Interesting bit with the pressure measur ... / 2011-05-01

2011-05-01 Interesting bit with the pressure measur ...
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Interesting bit with the pressure measurements at the moment. I now have two running sensors, the old one from the Conrad weather station and the new one from Hobby-Boards. The new one is calibrated for 2 meter above sea level which matches the height reported for our street and backyard at Actueel Hoogtebestand Nederland. And as a reference there is the measurement from De Bilt and Cabauw at the actuele waarnemeningen Nederland knmi. And there is a big disagreement between our shed and the officials. For today 12:00:
  • The 'old' sensor (Conrad) in the shed: 960 hPa
  • The 'new' sensor (Hobby-Boards) in the shed: 988.8 hPa
  • KNMI measurement De Bilt: 1012.8 hPa
  • KNMI measurement Cabauw: 1012.5 hPa
I've been mapping the difference between the old sensor and the measurement from De Bilt at the weather station page and it seems to be nearly constant. I first thought the age of the old sensor was showing. But with two sensors here showing about the same difference with official sensors I'm not sure what the cause is.

First thought after I finished typing: move the sensor outside. But that does not 'fix' the problem.
Update 2011-05-02: More thinking and reading made me wonder if 'above sea level' has different meanings between the US (where hobby-boards calculated the offset for me) and the Netherlands, but that doesn't seem to be the right way. If I try to follow the calculations at Air Pressure and Altitude above Sea Level - The Engineering Toolbox it seems I miss 19 meter height above sea level.

Difference: 21 hPa, using the formula above I can get the closest to that difference with a height of 19 meters in that formula:
101325 * (1 -2.25577*(19/10000))^5.25588
Runtime warning (func=(main), adr=42): non-zero scale in exponent

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