"gszla...@gmail.com" <gszlagow...@gmail.com> writes: > If you are comparing your external sensors to your console's barometer keep > in mind that the offset does not mean the sensor is out-of-spec. An > absolute offset is not a specification. It is just the sensor correction > offset so that all your barometers read the same absolute pressure. I am > assuming you using the console as your reference and you have calibrated it > with a close-by airport using METAR or mesowest readings or equivalent.
What I have done so far is that I entered an elevation that I knew was very approximate, and watched the barometric pressure compared to official readings at airports around me, and on days when the pressure was steady and similar adjusted by elevation (because that's all you can do on VP2 console) to get a matching value. I knew that was bogus because it's doing station pressure calibration by having the wrong altitude, but it got me decent values. However what I should be doing is having a calibration offset for station pressure, and then using the correct altitude (which I now know). The pressure sensor has some spec, and if the calibration magnitude is higher than that, then yes the device doesn't meet spec. The VP2 is specified to +/- 1.0 hPa. If the calibration offset were 1.5 hPa, then it would be out of spec. But my best guess this minute is that I need to add 0.1 hPa to the station pressure to get the true value. > To answer your question if QNH = Altimeter? Altimeter includes a landing > gear offset of 10 feet = 0.3mb/.01 inHg. I believe that QNH does not have > that peculiar offset. Although most sources equate the two, I think the > equations are different. Interesting. I'll try to understand this better, but I suspect that the altimeter pressure used by CWOP (US again) and calculated by weewx does not have this offset. > Lastly you were asking about different definitions of SLP? No, I was asking "what does sea level mean, when weather people say 'pressure reduced to sea level'". How to reduce from a known elevation is less confusing. > It is easy to get confused here because it is confusing. Yes, and particularly because height and sea level are both very difficult subjects if you want to get them right. > I know of at least 6 different equations for Altimeter and there might be > dozens of different country-specific equations for (M)SLP. > To make things more interesting both Altimeter and (M)SLP can be referred > to as "SLP".as both are reduced to sea level elevation. They are, but in US usage "Sea Level Pressure" means "barometric pressure" meaning a reduction equation that uses temperature. > As you are aware, if you hook up your console to WeeWX, WeeWX will > automatically calculate Altimeter and (M)SLP using the proper algorithms. > Need to adjust the current algorithm for your region? You can modify it. Sure, but with VP2 it defaults to using hardware calculation for barometric pressure. I thought station pressure was also hardware, but https://www.weewx.com/docs/hardware.htm#vantage_notes says S. That may be the next missing link in my understanding. -- You received this message because you are subscribed to the Google Groups "weewx-user" group. To unsubscribe from this group and stop receiving emails from it, send an email to weewx-user+unsubscr...@googlegroups.com. To view this discussion on the web visit https://groups.google.com/d/msgid/weewx-user/rmik09q10hk.fsf%40s1.lexort.com.
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