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Altimeter prob...(6 posts)

Altimeter prob...must_pedal_harder
Apr 17, 2001 10:18 AM
This is on a Spec P-Brain but I'm sure any computer with an alitmeter function works in a similar manner.

Anyway, my altitude jumps around ALOT. I do live in a hilly area where flats are almost non-existant but when your looking at your computer while it goes from 700ft to 400ft in 3 seconds, well...

Anyone have any suggestions on how to fix this? Anyone have any probs with their own altimeters?
Hey, at least you can predict the weather with it.Jim A
Apr 17, 2001 1:55 PM
The trouble with altimeters is that they calculate altitude based solely on measurements of ambient barometric pressure, a parameter which is almost constantly changing. The amount of drift you report seems high, but if a high pressure or low pressure front is moving through, you can expect significant change in the altitude reading. Even with precision altimeters you have to correct for natural pressure drift. You have to calibrate the instrument at a known altitude (bench mark) at the beginning of the day, travel around and measure your altitudes, then return to the benchmark at the end of the day to see how much pressure change occurred at the bench mark since you calibrated. Then you correct all your data based on the change. Although it's capable of reasonably accurate readings, the little altimeters we use for biking are subject to substantial error depending on the weather. Sounds to me like maybe a cold front (high pressure) was moving in (and movin fast!).
Hey, at least you can predict the weather with it.must_pedal_harder
Apr 17, 2001 3:19 PM
Thanks for the input... I would attribute it to something natural, but it was really jumping around, it seemed to have a general trend, but almost everyother sampling was far off...

They're notoriously touchyCory
Apr 18, 2001 8:12 AM
I've had three or four altimeter computers/watches, and they've all had the same feature, which I've tried to stop thinking of as a problem. Jumps of 100-300 feet are common. Avocet seems to do it a little worse than Casio, but I'm not even sure about that.
I finally quit worrying about it--when you think about the tiny pressure change involved in a 20- or 50-foot climb or descent, it's not realistic to expect a $100 instrument to be able to measure that, but ignore the normal variations in a given day. Just reset whenever you pass a known altitude and try not to think about it.
Vertech?grz mnky
Apr 18, 2001 9:57 AM
I've had very good luck so far with the Avocett Vertech. True, it's not a bike computer, but when it comes to bike electronics less seems to be more. The less extra functions you have the more reliable they seem to be. Load up a bike computer with a bunch of functions and the more likely it is that some won't work right. the Vertech can then be used for other veritcal pursuits (i.e. MTB, hiking and skiing). My wife has one of the Cateye units, as does a friend, and theirs are always messing up.
re: Altimeter prob...steveF
Apr 18, 2001 5:44 PM
My friends and I have several Cateye AT-100's that have an altimeter. I like the altimeter to give a good idea of how much altitude gain I get in a ride or segment of a ride. I zero the gain before each ride but don't pay much attention to current altitude readings. I have found, over 5 years or so with the computers, that current altitude readings fluctuate quite a bit, but the altitude gains for known rides and climbs are quite consistent, i.e., fairly accurate. I would suspect that your computer would show the same (wide fluctuations of current altitude but fairly accurate total gains).