Question Diving before flight

[Dan G]

It would be difficult and/or expensive to get a hold of 40% EANx. In the upper Keys, 30% seems to be what gets banked and it's rare that anyone uses Nx on the shallow reefs out of Key Largo, for example. On some of the dives, you would need a shovel to get deeper than 30'.

To repeat what was stated earlier, if the 18 hr recommendation did not apply to Colorado, then there would be many many divers getting bent upon return. Secondly, DAN's recommendation is not based on the FO2 we use, but whether or not we dove within NDL and if we did multiple dives or not. As a new diver, I do not think it is a good idea for the OP to get the idea that using Nx is a good way to reduce their Time to Fly countdown.

 

[Lorenzoid]

I've been pondering this some more. DAN's recommendations are not (at least not explicitly) based on the length of the flight. A diver might have short flight, but another diver could conceivably have an 18-hour flight. Could DAN's recommendations already factor in the possibility the diver will be spending a "long" time at cabin pressure (typically 8000 ft)? If DCS doesn't manifest within something like 24 hours, it probably isn't going to. In other words, maybe DAN's recommendations do not presume a return to sea level after the flight but rather factor in, in effect, remaining at altitude a very long time--longer than the time it would take for DCS to manifest?

Has anyone asked DAN yet?

 

[goldfishtornado]

He had done some light deco, but did wait 24h; they said, not long enough

This is almost entirely irrelevant to the discussion of a single recreational dive within limits. From Flying After Diving "There is little experimental or published evidence on which to base a recommendation for decompression dives. A preflight surface interval substantially longer than 18 hours appears prudent." Compare to the section discussing what the recommendation of 12 and 18 hours are based on: "For single no-stop dives to 60 fsw (18 msw) or deeper, there were no cases of DCS with surface intervals of 11 hours or longer."

You are supposed to wait because you are flying an airplane pressurized to 7,000-8,000 feet. That is where the concern is. Descending to Denver's altitude of 5,280 should not be a problem after that.

This is exactly what is assumed in all the models used to make the recommendation. The navy dive manual explicitly discusses the cases of altitude diving and driving over a mountain pass. The maximum risk is assumed to be from the maximum increase in altitude. On 32% table 10-1 gives an EAD for 60 ft of 50 ft. If you then use table 9-7 for air at 50 ft the NDL is 92 minutes which puts you in navy group M.
Going to table 9-6 (assuming note 3 holds) the required surface interval is 18 hours, which is the DAN recommendation for moderate exposures. Not really a coincidence. It's because all the decompression models are based on the same trial data and simmilar assumptions about the physiology. In practice no dive op is going to give you 92 minutes, if you assume you spend less than an hour then the maximum group you could end up in is K. Then the time to fly is 15:35.

Also note the manual address the case of decompression dives "Exceptional Exposure - Wait 48 hours before ascent"

As a new diver, I do not think it is a good idea for the OP to get the idea that using Nx is a good way to reduce their Time to Fly countdown.

Maybe not to reduce their time to fly, but if you use the same time at depth and surface interval using Nitrox does reduce risk. From Guidelines for Flying After Diving, "Diving nitrox and pre-breathing oxygen reduces risk of DCS in flying after diving."

DAN's recommendations are not (at least not explicitly) based on the length of the flight.

They have studied this. The original 90's trials used a 4 hour flight. The more recent reports concluded "For example, it has always been believed that a flight of long duration poses greater risk in respect to a flight of medium duration; instead it is exactly the opposite. This is most likely due to the altitude of the aircraft pressurization, approximately 1500-1800 metres above sea level for travel to the Maldives, and 2400 metres above sea level (maximum allowed) for trips to closer destinations."

Has anyone asked DAN yet?

I would be interested in their response, but they don't have a magic-8 ball to see if you individually are prone to bubbling. With a 48 hr surface interval there is virtually 0 risk. with a 12 hour interval there is a small, but hard to quantify non-zero risk. Between that it's down to your own risk tolerance and physiology, with the risk falling off exponentially with interval.

