A question about computers and airplanes

[LaurenW]

A quick peek at my computer showed that the dive counter had incremented from 0 to 1. A review of the dive log shows a dive with a max depth of 8ft. for less than 1 minute.

Interesting! I'm really surprised it would record a dive without the moisture sensor being triggered, very cool. I'll try a similar test with my own computer and see if I get the same result.

I looked this up since I was curious: The shore of the Dead Sea is the lowest dry point on earth, and is 1385 feet below sea level. According to google that gives you an air pressure of about 15.5 psi instead of 14.7 psi at sea level, so almost certainly not enough to trigger your dive computer. Too bad.

It will probably be a pain to ship the beer across the Canadian border, so I won't hold my breath while waiting.

Probably true, but if you're ever in the San Francisco area let me know!

 

[GrandpaScuba]

I think the Canuck has it right - some computers won't read anything if you are above 1 metre - otherwise snorkelling would add to your bottome time - which is silly.

Actually, it's not because of snorkeling. There is enough legitimate variation in the barometric pressure, that they have to really be sure that you are actually underwater, not just in a nice high pressure zone.

 

[sschlesi]

I looked this up since I was curious: The shore of the Dead Sea is the lowest dry point on earth, and is 1385 feet below sea level. According to google that gives you an air pressure of about 15.5 psi instead of 14.7 psi at sea level, so almost certainly not enough to trigger your dive computer. Too bad.

At 0.445 psi/fsw that's not even 2fsw. My computer needs 4fsw to start a dive.

Though that is quite a bit of difference considering it only takes ~1 inch of water pressure to support an inflated stadium roof.

 

[ishmaelcat]

Hey this is all great folks, but I'm gonna take one more stab at getting an answer for my original question:

My computer does NOT register a dive after a plane ride, but gives you one pixel of N2 loading and an hour or so of desat time even after you return to sea level. I'm looking for some sort of SPECIFIC, DETAILED explanation -- or at a least reasonable hypothesis -- actually framed in terms of the computer's possible algorithms.

Hoping someone with technical knowledge about dive computers might be out there. . .

 

[Web Monkey]

My computer does NOT register a dive after a plane ride, but gives you one pixel of N2 loading and an hour or so of desat time even after you return to sea level. I'm looking for some sort of SPECIFIC, DETAILED explanation -- or at a least reasonable hypothesis -- actually framed in terms of the computer's possible algorithms.

What about "It's broken" (hardware or bug in the software)?

Neither my SmartCom or Galileo do that.

Terry

 

[Dave1981]

Here's a thought. I think it's to do with the washout model.

If it gave you the full credit for being at the same altitude you started from, ie reset to 0 N2 loading after having off gassed the longer compartments to saturation at altitude, then if you were to do short deep dives the washout would be so fast as to falsely indicate no loading in the longer half time compartments and it would allow you to make repetitive deep dives dangerously.

I think it's to do with the model allowing quick washout of the short compartments for short S.I but making sure you can't accidentally overload the longer compartments.

Edit: Not that I mean it's for diving straight after flying, I mean it's a function for preventing danger on short repetitive dives or sawtooth profile.

 

[Cave Diver]

What doesn't seem to make sense to me is that when the plane lands, at sea level, the computer still shows 1 pixel of N2 and about 2 or 3 hours (I forget exactly how much) time until total desaturation. And that pixel remains until those 2 or 3 hours have passed. It seems to me that any tissue in my actual body at this point should be picking up more N2, not offgassing, no matter which "compartment" it is. Strangely, just to add another oddity, the Cressi also seems to "understand" after landing that I haven't really been diving, as the "No Fly" icon does NOT appear.

You incur nitrogen loading of tissues when you breathe nitrogen under pressure. Since sea level is normal atmospheric pressure (1ATA) nitrogen is not being forced into your tissues, therefore your computer does not track it as nitrogen loading even though you have "returned" from an elevation of 8000'.

That's the same reason you can dive to 10-15' for an indefinite period of time. There's not enough of a pressure gradient to induce nitrogen loading.

 

[Cave Diver]

Hey this is all great folks, but I'm gonna take one more stab at getting an answer for my original question:

My computer does NOT register a dive after a plane ride, but gives you one pixel of N2 loading and an hour or so of desat time even after you return to sea level. I'm looking for some sort of SPECIFIC, DETAILED explanation -- or at a least reasonable hypothesis -- actually framed in terms of the computer's possible algorithms.

Hoping someone with technical knowledge about dive computers might be out there. . .

That's because you haven't been diving. The fault is not in the computer, it's in the line of thinking being pursued.

 

[Cave Diver]

Going to a higher elevation should cause you to desaturate faster. That's the reason why it's advised you wait before you fly after diving. The decrease in pressure causes the nitrogen to offgass at a faster rate. Returning to sea level and off gassing continues, but at a normal rate. If you off gas too quickly, you get DCS.