Altitude Diving & Calibration

lizardland

Hi Folks,

I'm going cave diving in the French Jura this week (assuming I get a decent weather report ) and will be diving CCR there for the first time. I've been there many times on OC before and OC altitude diving is not rocket science. A lot of the sites are quite high up (900m+) and also quite deep (50m+). So... any tips for CCR diving at altitude?

I'm thinking mostly calibration. My instincts would say take the atmospheric pressure and work out the actual ppO2 at the surface and for 100% O2 for calibration (I can get an altitude reading from my GPS and estimate air pressure using a Palm program or buy a small barometer). Calibrate the handsets for, say, 0.19 for air and 0.98 for O2 (or whatever it works out at). I'm talking KISS style displays, not fancy thinking controllers like the Desperation.

Am I on the right track or am I just being anal and should say sod it and calibrate for 0.21 and 1.0 (which just seems wrong to me but that's what someone told me they do).

Cheers,

Stuart
(Only 54m above sea level )

jradomski

Stuart,
Here are two examples for different altitudes as well as calibrating the units for either Bar or ATAs

305m above sea level the nominal ambient pressure is 977 mbar (if you can get the exact pressure the better)
just multiply this by the calibration gas percentage..

.209 x .977 = .204 for air or .977 for 100% Thats if you are setting your po2 for bar (that means 100% oxygen at sea level (nominal) is 1.013 )

if setting it for ATAs
the nominal pressure is
.964 atas for 305 m, so air would be .201, and 100% oxygen would be .964

for the altitudes you are diving

for 914m the nominal ambient is 908 mbar or .896 atas

For BAR
so .209 x .908 = 0.19, and 100% would be .908
or
FOr ATA
.209 x .896 = .187 and .896

adjust for you actual oxygen.. alternatively, if the unit holds calibration well, just calibrate it at sea level and bring it with you.. then you could always check it against the above numbers. If you use nominal sea level values your loop will be off by ~10% on the PO2

Personally If I was doing the calibration at altitude I would do it using 100% as long as I could insure the loop was flushed correctly since you have a bit more room for calibration error.. Personally I rather be off on the low side (than the high side) since I don't ever anticipate allowing the loop anywhere near the low side calibration..

__________________
Joe Radomski
CCR Trimix Instructor Trainer
ANDI Instructor Trainer Director #10

All posts are personal opinions and DO NOT reflect any affiliated agency unless specifically stated.

lizardland

Thanks for that!

It's all academic now though, the weather is too bad

jradomski

Anytime,

Correct calibration procedures is one thing I really try and stress in my classes, ESPECIALLY at altitude..

I guess others find my post helpful as well(by adding rep points), Its nice to see that people appreciate it, and makes me try harder...

__________________
Joe Radomski
CCR Trimix Instructor Trainer
ANDI Instructor Trainer Director #10

All posts are personal opinions and DO NOT reflect any affiliated agency unless specifically stated.