Gas Management Calculations at Altitude

[mplooy]

My wife and I are newly certified AOW and drysuit divers and have primarily been diving mountain lakes at altitude. I've become more interested in gas management for dive planning (starting small and slowly working upwards, preferably with a mentor, of course).

After reading NWGratefulDiver's website article on Gas Management, I was curious to know if/how the calculations would differ diving at altitude. I posed the question to him in a PM, and his honest reply was, "I don't know" since he lives and dives at sea level.

So, now I'll pose the question to everyone: does the calculation of SAC, rock bottom pressure, etc. change due to decreased atmospheric pressure diving at altitude? I'm sure the method of calculation would be the same, just curious to know if/where a correction factor would be applied.

The lakes we've been in are around 5000ft elevation which translates to an atmospheric pressure of 0.832ATA.

 

[eponym]

Good question.

The simplest answer is that your consumption underwater won't be affected by altitude. The primary drivers are exertion, cold, mental state, lung volume, and metabolic rate.

When you're cruising at two atmospheres ambient pressure, you'll still be breathing twice your surface rate. Of course, on a 5000-foot lake you'll have to go a few feet deeper to get to two atmospheres absolute (see postscript). That small difference is unlikely to affect your planning.

And depth gauges (including dive computers) can only sense ambient pressure and convert that to a "depth" display. If you have a dive computer that senses altitude before the dive and calibrates itself for fresh water, the reported depths will be close to accurate. If you have a depth gauge that doesn't take altitude into account, you'll have to drop a few feet before it starts to rise off its "zero."

So while a sea-level-calibrated depth gauge will mis-report depth, it will be accurately reporting pressure, which is what affects your consumption rate . . . which is good for purposes of following the gas plan.

Hope this helps,
Bryan

PS. In theory, at .83 atm surface, you need to be 5.8 feet under (.17 times 34 feet) to get to one atmosphere, where an analog gauge (which is calibrated to ignore the first 14.7 psi) will start to rise off the zero.

 

[DivemasterDennis]

I totally agree with eponym, having about 350 dives and altitude and more than that at sea level. The dive planning issues for altitude dives are adjusting for altitude per the altitude table. That is not a gas mangement issue. It is a no decompression limit issue.
DivemasterDennis

 

[mplooy]

Thanks guys, appreciate your answers. I figured the difference in calculated values would be fairly minimal if they were actually necessary, plus any conservatism in the gas plan would probably encompass the small difference anyways, right?

Thanks again for your input.

 

[eponym]

Thanks guys, appreciate your answers. I figured the difference in calculated values would be fairly minimal if they were actually necessary, plus any conservatism in the gas plan would probably encompass the small difference anyways, right?

Yes. Some have summarized pre-dive gas planning as "measure with a micrometer, mark with chalk, cut with an axe."

So by all means plan your gas for the dive, and plan generously. And also plan for the unexpected.

 

[Necklinsberg]

Being an engineer by training and thus a data nerd, I went back and analyzed my consumption on altitude dives vs. sea level dives.

Once adjusting for exposure protection, no statistical difference between gas consumption at sea-level dives, dives at 1000 ft and dives at 3200 ft

 

[mplooy]

Being an engineer by training and thus a data nerd, I went back and analyzed my consumption on altitude dives vs. sea level dives.

Once adjusting for exposure protection, no statistical difference between gas consumption at sea-level dives, dives at 1000 ft and dives at 3200 ft

Cool, thanks for doing the number crunching to check the differences (or rather, lack thereof).

 

[windapp]

I suppose you could always adjust your sea level SAC by multiplying by 0.832, and using that in your planning, if, other than the altitude, the dive was under exactly the same conditions as your sea level dives. However, given that your SAC is bound to be different, you are probably going to recalculate it for the new conditions anyways, and all the other calculations start with your SAC anyway.