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<DIV>See comments below...</DIV>
<DIV> </DIV>
<DIV>Dave</DIV>
<DIV>CW0351</DIV>
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<DIV>> <BR>> Well I'm a little more enlightened and a little more confused. <BR>> <BR>> Using Phillip Gladstone's calculation for altitude correction <BR>> gives me different numbers than using what I think David Helms <BR>> was suggesting (0.001 inch per foot of elevation change). <BR>> </DIV>
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<DIV>--- see below on discussion of QCMS statistics.</DIV>
<DIV><BR>> Comparing my station with Fort Collins, it looks to me like <BR>> I'm currently somewhere between 0.69 and 0.89 high with my raw <BR>> inches-of-mercury reading. Then I'm applying an altitude <BR>> correction that leaves me with a reported mb figure that is <BR>> somewhere around 10-20 mb low compared to the new other CW <BR>> station in town. </DIV>
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<DIV>--- That CW is probably not calibrated yet (fact: at lest 25% of all CWOP stations get flagged for pressure issues, very problematic). The thing is, adjusting pressure is a rather trivial thing to do; unfortunately, people don't have enough information to make an accurate adjustment (the wrong answer is to listen to the radio). <BR></DIV>
<DIV>> <BR>> So if I calibrate my raw readings to be 0.79 lower and then <BR>> use Phillip's altitude correction I would come out slightly <BR>> higher than Fort Collins for reported mb figure. Is being <BR>> +/- 0.1 inches of mercury considered close enough? </DIV>
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<DIV>--- 00.03 inches or 1.0 mb of actual pressure is the target accuracy, I won't say +/- 00.10 inch is good or bad when looking at KFNL as atmospheric pressure variations may cause that difference to dramatically vary. At least it shows you are getting close.<BR></DIV>
<DIV>> <BR>> I don't know how confident I am with the idea of comparing <BR>> with Fort Collins. It's 30 miles away and on the plains at <BR>> ~5000 feet. I'm in a high mountain park at ~7800 ft. <BR>> But that does sound like the nearest calibrated point. <BR></DIV>
<DIV>--- That is what is available through the Internet anyway. Your elevation difference is similar to the Lararie (KLAR) and Cheyenne (KCYS) mountain vs. high plains situation elevation difference (I think they are 00.02 inches different today). If you have to use an airport that is more than 10 miles and a few hundred feet different elevation, do so with care. Always do your comparisons during light winds (high pressure) to minimize the pressure differences, especially when using a more distant "reference" pressure. You might try the Estes Park Medical Center's Heliport, 970-586-2317, and see if they have a calibrated altimeter for the inbound Medevacs (make sure they aren't busy!).
<P align=left>> <BR>> Do these sensors drift over time? or you think I have <BR>> been off by this much from the very beginning? (I bought <BR>> the thing in 1996.) <BR>> </P>
<P align=left>--- Definitely. You should reaccomplish pressure calibration checks every 6 months or better. Hopefully, the QCMS data will give you a heads-up when things get out of whack. The problem with QCMS is it relies on your buddy station's pressures to create the "analysis" field, if the analysis includes several poorly calibrated pressures, then the analysis is not going to be very useful. That's why I say use a pressure sensor you know is professionally maintained which in most cases will be the local airport ASOS/AWOS.</P>
<P align=left><BR>> Chris Howard <BR>> chris@yipyap.com <BR>> _______________________________________________ <BR>> wxqc mailing list <BR>> wxqc@lists.gladstonefamily.net <BR>> http://pond1.gladstonefamily.net:8080/mailman/listinfo/wxqc </P></DIV></body></html>