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Watches on a Plane
January 7, 2010 6:14 PM   Subscribe

Was my Casio Pathfinder's altimeter function working on the plane?

So I got a Pathfinder Triple Sensor for Christmas (this one, for reference) while on vacation on the other side of the country. On the commercial flight back (a Delta MD-88 or something), I clicked onto the altimeter just to see what it would read. It gave a reading of 32,800 ft (its maximum reading). I asked a flight attendant what our current altitude was, and was surprised to hear we were at 33,000 ft. After that I updated it frequently, and was able to watch the altitude slowly go down as we descended, stopping as we circled the airport for several minutes (reading about 6000 ft), and returning almost to zero after we touched down. My understanding of how this watch works (pressure/temperature of surrounding atmosphere) and how airplane cabins work (pressurized to approximately 7000 ft) dictate that this shouldn't have happened.

As far as I know, no one died of hypoxia on the flight, so, what gives?
posted by nzero to Technology (8 answers total) 2 users marked this as a favorite
 
Well, planes aren't fully pressurized, hence the ear-popping you experience. That said, you're right that it is pressurized well above what you'd experience at 32k feet. My guesses:

1) The watch has a range of values for which it's accurate,. Once you're outside that range, all bets are off. It specifically mentions in your manual that you shouldn't rely on it in an aircraft.

2) Your reference altitude is off. The manual talks about how you can calibrate the altimeters settings by saying "I know I'm at 10,000 feet". You may have inadvertently changed this setting at some point, which caused things to be out of whack.
posted by chrisamiller at 6:32 PM on January 7, 2010


Ah, chrisamiller, we meet again (remember the antibiotics thing?)!

I did think about the possibility of an adjusted calibration, but as I said, it returned nearly to zero on the ground.

I also considered the idea that the watch was just off in lala land, and that it was just a coincidence that the maximum reading corresponded to a standard flying altitude, but I got such a nice graph of the descent (it shows the last 26 readings or so in a graph format) it's just hard to believe that's all there was to it.
posted by nzero at 6:37 PM on January 7, 2010


Well if you flew into Hartsfield Jackson, it should have read 1000ft or so when you landed, not zero.

So your calibration is certainly out to some degree.
posted by Brockles at 6:55 PM on January 7, 2010


So your calibration is certainly out to some degree.

No doubt about it. I've found it to be pretty inaccurate on the ground in general. However, it was calibrated to zero on the ground at SAT, which is only a couple hundred feet difference.
posted by nzero at 7:09 PM on January 7, 2010


Altitude isn't the only factor, pressure changes throughout the day and weather system (think the High and Low pressure systems on your evening weather forecast), a calibration at SAT may well be way off.
To answer the original question though... no idea. Plane pressure is normally set to about 8000ft or so (http://en.wikipedia.org/wiki/Cabin_pressurization), as the aircraft drops below this pressure, I think I read somewhere that cabin pressure increases to correspond with the external pressure. (I could be wrong, I can't find a reference right now)
posted by defcom1 at 8:49 PM on January 7, 2010


I've done this with a Suunto altimeter watch. In mid flight, the altimeter read 6500 feet or so. It seemed to work through the descent of the flight.

It doesn't seem like there's a GPS in the watch, so it wasn't registering actual altitude.

My guess is that the watch was thrown off by the suddenness of the ascent. They're pretty sensitive, and more designed to work with slower ascents/descents.

Or it could be a battery thing.

One thing's for sure. If the air pressure corresponded to that altitide, you wouldn't be here to tell us about this!
posted by thenormshow at 8:58 PM on January 7, 2010


I had a brief look at this earlier today and wondered whether the temperature could have been throwing it off, and considered asking whether the watch was on your wrist the whole time, but even on a marginally-closer inspection, it would have taken a very uncomfortable cabin temperature to throw it off that far.

I say you write the whole thing up in as much detail as you can remember (perhaps appending a summary or printout of this thread), send it in to ask Casio as a satisfied customer who's wondering what was up, and post their reply in this thread, because I'm getting really curious.
posted by tellumo at 12:01 AM on January 8, 2010


All marked as best answers, because everyone had valuable contributions and good ideas.

And now the embarrassing conclusion:

Turns out there was no mystery, and the watch was (more or less) accurately reading the cabin pressure as 6560 ft. I made the erroneous assumption that there was a x5 multiplier due to a misinterpretation of the manual. The nice descent registered was presumably the increase in pressure due to the actual descent. The key point of my confusion was the coincidence of 6560x5 being exactly 32800, which is both the watch's maximum altimeter reading and the approximate altitude of the airplane at the time. I really appreciate all the valuable insight, and I apologize for wasting everyone's time.

Credit for figuring out my mistake goes to my sister who had that little bit of extra information I should have provided in this thread but didn't think was important enough.
posted by nzero at 1:09 PM on January 10, 2010


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