Altimeter Athmospheric Lapse Rate

[crinkles]

Hi,

 

I am working with a 62s and have been fiddling around with the altimeter.

 

My issue is that I can't calibrate the altimeter Correctly if I use the sea-level reported Barometric pressure of the towns surroundign me (even an average of the lot, two within 10 miles and 2 within 50 miles).

 

My known elevation is 145m.

 

If I calibrate using "known elevation", I will always get a lower barometric pressure (corrected to sea level) than the reported measurements in the town (also corrected to sea level).

 

For instance, calibrated to 145m this AM, 62s said barometric pressure is 1021.1 hPa. Check of the baro pressure in twon shows 1023.0, next town across is 1022.6 ((I am between the two towns).

 

So If I were to claribrate using known barometric pressure, from the weather service, then my elevation reading on the 62s will be something like 160m +.

 

I have found this on a few occasions now.

 

In my mind this raises questions as to the accuracy of the lapse rate used in the atmospheric model by the 62s. if over a real 150m rise an extra 10m could be added by the unit, how much of a difference would this model make if I were to go up the dividing range (1000m+)?

 

As far as I am aware the auto calibrate adjusts for barometric pressure changes only.

[+fizzymagic]

If I calibrate using "known elevation", I will always get a lower barometric pressure (corrected to sea level) than the reported measurements in the town (also corrected to sea level).

 

I don't think you understand how the calibration works.

 

The pressure sensor in the 62s reads absolute barometric pressure. Not adjusted to sea level. And it is not adjustable via calibration.

 

What the calibration does is adjust the offset of the altimeter to the known (or GPS-derived) elevation. There is no way to view the resulting calibration factor on this device.

Edited August 26, 2011 by fizzymagic

[crinkles]

To expand a bit to the information I provided, for the Barometric Pressure I mentioned of 1021.1 shown by the unit (when calibrated to the known elevation) the ambient pressure measured by the unit was 1006.0 hPa (which is the absolute pressure you refer to?).

 

Isn't the "Barometric Pressure" referenced to sea level per definition?

Edited August 26, 2011 by crinkles

[crinkles]

OK. I have done a check using the standard atmosphere model (1976). Fully aware of the limitations etc.

 

At 145m elevation the pressure should be 995.95/1013.25 ca. 98.29%.

 

Referencing the weather report, with MSLP 1023.0 hPa, gives 1005.5 hPa for ambient pressure measurement. This agrees well with the ambient pressure measurement by the 62s of 1006 hPa.

 

So why would the "barometric pressure" on the unit read 1021.1 hPa?

Edited August 26, 2011 by crinkles