Altitude diving for a "knock-off" dive

[boulderjohn]

his approach was to convert altitudes to sea level and fake the stops with a Perdix.

I don't know what this means. The Perdix will adjust to altitude and calculate the dive accordingly. Why do anything else?

 

[lowviz]

Crutch. I've always tried to enforce basic understanding. He did the first part incorrectly.

 

[DiveProKoko]

Have him set his Perdix to auto mode and it will adjust to his current elevation. ....There are some other settings he may want to consider like fresh water versus salt. ....... diving above 6,000 feet so he just needs to be aware of a few things. If he plans on driving the same day, it might be best to stick to non-deco dives.

This! Modern technology takes most of the guess work out of diving in conditions that are out of the "norm" for you. If he sets to water type, gas type, and altitude, he should be good to go. Again, it would be good for him to at least learn and be aware of how altitude can affect divers, etc. I'm "altitude certified" due to work so I encourage that, but the classes can teach you about things that most who don't dive at altitude probably don't think about, like getting bent driving over even higher mountain passes after diving, etc. Yes, I know a guy who got bent doing this! But diving at altitude usually just means setting computers and going in.....then not driving the scenic route over an 11,000 foot pass until stopped for a long lunch!

 

[lostsheep]

(Yes, he called the police and recovery peeps before diving.) (Thoughts?)

Shirley, you must be joking.

 

[mbrann]

Shirley, that’s a joke?

 

[Jonn]

What do you mean he was faking stops with his Perdix? Doesn’t the computer know what the surface pressure is and adjust accordingly?
I recall there was a bug like that in one of the early nitrox computers: it kept calculating as if you were still breathing enriched during your surface interval, the logic to swap over to air when the dive ended never got coded in, and repetitive divers were bent. Does the Perdix NOT assume you’ll be surfacing to the same altitude you started the dive with, even if that is something other than one atmosphere at sea-level?!
I understand why altitude diving needs greater conservativeism, I learned to use those special charts, this decadent digital age has made me complacent though: I trust the black box to sweat the details of it. Maybe we here at Scubaboard could all use a dose of reality on this subject?

 

[DAJ]

Yes, the computer will automatically adjust. Maybe the person in question did not realize that the computer had this function. While the Perdix is set to run at sea level (1013 mBar), the computer will automatically adjust if the pressure drops below 965 mBar (Just over 1,000 feet in elevation). You can also set the altitude to “auto” and it will work the same. I leave mine in auto mode as I dive at different elevations.

 

[Miyaru]

I understand why altitude diving needs greater conservativeism, I learned to use those special charts, this decadent digital age has made me complacent though: I trust the black box to sweat the details of it. Maybe we here at Scubaboard could all use a dose of reality on this subject?

Digital age or not, physics haven't changed since they were defined.

At altitude, the ambient pressure is lower.
Theoretically, every 8.3m of altitude increase above sea level (1013.25mbar without weather influence), the ambient pressure is reduced by 1mbar.
At 300m, ambient pressure is ~977.1mbar, a reduction of 3.6% from sea level. Older computers should be set for this altitude.
At 1600m, ambient pressure is ~835.24mbar, a reduction of 17.6% from sea level.
At Lake Tahoe, where @lowviz son went for a dive, ambient pressure is 805.17mbar, a serious reduction of 20.6%. You can fool your dive computer but you can't fool physics and nature.

Under water, the pressure exerted by the water column above you doesn't change, that's still 1bar for every 10m of depth.
During the dive, you're on-gassing nitrogen as during any other dive.
The risky part of every dive is during the last 10 meters to the surface. At sea level, the pressure goes from 2bar to 1bar - the largest pressure change of every dive.
At altitude, that pressure change is even larger, which means that more nitrogen has to off-gas. Which means a slower ascent and/or longer stops.

Above 2500m is where the risks are becoming significant, especially after a rapid altitude change: altitude sickness.

At cruising altitude, the cabin pressure of an airplane is ~750mbar, which corresponds to an altitude of 2450m. That is a pressure reduction of 25%. Every passenger is decompressing during the initial climb, which takes roughly 10 minutes.
Although you're still breathing 20.9% oxygen, the partial oxygen pressure is 0.16 - at this partial pressure, people can still function.
Rapid reduction of ppO2 below 0.16 can have nasty effects.

 

[lowviz]

What do you mean he was faking stops with his Perdix? Doesn’t the computer know what the surface pressure is and adjust accordingly? ...//...

Yep, the Perdix is all that you need to do such a dive.

...//... Maybe we here at Scubaboard could all use a dose of reality on this subject?

This is my central point. I don't fly my computers as I don't trust the simple "cheat" of just depending totally on them without any innate sense of the forces at play.

It wasn't until I got my BSAC tables that it began to make sense how to skew stop depths SHALLOWER at altitude. IMHO, two things need attention at elevation:

1) shallow depths/stops/ascent rates

2) muscle memory from sea level