we assume some micro bubbles exist in all dives then suddenly dropping the ambient to 0.75 is likely to encourage them.

In practical terms there will be time between the end of the dive and getting into the air so comparing the dives directly is not like for like. But similarly just because the dive would be ok at altitude does not mean it is safe to immediately ascend to that altitude. The safe thing to do is look at the tissue pressure at the time of ascent and see if that will be safe at the desired altitude. That is what the wrist computers are doing, and if you breath nitrox for your diving they will have recorded a lower tissue ppN2 and so give a shorter no fly time.

To the OP, personally I do not want to be bent in a plane so plan not to dive for at least 24 hours before flying. Being stuck on a long flight is bad enough healthy.

 

[tbone1004]

@KenGordon you are correct, same dive to 50ft for 50 minutes or whatever gives you some amount of O2. If you do that same dive at altitude, you basically have to tell the tables that you are diving at a deeper depth in order to get the ppN2's to be the same for the NDL's if diving sea level tables.

For this scenario, you do the same, tell the tables or computer that you are diving at altitude, when in fact you aren't, so it thinks you have more N2 in your tissues than you actually do. That way when you go to altitude, all will be well in the world

 

[KenGordon]

But when you dive at altitude you have LESS N2. Both at the start of the dive if acclimatised and at the end of the dive due to the lower contribution of surface pressure to the ambient pressure during the dive.

The planning of an altitude dive MUST account for the lower ambient pressure at surfacing and MAY account for the reduced ppN2 before and during the dive. If it is doing the second then it is not appropriate to use it as you describe.

All dives have a ceiling. We introduce stops to avoid going above the ceiling. If the ceiling is between the surface and -2.5m (8000ft of air - i.e. Above the surface of the water) you need to stick at the surface stop until it reaches your next depth, i.e. minus 2.5m of water before flying. That surface stop is what your computer is telling you about with the no fly indicator.

 

[tbone1004]

@KenGordon you're not getting it Ken. You are tricking your computer to tables into thinking that you have more N2 than you really do. When you plan an altitude dive using sea level tables, you tell that computer that you are diving deeper than you are in order to make sure that the depth gauges are reading accurately and that the N2 tracking is accurate.
When you tell a computer or tables that you are diving at altitude when you are really diving at sea level, you are making your dive more conservative because they think you are deeper than you really are. With this, instead of making the dive more conservative when you surface, you are planning on "surfacing" at the max altitude of the cabin. It literally works exactly like what I described, and yes, it really is that simple. I obviously can't officially advocate for it, and if you don't understand how it works then you have no business trying to plan it, but I can promise you, it really is as simple as planning your last dive as if it was 8,000ft altitude *assuming you're at or close enough to sea level* and then getting on a plane when you finish the dive.