EAN versus "no-Fly" delay

[Charles2]

I obviously can't officially advocate for 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.

I was with you until you threw in the *assuming you're at or close enough to sea level* part. Do you mean that if diving in Lake Tahoe (6,000 ft.) and planning it as an 8,000 ft. dive would give a different outcome in terms of "time to fly"?

By the way, thanks for the comments. It's a different way to look at the problem.

 

[tbone1004]

I was with you until you threw in the *assuming you're at or close enough to sea level* part. Do you mean that if diving in Lake Tahoe (6,000 ft.) and planning it as an 8,000 ft. dive would give a different outcome in terms of "time to fly"?

By the way, thanks for the comments. It's a different way to look at the problem.

you have to compensate for the delta, if you have already compensated for the altitude, then I think you'd be fine. PADI says to start altitude diving at 1,000ft or 0.96ata, the delta between 6,000 and 8,000ft is 0.8ata down to 0.74 ata, so I would probably not change, but you can always make it more conservative by planning at 8,000ft.

There are a lot of considerations when doing this, and you are putting your hands in the faith of the decompression algorithms and your computers on whether you trust it. I.e. on my Petrel, I don't think I can manually override the altitude for anything except for fixed sea level. In this case, you have to do a bunch of math and figure out which gradient factors are going to correlate. I usually dive to a GF-High of .8, and to get an equivalent GF when at a pressure of 0.74, it has to be lowered to a GFhigh of 60 *rough math, haven't sat down and calculated, use it as an example only*. That's going to be a LOT of deco or a very short dive. In that case I'd try to plan for a morning flight, have at least 12 hours of SIT, and not worry about it.

 

[Charles2]

you have to compensate for the delta, if you have already compensated for the altitude, then I think you'd be fine. PADI says to start altitude diving at 1,000ft or 0.96ata, the delta between 6,000 and 8,000ft is 0.8ata down to 0.74 ata, so I would probably not change, but you can always make it more conservative by planning at 8,000ft.

There are a lot of considerations when doing this, and you are putting your hands in the faith of the decompression algorithms and your computers on whether you trust it. I.e. on my Petrel, I don't think I can manually override the altitude for anything except for fixed sea level. In this case, you have to do a bunch of math and figure out which gradient factors are going to correlate. I usually dive to a GF-High of .8, and to get an equivalent GF when at a pressure of 0.74, it has to be lowered to a GFhigh of 60 *rough math, haven't sat down and calculated, use it as an example only*. That's going to be a LOT of deco or a very short dive. In that case I'd try to plan for a morning flight, have at least 12 hours of SIT, and not worry about it.

Thanks for the explanation - it makes sense. I'll probably do a little of the math just for some grins and confirmations, but will probably see this in a slightly different light from now on. Thanks

 

[BRT]

you have to compensate for the delta, if you have already compensated for the altitude, then I think you'd be fine. PADI says to start altitude diving at 1,000ft or 0.96ata, the delta between 6,000 and 8,000ft is 0.8ata down to 0.74 ata, so I would probably not change, but you can always make it more conservative by planning at 8,000ft.

There are a lot of considerations when doing this, and you are putting your hands in the faith of the decompression algorithms and your computers on whether you trust it. I.e. on my Petrel, I don't think I can manually override the altitude for anything except for fixed sea level. In this case, you have to do a bunch of math and figure out which gradient factors are going to correlate. I usually dive to a GF-High of .8, and to get an equivalent GF when at a pressure of 0.74, it has to be lowered to a GFhigh of 60 *rough math, haven't sat down and calculated, use it as an example only*. That's going to be a LOT of deco or a very short dive. In that case I'd try to plan for a morning flight, have at least 12 hours of SIT, and not worry about it.

So you still think staying underwater doing deco that would be required at 8000 feet but is not required at sea level is going to leave less nitrogen in your body than just getting out of the water?

Suppose you were diving at 8000 feet using tables for 8000 feet vs diving at sea level using tables for 8000 feet. Which one will leave less nitrogen in the body?

 

[tbone1004]

So you still think staying underwater doing deco that would be required at 8000 feet but is not required at sea level is going to leave less nitrogen in your body than just getting out of the water?

Suppose you were diving at 8000 feet using tables for 8000 feet vs diving at sea level using tables for 8000 feet. Which one will leave less nitrogen in the body?

if doing accelerated deco, yes. You will get rid of more nitrogen using O2 at 15ft than air at the surface. If you are doing NDL dives, you get out of the water sooner, ergo less nitrogen loading.

