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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:
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.
- 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
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 99172.15726627368573690675