Diving at Altitude

[ReikiMaster]

Just spotted a thread, somewhere else, about flying after diving and just wondered what the GUE stance is on diving at altitude. and what the result is on the deco obligation.

[amascuba]

Quote: (Originally Posted by ReikiMaster)

Just spotted a thread, somewhere else, about flying after diving and just wondered what the GUE stance is on diving at altitude. and what the result is on the deco obligation.

It's my understanding that GUE does not compensate for altitude because your body would be used to the pressure exerted on the body by the time you get into the water at a location at altitude. Confused yet?

If I'm wrong then somebody please correct me.

[ReikiMaster]

Quote: (Originally Posted by amascuba)

It's my understanding that GUE does not compensate for altitude because your body would be used to the pressure exerted on the body by the time you get into the water at a location at altitude. Confused yet?

I would kind of agree with that if you had been there for in excess of 48 hours or so but not if you have just arrived there. Much the same as you dont off gas as soon as you hit the sruface. In theory you already have a nitrogen load en excess of the surroundings. But am just kicking ideas around.

[boomx5]

We don't really do any altitude compensation for our dives. A normal dive consists of driving up from sea-level over a 7,000' summit, doing a dive in the 225' range at an altitude of about 6,200' for about 20-30 minutes, driving back home over the summit and arriving 6-7 hours later a sea-level. These are dives we do all the time in Lake Tahoe, where the bottom temps get around 37 degrees at depth.

I remember when we were training for T1 a couple of years back, we would do all our training at altitude just because it was close and the vis was good. On a normal day we would work on anywhere from 5-15 bounce dives in various ranges from 40'-100' just working on ascents and skills and never had a problem.

What I really find obsurd is how many of the shops around here will sell these altitude specially classes and then make the student drive up to the lake the night before and then spend the night so they can get certified to dive at altitude. By the time the student is done with the class, they've spent a butt load of cash just so they can get a piece of plasic that says they can now dive at altitude...what a joke.

[Joe Hesketh]

I must admit I’ve never understood GUE’s stance on this (if indeed it is that of the agency, and not just imputed from various GI3 posts over the years saying he “doesn’t believe in it”). As in the posts thus far, the usual reasons are vague references to adjustment time in the body at altitude.

However, this is only a minor part of “traditional” thinking on altitude diving, which focuses on pressure differentials between absolute pressure at depth and the reduced atmospheric pressure at the surface, which I’ll try and summarise below, and altitude adjustment is just a complicating factor to that.

To use the Tahoe example above (and sorry I’m going to have to be metric here), the atmospheric pressure at an elevation of 1,900m is 0.80ATA. Given that for all intents and purposes the hydrostatic pressure doesn’t change in a body of water at altitude, the differential between, say, 10m depth and the surface in pressure terms is now not 2:1 (as it would be at sea level) but 1.8:0.8 (or 2.25:1). So in terms of ratios of pressures (which is basically what deco is) that 10m depth is equivalent to a 12.5m dive at sea level.

Those same ratios should also theoretically govern ascent rates and stop depths amongst other things. An ascent rate is a change in relative pressure over time. So given that the pressure change is greater for the same height of water column, the equivalent ascent rate will be correspondingly slower. Same in reverse for stop depths, the water column is shorter for any given pressure relative to the surface, so the stops will be shallower. At 1,900m ASL, an equivalent to 10m/min ascent rate would be 8m/min and the 6m stop would be a 4.8m stop.

Obviously, the greater the elevation and depth the more this becomes pronounced. The above Tahoe dive cited (225ft) worked out in this way would be equivalent in traditional thinking to a 270ft sea level dive!

I certainly don't wish to discredit the practical experience of divers such as boomx5 (at the end of the day, if it works it works), but just haven’t seen a decent explanation given for why the above might be wrong. There’s certainly plenty of detail explaining the traditional approach (for example, see Bruce Wienke’s chapter on Altitude for a meaty read).

Anyone have the answers?

Joe

[nadwidny]

One possible reason is that depth gauges don't actually measure depth but pressure. Therefore an ascent from what the depth gauge says is 10m to the surface should still be a 2:1 change.

We regularly dive in the 4-5000' range and don't make any adjustments for altitude.

[ReikiMaster]

Quote: (Originally Posted by nadwidny)

One possible reason is that depth gauges don't actually measure depth but pressure. Therefore an ascent from what the depth gauge says is 10m to the surface should still be a 2:1 change.

That one i like, and could sign up to, has your guage been calibrated at the altitude? what does it read at sea level (0m) and then at altitude, from the examples eariler this would be -3m or something.

[Chrisch]

Quote: (Originally Posted by boomx5)

... By the time the student is done with the class, they've spent a butt load of cash just so they can get a piece of plasic that says they can now dive at altitude...what a joke.

At least there is no PADI Altitude diver DVD.. :D That would be fun to watch.

Chris

[Joe Hesketh]

Quote: (Originally Posted by nadwidny)

One possible reason is that depth gauges don't actually measure depth but pressure. Therefore an ascent from what the depth gauge says is 10m to the surface should still be a 2:1 change.

That’s true and might balance things out if you ran a profile based on gauge readings. Some gauges automatically zero out surface pressure though, in which case that advantage is lost (and worryingly in a DIR context the UWATEC bottom timer is, IIRC, one of them).

Having said that, I’ve just thought of something else which may weight heavily against “traditional” thinking. For ratio comparison purposes this approach would give you an Equivalent Sea Level Depth, which makes sense. However, it would then advise you to follow a decompression schedule for that new depth. Whilst that new depth might be an equivalent in ratio terms, you are not actually at that new depth in terms of absolute pressure and cannot therefore have the increased tissue loading that would result if you were.

To take the 10m example to extremes, imagine a 10m dive done at 10,000m ASL. That would be “equivalent” to a 33m dive, but whatever the atmospheric pressure at the surface, I can’t see how you can have taken on as much inert gas at 1.3ATA (your actual absolute pressure) as you would have done at 4.3ATA (the ESLD pressure).

Dammit, now I’ve confused myself…

[Howard Payne]

Football was good last night wasn't it?