 

[BRT]

if doing accelerated deco, yes. You will get rid of more nitrogen using O2 at 15ft than air at the surface. If you are doing NDL dives, you get out of the water sooner, ergo less nitrogen loading.

So if not using a high percentage of O2..no. How about the 2nd question

 

[tbone1004]

I haven't looked at the altitude tables to check, but altitude tables at sea level should leave less N2 in the system.
If using normal sea level air tables when diving at altitude, you calculate the equivalent sea level depth in the same way that you calculate EAD for nitrox. With nitrox, you get a shallower equivalent air depth than your gauge is reading, and with altitude you get a deeper depth.

 

[KenGordon]

But you are not planning an altitude dive. You are planning a sea level dive. No amount of messing will change how much n2 you enter and leave the water with and it is not the same as at altitude, it is more.

When you move to altitude you may have more nitrogen than you can cope with because the exposure will have been greater than the plan would have given at altitude.

Also please keep in mind that only some tables work by taking air sea level tables and mangling them for altitude or nitrox. Planners too.

 

[tbone1004]

But you are not planning an altitude dive. You are planning a sea level dive. No amount of messing will change how much n2 you enter and leave the water with and it is not the same as at altitude, it is more.

When you move to altitude you may have more nitrogen than you can cope with because the exposure will have been greater than the plan would have given at altitude.

Also please keep in mind that only some tables work by taking air sea level tables and mangling them for altitude or nitrox. Planners too.

what? if you use air tables, and adjust the depth to account for 8,000ft of altitude, then you have a shorter dive time because your nitrogen loading is set to account for a surface at 8,000ft. The exposure is the same as it would have been as if you were at altitude in terms of actual water depth and dive time. The difference is because the exposure is much shorter than it would have been if you hadn't accounted for altitude.

example using naui tables.
I'm flying later this afternoon and want to plan one last dive where I have to go to 60ft. I'm not deco certified and I haven't dove since last night so I am an A diver.
Normally I would have an NDL of 50 minutes based on 5 minutes of residual nitrogen time and a maximum dive time of 55 minutes per my tables.
Since I'm planning on flying, I want to have adjusted for that altitude. No different than using nitrox on air tables and having to calculate an equivalent air depth to get the new NDL times, I have to adjust for an EOD *equivalent ocean depth*. My EOD is now 79ft because of the altitude adjustment. Instead of having an NDL of 50 minutes as an A diver to 60ft, my EOD is 79ft and my new adjusted maximum dive time is 31 minutes *35 minutes MDT - 4 minutes of residual nitrogen*.

You have 31 minutes of dive time instead of 50 minutes, so are coming out with much less nitrogen in your body. Assuming you got right out of the water and then got on a plane immediately, when the cabin hit maximum altitude, you would theoretically have the same amount of nitrogen in your system as if you did that 60ft dive for 50 minutes and stayed at sea level. Same concept applies to decompression diving

 

[BRT]

what? if you use air tables, and adjust the depth to account for 8,000ft of altitude, then you have a shorter dive time because your nitrogen loading is set to account for a surface at 8,000ft. The exposure is the same as it would have been as if you were at altitude in terms of actual water depth and dive time. The difference is because the exposure is much shorter than it would have been if you hadn't accounted for altitude.

example using naui tables.
I'm flying later this afternoon and want to plan one last dive where I have to go to 60ft. I'm not deco certified and I haven't dove since last night so I am an A diver.
Normally I would have an NDL of 50 minutes based on 5 minutes of residual nitrogen time and a maximum dive time of 55 minutes per my tables.
Since I'm planning on flying, I want to have adjusted for that altitude. No different than using nitrox on air tables and having to calculate an equivalent air depth to get the new NDL times, I have to adjust for an EOD *equivalent ocean depth*. My EOD is now 79ft because of the altitude adjustment. Instead of having an NDL of 50 minutes as an A diver to 60ft, my EOD is 79ft and my new adjusted maximum dive time is 31 minutes *35 minutes MDT - 4 minutes of residual nitrogen*.

You have 31 minutes of dive time instead of 50 minutes, so are coming out with much less nitrogen in your body. Assuming you got right out of the water and then got on a plane immediately, when the cabin hit maximum altitude, you would theoretically have the same amount of nitrogen in your system as if you did that 60ft dive for 50 minutes and stayed at sea level. Same concept applies to decompression diving

Same amount of nitrogen in your system after a 31 minute dive to 60 feet as you would have doing a 50 minute dive to 60 feet? Better rewrite that.

You are forgetting that you have more nitrogen in your system at sea level than you do at 8000 feet. Period.