 

[tursiops]

This is almost entirely irrelevant to the discussion of a single recreational dive within limits. From Flying After Diving "There is little experimental or published evidence on which to base a recommendation for decompression dives. A preflight surface interval substantially longer than 18 hours appears prudent." Compare to the section discussing what the recommendation of 12 and 18 hours are based on: "For single no-stop dives to 60 fsw (18 msw) or deeper, there were no cases of DCS with surface intervals of 11 hours or longer."


This is exactly what is assumed in all the models used to make the recommendation. The navy dive manual explicitly discusses the cases of altitude diving and driving over a mountain pass. The maximum risk is assumed to be from the maximum increase in altitude. On 32% table 10-1 gives an EAD for 60 ft of 50 ft. If you then use table 9-7 for air at 50 ft the NDL is 92 minutes which puts you in navy group M.
Going to table 9-6 (assuming note 3 holds) the required surface interval is 18 hours, which is the DAN recommendation for moderate exposures. Not really a coincidence. It's because all the decompression models are based on the same trial data and simmilar assumptions about the physiology. In practice no dive op is going to give you 92 minutes, if you assume you spend less than an hour then the maximum group you could end up in is K. Then the time to fly is 15:35.

Also note the manual address the case of decompression dives "Exceptional Exposure - Wait 48 hours before ascent"


Maybe not to reduce their time to fly, but if you use the same time at depth and surface interval using Nitrox does reduce risk. From Guidelines for Flying After Diving, "Diving nitrox and pre-breathing oxygen reduces risk of DCS in flying after diving."


They have studied this. The original 90's trials used a 4 hour flight. The more recent reports concluded "For example, it has always been believed that a flight of long duration poses greater risk in respect to a flight of medium duration; instead it is exactly the opposite. This is most likely due to the altitude of the aircraft pressurization, approximately 1500-1800 metres above sea level for travel to the Maldives, and 2400 metres above sea level (maximum allowed) for trips to closer destinations."


I would be interested in their response, but they don't have a magic-8 ball to see if you individually are prone to bubbling. With a 48 hr surface interval there is virtually 0 risk. with a 12 hour interval there is a small, but hard to quantify non-zero risk. Between that it's down to your own risk tolerance and physiology, with the risk falling off exponentially with interval.

I agree with everything you've said, especially the last paragraph. Flying to Denver (which has NOT been studied) is more risk than flying to sea level (which HAS been studied). Enough more risk to worry about? My personal decision is to wait 24h....but then I prefer that to 18h in all cases anyway.

 

[Lorenzoid]

They have studied this. The original 90's trials used a 4 hour flight. The more recent reports concluded "For example, it has always been believed that a flight of long duration poses greater risk in respect to a flight of medium duration; instead it is exactly the opposite. This is most likely due to the altitude of the aircraft pressurization, approximately 1500-1800 metres above sea level for travel to the Maldives, and 2400 metres above sea level (maximum allowed) for trips to closer destinations."

That's very interesting that DAN has studied this, but apparently DAN chose not to factor it into their consensus recommendations, no doubt to keep their recommendations as simple as possible for traveling divers to follow.

DAN knows that divers might fly for any number of hours--2 hours, 18 hours, or whatever--and knows what typical cabin pressures are. So, if the OP in this thread is going to be at a typical cabin pressure (8000 ft for flights across the US?) for several hours, then it would seem to me that arriving in Denver (5000+ ft) after that is irrelevant--as I see it, it's no different than continuing on a longer flight.

I'm not disagreeing with you that the risk in following DAN's longstanding guidelines is non-zero and hard to quantify, but I have always interpreted DAN's advice as being conservative, risk-averse. They're targeting recreational divers, a broad group. I'm focusing on the Denver issue, and I suspect it is not an additional risk factor at all; I think it's a red herring in this question. But again, I'd love to know what DAN says about that.

I respect tursiops' reasoning, but I'm not persuaded Denver is relevant.

 

[MarkA]

Wikipedia has a list of the highest airports.

