Why the wait to fly?

[inquis]

Accelerated deco dives (even with 50% O2) quite often end with GFs quite lower than the typical multiday NDL dives.
Doesn't that matter in time before flight considerations?

It certainly does, because the controlling tissue for a deco dive has a slower off-gassing rate than that for an NDL dive. A 60 ft NDL (44 min bottom time) might exit at GF 85% and fall to 5% after an hour. An 88 min bottom time that exits at 85% would only fall to 28%.

Forgot to say that multi-day NDL does load the slower tissues more, and in that respect is similar to a deco dive. It's possible that multi-day NDL diving could be riskier than a single deco dive, depending on the times involved. (ETA: my feeling is you'd have to hit it pretty hard to do so, 4+ dives per day.)

 

[tursiops]

Thanks to everyone whose replied. I thought this report really interesting, particularly the below. 3 out of 36 people with a 3 hour surface interval ended up bent. yikes....
View attachment 796825

Scary, and intriguing, but somewhat difficult to interpret with certainty. What if those same 3 people had NOT flown, and had just sat on a beach. Would the DCS symptoms still have appeared? DCS might emerge anytime within 24h after surfacing, which is the most compelling reason to wait 24h before flying. I'd rather get DCS at sea level -- near medical support and maybe a chamber -- than trapped in an airplane.

 

[inquis]

For those who like numbers, the dive in the Duke study surfaced with a GF99 of 97%, so definitely on the aggressive side by today's standards (to @tursiops' point). After 3 hours, the GF99 was at or below 0 at sea level, meaning all tissues were below ambient pressure at the "dive" site.

However, at the ambient pressure of 8000 ft, the GF99 was 142% (0 hr), 38% (3 hr), 26% (6 hr), 20% (9 hr), 18% (10 hr), 17% (11 hr), 16% (12 hr). For reference, the Navy table ascent time is 14.25 hrs (GF99 of 14%).

 

[OMyMyOHellYes]

Most commercial aircraft (pressurized), when at cruising altitude have cabin pressures equivalent to standing on a mountainside at 6,000 to 8,000 ft above sea level.

Add in the potential complication of loss of pressurization taking the cabin altitude to FL 350 and I assume problems with DCS would be enhanced.

I guess the thought is 8.0 pounds of prevention beats a whole lot of DCS exposure.

Medevac back to the US means aircraft burning crap tons more fuel because they can't climb to an efficient cruise altitude. They have to keep the cabin at pretty close to -0- ft above sea level.

 

[John C. Ratliff]

Not all commercial flights are in fully pressurized to 8,000ft cabins. The turboprops flying to smaller regional airports in the Rockies are pressurized to about 12,000ft. The difference is marginally noticable since nobody is exercising.

Those turboprop pilots are probably on 100% oxygen with masks on. Oxygen is required in the USAF for altitudes over 10,000 feet unpressurized.

SeaRat

 

[John C. Ratliff]

This is correct.

DAN Europe has called for 24 hours for a long time, and few years ago they confirmed their belief with an absolutely maddening study in which they tested divers for bubbles immediately after a dive and then 24 hours later. The divers tested 24 hours later were bubble-free, so they said that this proved that 24 hours was indeed sufficient. Yes, 24 hours was shown to be sufficient, but it does not mean it was necessary. Why they didn't test them sooner than 24 hours is a mystery.

There have been very few cases of DCS after flying, and it was a couple cases that occurred before there was any such policy that spurred things along. The problem with using any cases of DCS after flying to prove a point is that people get DCS after diving without flying, too. In many cases we don't know why they got DCS. How do we know that flying was a factor if people flew after their dives?

In one of the many threads on this topic years ago, a well-regarded participant named Lynne Flaherty (TS&M) said she had a suspicion that people who get bent after flying were already bent when they got on the plane. Of course, there is no way to prove this. (Emphasis added, jcr)

BoulderJohn,

That could be because of micro-bubbles that can be picked up by Doppler, but are asymptomatic otherwise. If those are circulating, and the diver gets into an airplane, they may become symptomatic with the decreasing pressure at altitude, especially in an unpressurized airplane or helicopter (most of which are unpressurized).

SeaRat

 

[DiveProKoko]

I always find it interesting how low my "no fly" time is on my computer at the end of a dive trip. We just did 6 days of diving and got 16 dives in on Nitrox. My no fly time after my last dive?......8 hours. LOL.

 

[tursiops]

I always find it interesting how low my "no fly" time is on my computer at the end of a dive trip. We just did 6 days of diving and got 16 dives in on Nitrox. My no fly time after my last dive?......8 hours. LOL.

Which computer?

 

[wetb4igetinthewater]

If the plane were to lose cabin pressure at altitude, there is potential to get bent.

I think that's a risk at cruising altitude. When I went for my interview for a NEXUS pass, the Canadian official was an ex-air force pilot who had a mechanical failure (I didn't ask what altitude) who ejected and got bent.

 

[DiveProKoko]

Which computer?

EON Core set to "aggressive". I always adhere to at least an 18 hour rule, but depending on the surface intervals between dives, etc. and how the dives are spaced out, it can sometimes be on the very low end for a no fly window.