A question about computers and airplanes

[ishmaelcat]

OK folks, here's a real puzzle I'm hoping someone knows the answer to:

First, the background (don't get excited yet, this isn't the puzzle) -- Let's say I haven't been diving for several days. Now I take my dive computer (Cressi Sub Archimedes II) up in a commercial airliner. At cruising altitude, the cabin is pressurized to what is equivalent to about 8000 feet above sea level. My Cressi shows two little mountains to tell me I'm at altitude; it also show 4 pixels on the N2 loading graph.

This all makes sense, because I started at sea level with my blood saturated at 1 atm, and now I'm in a lower ambient pressure environment, so I'm now supersaturated and off-gassing. As a matter of fact, on a long flight you can watch the saturation level on the N2 graph go down -- just like after a dive -- to 3 and then 2 pixels and so on. This all makes sense too.

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.

So what's the explanation? And if you're going to say it's a glitch, what I want to know is, how you think the computer's algorithms arrived at this particular glitch. Onthe other hand, if it's intentional, what's the rationale.

Finally I'd be curious to know if other people had brought their computers with them on a fight and noticed the same thing -- especially if you have a Suunto or Oceanic or something else that has different innards than mine (i.e. not a Nitek Duo or Tusa whatever it is).

I've been wondering about this for over a year; any insight would be appreciated.

 

[Coldwater_Canuck]

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.

Purely a guess, but I think you may have answered your own question with that last sentence. The computers can tell when you've, entered into a dive, I assume either by the total pressure being so great or the rate of pressure change it can tell this fairly easily. If it couldn't do this, it would be kind of fun to watch your computer during the end of your airplane descent and see what depth it thinks your at, and then once you're on the ground watch as it tacks on years of deco time since it thinks you're still at the bottom

Since it knows you aren't diving, I wouldn't expect it to start thinking you're taking on Nitrogen instead of off gassing since it knows you're never actually in pressure greater than sea level (land).

 

[ishmaelcat]

Since it knows you aren't diving, I wouldn't expect it to start thinking you're taking on Nitrogen instead of off gassing since it knows you're never actually in pressure greater than sea level (land).

Exactly! So why does it show a pixel of N2 loading -- that's the question.

 

[knotical]

Computers tend to do what they are told, and no more.

The computer was probably programmed with some sort of an off-gassing clock. When you land, the clock is still running.

 

[scuba_frog]

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.

I know I'm not much help, but it probably knows you were not diving because it did not get wet. I have an Oceanic which has H2O activation. Without getting wet, it "knows" you were not diving.

 

[Coldwater_Canuck]

Exactly! So why does it show a pixel of N2 loading -- that's the question.

And that pixel wouldn't be there if you'd stayed on land? Or it would but you think going to altitude should reduce the desat time and make it go away?

I know I'm not much help, but it probably knows you were not diving because it did not get wet. I have an Oceanic which has H2O activation. Without getting wet, it "knows" you were not diving.

I don't know if it would just be that though. Even if you placed it in a Tupperware container or something with water in the plane (back in the days when you could bring water on a plane ), I still don't think it would enter dive mode because the water pressure would never become high enough. That's just a guess though.

 

[Crowley]

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. Yes, they can be configured for altitude diving but as far as I know - you have to preset them, depending on the altitude at which you're diving.

I think the simple explanation is that dive computers are designed to work whilst diving, not flying. With regards to the N2 loading - it may be that as you descend, cabin pressure actually exceeds ambient pressure so therefore you have more pressure in the cabin when you land than is actually outside. Cabin pressure is generated by the engines and programmed via the plane's flight systems so potentially it might be playing "catch up" and hence yes, you are therefore offgassing (albeit minimally but statistically significant enough for the computer to clock it). This is more of an educated guess but this article: How Things Work: Cabin Pressure | Flight Today | Air & Space Magazine has a brief description of how cabin pressure is managed.

Hope that helps

C.

This article has a basic explanation

 

[Coldwater_Canuck]

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. Yes, they can be configured for altitude diving but as far as I know - you have to preset them, depending on the altitude at which you're diving.

Kind of a funny thing that happened to me because of this. I was doing my night diver cert and we were just ascending by swimming along the ground towards shore. I hadn't looked at my computer for a minute, knew we were probably at about 10 feet or so, but night + Seattle vis, had no way to see the surface. All of a sudden I looked down and saw "55 feet". It scared the crap out of me, I thought I'd blacked out or my mind had completely lost it or something, I didn't know what to do for a second since I was just so confused. I then noticed that beside the depth it said "Max depth": we were actually at around 5 feet and my computer had exited dive mode and was displaying the "max depth" of my last dive

 

[Sinbad the Diver]

I

I think the simple explanation is that dive computers are designed to work whilst diving, not flying.

Reminds me of a few years ago I was a network admin and was getting strange messages from an antivirus program (false positives). Called tech support, guy told me that I had it configured wrong (bad instructions of course ). So I changed the confiruration and thanked him, then asked, OK, so why am I getting all these messages. He paused and as nicely as possible asked: so, you would like me to explain why your getting error messages from the system that was configured incorrectly? I thought about that for a second, thanked him and moved on.

 

[ishmaelcat]

Thanks for all the replies, but with due respect, I don’t think we have an explanation yet. I do think Scubafrog is right that the computer knows I haven’t been diving because it didn’t get wet (explains the “no fly” at least).

And Crowley’s theory would at least theoretically explain the central mystery here -- the pixel of N2 loading we see on the ground. If the cabin was pressurized to higher than 1 atm at some point, then landing would put us into a supersaturated state again. Unfortunately I think there are several reasons to doubt that this is in fact happening. First, I don’t believe airliners are ever intentionally pressurized to higher than 1 atm, as it would kind of defeat the purpose of ameliorating the extreme pressure gradient between the ground and 30,000 feet. My understanding is that they start at 1 atm and then as they gain altitude gradually add some pressure to minimize the pressure drop. I should note here that I’ve watched my computer go through the behavior cited in my OP on several (at least 5) occasions. Secondly, if for some reason airplanes did routinely pressurize above 1 atm, I think the passengers would feel it in their ears, and I've never noticed this.

Thirdly and most importantly, though, if Crowley’s theory were correct, my computer should at some point during the descent (when the pressure climbed above 1 atm) clear completely – as I would now no be longer offgassing at all, and as we’ve established it “knows” I’m not underwater; then when the cabin door opened on the ground, or the pressurization machinery turned off, the single pixel should reappear – but this is NOT what happens. The pixels recede as the plane descends – and the altitude graphics “mountains” disappear, but one pixel and more than 1 hour of desat time remain on the display at all times. Maybe there’s something I’m missing here, but for now I’m gonna say, “nice try” but no cigar yet.

As for some of the ideas but forward by others, briefly: Knotically posits some sort of offgassing clock, which is still running when the plane lands. Maybe, but WHAT sort? It can’t be a simple timer. I mean, at 30,000 feet my computer tells me I have several hours until desaturation, and as we descend and cross some preprogrammed pressure boundary (probably the same one that makes the “mountain” graphics appear and disappear) these several hours disappear instantaneously, as do most of the pixels – but not the last one.

As far as computers not reading anything shallower than one meter – yes, of course, but I really can’t connect that in any way to the residual pixel after flying.

I have heard one theory from a member of my local dive club that seemed vaguely possible, but I’m going to hold off on airing it here yet to avoid contaminating people’s thinking. In the meantime I’m still anxiously awaiting a plausible – and detailed – putative mechanism. I KNOW someone out there is scubaboardland can figure this out.