List of highest airports - Wikipedia

en.wikipedia.org

They do not trike me as the destination airports for a lot of divers....

One caught my eye having caught a flight from a known diving destination to there. Galápagos to Quito, with the city being above 9,000 ft. What Would the recommendations be for this?

P.S. It was a non diving holiday.

 

[goldfishtornado]

To me, the issue is that any answer someone give is at best an extrapolation. Based on reputation, we expect DAN will give an answer solidly based on empirical observation and incorporating known dive physiology. The consensus recommendations balance the obvious fact that people often need to fly after diving at a destination with the caveat that "The recommended preflight surface intervals do not guarantee avoidance of DCS."

Not real numbers, I couldn't find the original paper from a cursory search. But say 0/400 divers had observed DCS after a single no-stop dive and 11 hour surface interval before flying. Then the estimated prevalence at 95% confidence is [0, 0.0095]. So the modal estimate is zero, but the upper bound is 'only' about 1/100. Completely made up, but say landing in Denver after 4 hours raises your risk by 1/1000. The change in risk is completely swamped by the existing uncertainty the recommendation is based on.

After a single one hour dive you are unlikely to have saturated any slower tissues. Even if you assume all but the slowest 3 compartments are saturated, some models would say you are "safe" to fly after only five hours. Other models would never clear you to fly.

If OP chooses to fly at exactly 18 hours the risk of that action is probably no greater than any of the other many risks you take while diving. This can be true while simultaneously increasing the cumulative risk of the trip. Then again the cumulative risk of life resulting is death is currently 1.

OP, if you are looking for a concrete answer for your trip: If you dive 30%, limit your depth to 60 ft, bottom time to 48 minutes or less, and take a nice long safety stop. If you were to do this, even if you were then were to be teleported to 8,000 ft the risk from that single dive will not substantially exceed any other taken to a GF high of 75%. Of course even people diving within those limits sometimes take "undeserved" hits, but you are well within the risk tolerance required for diving.

 

[goldfishtornado]

Galápagos to Quito, with the city being above 9,000 ft. What Would the recommendations be for this?

The most exact answer is no one can say for sure because this exact thing has never been studied, and most assuredly has never been studied on you under the conditions you will be experiencing on that day.

A reasonable estimate is to consult table 9-6 in the Navy Dive Manual, which is for maximum increase in altitude expected. That gives you 21:38 from their "Group M." Or maybe 26:14 if it's more than 9,000 ft.

A good rule of thumb is the one that DAN gives, which is that 24 hours is fine for most recreational divers under most circumstances.

If you want to be completely safe you could wait 48 hours, but this is probably inconsistent with the other risk you take in life.

 

[Lorenzoid]

A good rule of thumb is the one that DAN gives, which is that 24 hours is fine for most recreational divers under most circumstances.

Except that isn't the one that DAN currently gives, despite the more recent research you noted. Unless there is inconsistent information on their website, DAN has had this posted for years:

Consensus recommendations for flying after diving​

• A minimum of 12-hour surface interval was recommended for the single no-decompression dive.
• A minimum of 18-hour surface interval for multi-day repetitive diving.
• Substantially longer than 18 hours after diving involving compulsory decompression, or using heliox and trimix.

Guidelines for Flying After Diving

The recommendations for flying after diving come from a DAN study — read all about the study workshop and conclusions.

dan.org

Many of us do use a 24-hour rule of thumb for flying home after our dive vacations, but as far as I can determine, DAN did not say that is their rule of thumb.

 

[goldfishtornado]

Many of us do use a 24-hour rule of thumb for flying home after our dive vacations, but as far as I can determine, DAN did not say that is their rule of thumb.

I think I confused the section where they talk about "Subsequently, DAN proposed a simpler 24-hour wait after any and all recreational diving." with the final "official" recommendations, though you might consider the simpler rule as the rule of thumb.

In the proposed example an accent to 9,000 ft might be considered somewhat advanced altitude diving, so it probably is too casual to just throw a rule of thumb out there without further qualification.