The way flights used to be scheduled from here, we'd often end up with customers choosing to do five or six days of two 100-ft-plus dives a day, then fly (in a Twin Otter, unpressurised, at 10,000 feet) after an 18-hour interval. Nobody ever had a problem. On the other hand, we're not complaining now that the schedules have changed and the morning flight out is four hours later...
Keeping the deco theory stuff simple, vacation divers who are doing 2 Rec dives/day ... and probabably no-deco too ... are probably going to be close to the generic PADI simplified table in which the controlling compartment is the 1-hour group. As such, an 18hr SI is 18 "half life" times ... the KISS model would say that their residual Nitrogen has been (1 - (0.5)^18) = 99.99962% cleared.
I'd tend to agree with wedivebc. If you get symptoms on the plane, you were probably very, very close to bent before you took off. I'd be more concerned about the six hours a day in the water for three days than I would be about the flight, even if it's only at 30 feet.
Thinking the same thing here...changing to long & shallow dives will transcend the effective controlling group to a longer one; 2 hrs is what the old NAVY tables used; the amount that a 2hr compartment would clear in 18 hours would be 9 half lifes = 99.8% cleared.
Some of the early (1980s) dive computers had even longer compartments; IIRC, the original Orca EDGE had a 480 minute (8hr) group @ ~8ft depth, which would be 2.25 half lifes = only 78.98% cleared.
And the risk assessment isn't done yet, since what's not presently identified is what amount of clearing is required when one leaves sea level...
Flying only becomes a problem when the cabin suddenly decompresses..
Not quite: aircraft cabins aren't generally pressurized to Sea Level, so any flight functionally becomes the equivalent of an "Altitude Dive" (eg, 8,000ft cabin pressure).
I remember a thread on how a "safe" diver got bent on a flight out, when he was NOT expecting to, had respected ALL the guidelines and his computer said it was safe.
And yes, there's also the variable & risk from "Undeserved Hits" too.
I appreciate all of the responses. I haven't decided what I'll do yet, but I definitely have more info to help make a decision. These are all super-easy fossil hunting dives off of Venice Beach, FL. I descend to 30ft, stay for 45 minutes, surface to see where I am relative to boat, and remain on bottom for another 45 minutes. I do this trip several times each summer but usually I have 28hrs post dive. I understand that more is better, but there doesn't seem to be a way to determine the relative risk of 18 hours vs. 28 hours surface interval.
What's probably a reasonably safe rule of thumb IMO is "6 half life" time intervals for pragmatically saying that a diver is fully clear. However, the trick to the question is what is the controlling compartment for which the half life applies. For mundane recreational no-deco at 2-3 dives/day, the 1hr or 2hr compartments are most likely controlling the driver - - the modern PADI dive tables (no deco anywhere) use 1hr and the old (Navy) tables (some deco OK) used 2hrs, and based on six, this rule of thumb infers that the risk should be acceptably low after 6-12 hours. Of course, because people always push limits (and the models are known to have gaps), one can expect official guidelines to choose the 12 hours at the top of this range.
But in this case, the dive profiles are very very long (despite being shallow) and as such, I'd really be concerned over long compartments which are parts of the Deco theory that we recreationally don't generally venture into...and which aren't even present in some dive computers. I already mentioned the old Orca; IIRC, Suunto has/uses a 240 minute compartment ... and in looking at this 4hr compartment, to bump up from 19 to 28 hours puts that notional compartment from 4.5 to 7 half lifes of clearing, which would suggest that 24+hrs is what would follow this "use 6" rule of thumb.
To pull my ramblings to a quick conclusion, I'd say it looks like your risk at 18hrs is that you're probably not fully clearing your 4hr and longer compartments, even if your choice of dive computer doesn't have or track them in its software.
Using nitrox on the last day of diving would help for the No-Fly, but as was pointed out, that's just trading off one risk for another.
Hope this helps,
-hh
