going up mountains after diving

The dive resort I work for offers trips to a couple of destinations located up in the mountains with elevations anywhere from 300-500 meters. We won't allow guests to do those trips if he/she has already gone diving on the same day (e.g. dive in the morning, go up to the mountains in the afternoon), but we sometime have guests who insist that the elevation change isn't big enough to be worried of.

Most of our guests would be in the S to U pressure groups (PADI air table) immediately after 2 dives. If they were to go on a trip, they would be in at least 300m ASL after 2.5 hours.

I personally think that our present policy makes sense, 300m ASL is enough to make my ears pop, or are we just being worrywarts?

Well, you’re certainly being conservative. DAN's flying after diving recommendations apply to altitudes higher than 2000 feet (~600m) (see for example: Alert Diver | Flying After Diving )
Your divers are going to altitudes lower than that, which DAN doesn’t even address - at least as far as I am aware.

You might want to recalculate the divers&#8217; profiles using NOAA&#8217;s air tables: http://www.ndc.noaa.gov/pdfs/NoDecoAirTable.pdf < NOTE: the pressure group letters do not correlate with PADI&#8217;s. >
And then apply the recommendations from their Ascent to Altitude table: http://www.ndc.noaa.gov/pdfs/AscentToAltitudeTable.pdf

You could well find that many of your divers should be able to safely ascend to such elevations with little or no interval.

Also consider that these are based on square profiles. If the divers actually spend a modest time in the latter part of their dive at less than maximum depth their N2 loading is lower.

We had a local instructor about 10 years ago do a fairly conservative set of training dives with her students, then she drove to her boyfriend's house over a maximum elevation of 1,600 ft and got bent. There may have been other factors involved. However, I personally play it conservatively.

I think the issue isn't so much the altitude change but rather the ascent rate. I don't imagine that the elevator is all that slow.

nimoh:

I think the issue isn't so much the altitude change but rather the ascent rate. I don't imagine that the elevator is all that slow.

Click to expand...

As a diver you may remember off the top of your head that water is 800 times denser than air. That would make an aircraft ascent rate of 2400' per minute roughly comparable to a diving ascent rate of 3' per minute. Short of a very rapid ascent to altitude, I expect that the question is just at what point you're supersaturated enough that you bend.

drbill:

We had a local instructor about 10 years ago do a fairly conservative set of training dives with her students, then she drove to her boyfriend's house over a maximum elevation of 1,600 ft and got bent. There may have been other factors involved.

Click to expand...

Maybe a bunch of repetitive ascents? The decrease in pressure at 1600' is pretty small - perhaps 6 to 7%. That's comparable to an ascent from 99' to 91' or 50' to 44' and taking 3 minutes or more to do it (3 miles at 60 mph on a road with a grade of 10%).

Other than having a deco obligation and having just arrived at your deco ceiling, when would you be concerned about such an ascent? Granted, deco calculations with no safety margin could theoretically have you returning to the surface still at the very limit for surface pressure, so that any further immediate reduction is too much, but is that realistic? I'm not a deco diver and I realize that calculations aren't linear, but how long a "stop" would be required at sea level before ascending the equivalent of another 5 to 10' of sea water?

marku:

are we just being worrywarts?

Click to expand...

As a business, what you permit customers to do and what you'd do yourself are different things. Based on what I know (which may be incorrect or insufficient), I wouldn't hesitate to go to 500 meters or so on an afternoon following a morning of diving. I wouldn't run from a shore dive and immediately speed up a steep road, but I'd certainly do it at a reasonable pace. Get out of the water, deal with gear, get changed, maybe have lunch, and then a reasonable drive? If it takes 40 minutes to an hour that seems like plenty of time for offgassing.

FWIW, I've always been under the impression that the time to fly interval is conservative and based on the possibility of an unusual decrease in cabin pressure.

Thank you everybody! A lot of food for thought here. I guess the safest thing to do would be to stay conservative seeing that things CAN go wrong (I hope your friend's back to diving drbill). Maybe we'll do things in a case-to-case basis.

Living and teaching on the Big Island of Hawai'i we constantly have to go back over elevation after a dive. For a lot of places on Kona side, we have to climb back over 3000ft in order to get home to Hilo.

When I teach PADI courses through the University, our protocol is we cannot go to elevation until we are "B" divers on the RDP. Typically this is about a 1:45 surface interval following our 2nd dive (both dives are to about 35-40ft).

When I am conducting scientific dives under the University of Hawaii Dive Safety Program, we follow the NAUI tables. The protocol for the DSP manual is we cannot return to elevation until we are "D" divers. Under DSP protocol, if we are diving using dive computers (doesn't matter what algorithm), the minimum surface interval is 4 hours before returning to elevation.

There are some drives like Saddle Road which extend to over 7,000ft. For our protocol, anything exceeding 6,000ft is to be treated as "flying" and we have to wait 24 hours before ascending, even if it is a single dive. That's just what our diving safety program does. And to my knowledge we have not had a DCI incident following these guidelines *knocks on nearest wooden object*. Hope that helps!