Altitude after dives

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Yes, but remember . . . a 30 fpm ascent rate is acceptable in diving. If it takes me two hours to go up that .25 ATA, that's like taking two hours to go up 7.5 fsw.

Airplanes are different; there, you go from sea level to 8000 feet or more in five or ten minutes. Driving and flying have different dynamics.
 
I can agree with Hatul on the calculation of the percentage difference, but it still adds up to no more than 20% at a fairly extreme height of 1700m (5,000 ft), most airplane cabins are pressurised to about 800 mb, so the difference is roughly the same, and I agree with TS&M that the rate of change is faster. I would like to see any calculations for these 'no fly' limits all the dive operators seem to insist on, seems to me it means you either loose the last day of your holiday, or make sure you tell the dive operator you fly out the next day :)
 
Maybe I've missed something, but when you surface from a 30m dive you've been at 4 ATM pressure at 30m then off gas at 1 ATM on the surface at sea level. If you then drive, lets say to a height where atmospheric pressure is 800 milli bar (that's 1,700m), or 20% less than atmospheric pressure at sea level, you're total pressure change is 3.2 ATM as opposed to 3 at sea level, an increase of less than 10 percent, in theory I supose it means that you off gas ten percent faster, and as we know you need to off gas slowly, but the difference is still only 10 percent. Add in the time it takes you to drive to a height of 1,700m and I cant see it making any difference. I'm aware that this is a public forum, so if I'm speaking a load of nonsene, please correct me, with some calculations as to where I've gone wrong.
Altitude.org | Altitude air pressure calculator give some useful figures
You've already done some math, but in case there are readers who don't know where you got 20%:

It's the ratio that matters, not the absolute difference.

In your example:

From 4 atm to 1 atm is a difference of 3 atm, but a ratio of 4:1.

From 4 atm to 0.8 atm is a difference of 3.2 atm, but a ratio of 5:1 – a 25% larger ratio (the other ratio is 20% smaller).

Maybe still not huge, but sufficient to trigger DCS and should be considered, especially if one dived anywhere near limits.

Diving has risks. This is another. Those of us who dive at altitude and travel to even higher altitudes generally take these things into consideration.
 
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I can agree with Hatul on the calculation of the percentage difference, but it still adds up to no more than 20% at a fairly extreme height of 1700m (5,000 ft), most airplane cabins are pressurised to about 800 mb, so the difference is roughly the same, and I agree with TS&M that the rate of change is faster. I would like to see any calculations for these 'no fly' limits all the dive operators seem to insist on, seems to me it means you either loose the last day of your holiday, or make sure you tell the dive operator you fly out the next day :)

Another way to look at this:

We know there is a big difference between a dive to surface at sea level and coming up to 10 ft (3m), because the last deco stop is set to 10 ft. Yet the difference in pressure between 10 ft and sea level is not much.

Adam
 
But again, you are talking about going from 10 fsw to the surface in a very short period of time (scary short for some people). Time is the key element here -- as we know from deep saturation diving, you can offgas essentially ANY nitrogen load, if you take enough time to do it. Going from sea level to 3000 feet over a period of hours is a VERY slow ascent rate. You have lots of time to offgas whatever load you are carrying. And if you are doing recreational level dives and reasonable ascents, you should be fine.

Saying it's dangerous to drive over a pass after diving is like saying it's dangerous to surface from ten feet!
 
But again, you are talking about going from 10 fsw to the surface in a very short period of time (scary short for some people). Time is the key element here -- as we know from deep saturation diving, you can offgas essentially ANY nitrogen load, if you take enough time to do it. Going from sea level to 3000 feet over a period of hours is a VERY slow ascent rate. You have lots of time to offgas whatever load you are carrying. And if you are doing recreational level dives and reasonable ascents, you should be fine.

Saying it's dangerous to drive over a pass after diving is like saying it's dangerous to surface from ten feet!

You make a good point about rate of ascent being important and driving from 1000 to 2500 ft is probably not an issue.

The reason I used the analogy is in response to QatarDiver who wondered how the last small pressure change to make any difference to a deep dive.

Adam
 
You make a good point about rate of ascent being important and driving from 1000 to 2500 ft is probably not an issue.

You might want to be a bit more careful than assuming it's not an issue. The numbers may be close enough that it's worth checking.

Take the NOAA table that seaducer posted. (BTW, to determine group letters, you'll also need the NOAA tables, which don't necessarily have the same values as anyone else's.) If we use the example of diving at 1000 ft, if someone were to do a 60 minute dive to 60 ft, and then drove to 2500 ft, they'd want a 3:09 surface interval. If they hurried to pack and the drive wasn't too bad, they might be close to that limit. OTOH, maybe they decide to take it a bit easier, or the last part of the dive was shallow enough, or they don't need to be quite so conservative as to round all the way up to 3:09. Although it may not be a huge decision, it may still take a little bit of evaluation.
 
