[time-nuts] Weather/units question for European members

Mark Sims holrum at hotmail.com
Sun May 25 23:06:54 UTC 2014

I ran across this very issue when trying to calibrate my barometer chip against the NWS station located less than two miles away.  Their numbers for millibars and inches of mercury do not agree.  I sent them an email and asked what was going on.  They said their instruments read out in millibars (to three decimal places) The reported value is converted to sea level pressure and reported to two decimal places. They are also converted to inches of mercury for their reports.  Only problem is their conversion constant is NOT the proper value.  They consistently report around 0.02" too high.   I reported this back to them,  but have received no further responses.
Note that the conversion between true pressure readings and sea level pressure involves an equation with about a fifth power/root (depending upon the direction of the conversion) so it can be quite sensitive to true chip calibration.  The pressure chip that I am using (MP5611) is factory calibrated and has calibration constants stored on-chip (the Bosch BMP085 and BMP180 chips also do this),  but the soldering process can affect the chip so you need to do some final calibration.  The MP5611 can detect the air pressure change seen by raising the chip less than 6 inches...
Relevance of temperature/humidity/pressure sensors to time-nuttery?  We all know the comparatively massive effects of temperature on our equipment.  But humidity and air pressure also affect them in many subtle and not-so-subtle ways.  I'll post some recommendations/observations on various sensor chips in a while.
One funny thing about weather measurements is that the data that NOAA reports is not what it would seem. The standard ASOS data (which is what NOAA reports in its local current conditions) includes barometric pressure in inches of mercury and in hectoPascals. It turns out that neither is the actual barometric pressure. 


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