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Old Jan 09, 2010, 07:26 PM
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Carlyle Harper's Avatar
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Barometric altimeter giving erroneous readings

Sometimes but more often than not, when I plug in my fpv plane and then get ready to takeoff the barometric altimeter is giving me readings like -84, -50, -24, and so one. It's pretty much steady whereever it winds up at. But it hardly seems to put itself at zero before takeoff. And after I land I see that it has drifted around some more from where it started out at. When it was new it seemed to work just fine. What can I do about this?
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Old Jan 11, 2010, 08:36 PM
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Victoria, Canada
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Barometric altitude is a difficult thing to calculate. Just as full-scale pilots need to recalibrate their altimeters as the weather changes, the ET barometric altimeter needs to have a mechanism to update the mean sea level pressure in order to get accurate altitude readings. Unfortunately, I don't think the ET altimeter has any such setting, so the barometric altitude for a given point in space will change throughout the day.

For example, the altimeter reading at the end of your runway (which, for example, has a known altitude of 10 feet) may show 20 feet on a cloudy day, but show -10 feet on a sunny day. Sunny days usually correlate with days of high atmospheric pressure, so an uncalibrated altimeter will "think" that its "deeper" in the earth's atmosphere, and therefore show a lower altitude. On a day with less atmospheric pressure (e.g. on cloudy days where weather is moving into your neck of the woods), the altimeter will "feel" less pressure, and therefore believe that it's higher up in the atmosphere than it is really.

I'm not sure how ET compensates for the changes in atmospheric pressure (or even if it does), so I'll let Bill or somebody else chime in... just wanted to put in my two cents about barometric pressure and altimeters in general.

Cheers,

Victor
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Old Jan 11, 2010, 08:51 PM
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Hi, thanks for your explanation. It makes perfect sense to me.

The drift I'm talking about is within a 40 minute time frame from takeoff to land.

I reviewed some of my recordings and about half the time it got the zero reading correct, other times it could have been off by 20 - 50 feet in either directions.

Is there something in the firmware that's responsible for zeroing the altimeter? Or does it just pretty much read what you get as soon as it's plugged in?
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Old Jan 11, 2010, 09:11 PM
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Victoria, Canada
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Hello Carlyle,

I'm afraid I have no idea what ET does about calibration. The sensor itself automatically compensates for temperature, but I don't believe there's a way to set atmospheric pressure.

If the system zeros the altimeter, then all altitudes would be "tared" (i.e. relative) to the power-on location, while I think non-zero'd would show absolute altitude calibrated for standard sea-level pressure (29.92 inches of mercury or 101.3 kilopascals). Any deviation from standard pressure would cause an erroneous reading.

Hopefully Bill will set us straight!

Cheers,

Victor
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Old Jan 12, 2010, 12:30 PM
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Solihull, England
Joined Jun 2004
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A barometric device measures absolute pressure. We use it for height measurement as there is a known relationship between pressure and height, calibrated in ft or m and call it an altimeter.

Because the pressure at say ground level varies with weather conditions and areas of high and low pressure passing through we must have some way of adjusting for this variation by measuring the local pressure at ground level. Unfortunately any pressure change at your calibration point (ie the pressure where you set say 0ft) will show up as ground level being above or below 0ft unless you make a correction based upon the new pressure at ground level.

If you fly full size aircraft you ask for the local pressure at ground level and adjust your altimeter accordingly so that it will display 0ft when you are on the ground.

So without a local pressure sensor at ground level it is not possible to compensate for changes of pressure at ground level during a flight. ET does not have this facility.

Peter
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Old Jan 14, 2010, 01:06 AM
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Hi Carlyle, I think that Peter and Victor have summed it up pretty well (thanks!). If the barometer is changing rapidly, that will show up as a big swing in the altimeter readin.

There is one other thing that could be the culprit. If a *large* temperature change is occurring after you first turn on the altimeter, drift on the order of several feet can occur. This would normally only be an issue if you are (say) flying your plane on a cold day after just removing it from your warm car.

One way to greatly reduce this issue is to power the equipment for a few minutes, then turn it off and back on, which will re-set the zero altitude after the sensor has acclimated thermally.

Regards,

Bill, for Eagle Tree
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Old Feb 02, 2010, 01:30 PM
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Bill, thats answers my question as to why I was reading 20 feet higher then I was on my first FPV flight...need to let the plane cool down affter being in the car with the heater going before I fly.
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