Intersting that the NOAA website states;
'The cabin pressure in commercial aircraft is maintained at a constant value regardless of the actual altitude of the flight. Though cabin pressure varies somewhat with aircraft type the nominal value is 8,000 feet. For commercial flights, use a final altitude of 8000 feet to compute the required surface interval before flying.'
So they dont support the 24 no fly rule inposed by so many tour operators, just treat a flight as you would a 8000ft/2500m drive, which (I think!) means if you end your last dive in group C you can take off in 3 hours (practically impossible to do, 3 hours from end of dive to take off?) or 7 hours in group D, 24 hours would be group L! How many recreational divers ever end up in group L!
 
Intersting that the NOAA website states;
'The cabin pressure in commercial aircraft is maintained at a constant value regardless of the actual altitude of the flight. Though cabin pressure varies somewhat with aircraft type the nominal value is 8,000 feet. For commercial flights, use a final altitude of 8000 feet to compute the required surface interval before flying.'
So they dont support the 24 no fly rule inposed by so many tour operators, just treat a flight as you would a 8000ft/2500m drive, which (I think!) means if you end your last dive in group C you can take off in 3 hours (practically impossible to do, 3 hours from end of dive to take off?) or 7 hours in group D, 24 hours would be group L! How many recreational divers ever end up in group L!

I almost posted this last night, but since this 8000' thing keeps popping up, I offer this nugget for thought and discussion... Although I know very little about the NOAA system, it seems they are assuming that approximately 8000' is the maximum pressure altitude you are going to be exposed to on a commercial flight in order to calculate their flying after diving restrictions.

Before you dismiss the 24 hour no-fly rule in favor of the NOAA standards or treat the flight as an 8000' drive as discussed above, consider what might happen to you if that jet depressurizes at a significantly higher altitude... I understand that the jet will immediately descend if able, and that oxygen masks will magically appear from nowhere, but that is not where the story ends...

There could be weather under you that prevents a descent. The jet might only descend to 10,000' or 15,000' and stay there for quite some time based on terrain, weather, airspace restrictions, and dozens of other things out of the scope of this thread. Most people can survive pretty well at those altitudes with only mild symptoms of hypoxia. I go offroading almost every weekend here in Colorado above 10,000'. I do not, however, go up there with residual nitrogen in my system from a recent dive.

The mere difference between 8,000' and 10,000' of pressure altitude (2000') already has people talking on this thread... If guys are worried about getting bent from ascending 2,000' after a dive, I would hate to think what a depressurization from 8,000' to 30,000', and then back down to 10,000 or so would look like on the charts for a person still offgassing from a dive. If any of you have information on this extremely unlikely situation, I would be genuinely interested to read it.

Feel free to take this information at the value of what you paid for it, but at least consider it before you hop on an airplane prior to waiting the interval recommended by your certifying agency...
 
Before you dismiss the 24 hour no-fly rule in favor of the NOAA standards or treat the flight as an 8000' drive as discussed above, consider what might happen to you if that jet depressurizes at a significantly higher altitude... I understand that the jet will immediately descend if able, and that oxygen masks will magically appear from nowhere, but that is not where the story ends...

There could be weather under you that prevents a descent. The jet might only descend to 10,000' or 15,000' and stay there for quite some time based on terrain, weather, airspace restrictions, and dozens of other things out of the scope of this thread. Most people can survive pretty well at those altitudes with only mild symptoms of hypoxia. I go offroading almost every weekend here in Colorado above 10,000'. I do not, however, go up there with residual nitrogen in my system from a recent dive.

That's a very interesting though. Thanks. It certainly helps show how there are a whole lot of issues and scenarios that are worth thinking about.

FWIW, I asked a related question a few years back, and Gene Hobbs was good enough to point to a paper in the Rubicon archives (sorry, I didn't keep the reference). One of the authors might even have been our own Dr Deco. It's only one data point, but NASA flight medics were studying astronauts, who regularly dive in the simulators in Houston and then fly themselves back to Florida the same day. Since the military-heritage jets they fly have pressurization failures much more often than commercial planes, they wanted to see whether this was safe enough. IIRC, with prompt descents to low altitudes (spec is 2 minutes?), they didn't find any significant DCS issues in the hundred or so cases they studied.

However, there aren't any too many 10,000' mountains around the Gulf of Mexico. And maybe fighter jet pilots have more leeway and flexibility to stay away from weather than commercial flights on fixed routes and schedules.
 

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