No wonder the OP is confused. In Europe, METARs generally don’t have SLP, they have QNH stated as a “Q code” in the actual METAR report. An Altimeter /QNH reading of 1022 would be Q1022 in the METAR report.
So typically, QNH > QFF when current or average outside temps are greater than the ISA temp for his elevation. That is the reason why the OP remarks that he is “missing” 7 hPa: [I]> METAR for EPWR airport shows 1015 hPa. So I need to "collect" additional ~7 hPa > (1015 from METAR minus 1007.8 from Weewx calculations)[/I] The OP is smarter than us because WeeWX SLP is 1007.8 and Altimeter/QNH = 1015. Two different things entirely! So let’s take a step back and try this again. We want to match Altimeter/QNH. 1. The OP is trying to calibrate by matching QNH at airport (EPWR) 2. You need to use an Altimeter calculator or ISA calculator, not a SLP calculator. Let’s redo the REL offset for a 560m sensor elevation: 1013.25 – 947.760 = 65.49 (65.5 rounded) The 947.76 is the fixed ISA pressure for any 560m elevation. You can use a standard atmosphere calculator or Altimeter calculator to calculate it. Therefore, for a 560m elevation, QNH must always be 65.5 hPa higher than whatever the Fine Offset station pressure is at the moment or looking at it from a sea level perspective, station pressure must be 65.5 hPa lower than QNH. EXAMPLE: If EPWR is showing Q1022 then: 1022(REL) – 65.5(REL offset) = 956.5 (ABS) station pressure. i.e. REL= QNH at airport minus your offset = station pressure at your location (560m). To calibrate, you can either: 1. Set REL = 1022 and set your ABS = 956.5. or 2. Add 65.5 to your existing ABS value and then compare the REL value on your display to the QNH value at the airport. Adjust the ABS value up or down until the REL display is the same as QNH as the airport. ABS and REL on your display move lock-in-step. To change the REL value, you have to change the ABS value. REL must always show to be 65.5 higher than your current live ABS reading. There will be a lot of button pushing involved, but make sure the “spread” or pressure difference between REL and ABS is always 65.5. The first method is obviously a shortcut method. It is the simplest, and you still have to lookup your ISA pressure for your elevation, but you won’t learn much about the process. The second method is the method I usually recommend. I am assuming the OP is configuring his barometer on a display console and must be using the ABS/REL system instead of the offset system. The Fine Offset manufactured WiFi gateways use a different system. They are a display-less console that uses a ABS offset/REL offset system. If the OP is still running into calibration snags, I would encourage him to post his quesion at wxforum.net in the Ambient/Ecowitt/Fine Offset clone forum. On Friday, September 1, 2023 at 8:44:11 PM UTC-4 Greg Troxel wrote: > Tomasz Lewicki <barbap...@gmail.com> writes: > > > I'm back and starting my reply from Rainer's last question. Yes, I've > > read this topic: > > https://github.com/weewx/weewx/wiki/Barometer,-pressure,-and-altimeter > > on the very beginning. I use only 'pressure' and 'barometer' terms > > below, according to definitions from wiki. > > > > So, I'm even more confused than before. I've made the calculations > > with the calculator listed in > > https://www.wxforum.net/index.php?topic=40730.0 (direct link: > > https://keisan.casio.com/exec/system/1224575267 - BTW they will close > > at September 20) with my current conditions (pressure 944.5 hPa, > > temperature 20*C, height of sensor 560 m AMSL) and got result 1007.8 > > hPa - this is perfectly equal to what I see on my webpage - 1007.7 hPa > > calculated by Weewx algorithm (difference of 1/10th of hPa is > > neglible). I recall that I use 'software' option in weewx.conf for > > 'barometer' but 'prefer_hardware' for 'pressure'. > > This confirms that weewx's calculation matches the web calculator, that > IF your station pressure is 944.5 hPa then the barometric pressure is > 1007.7 or .8 hPa. It has ZERO information about whether the station > pressure measurement is accurate. > > > But very accurate numerical forecast for my location *and* METAR for > > EPWR airport shows 1015 hPa. So I need to "collect" additional ~7 hPa > > (1015 from METAR minus 1007.8 from Weewx calculations). Does it mean I > > should: > > This means that it is likely that the station pressure at your location > is about 7 hPa higher than the sensor reads. > > > a) add whole calculated offset - ~70 hPa (1015 from METAR minus ~945 > > of absolute pressure) - in my PWS console > > This will result in wrong station pressure readings, because you are > using the console to go from wrong station pressure to fake barometric > pressure. I would advise against this. > > > or > > > > b) add smaller offset - 63.3 hPa (1007.8 calculated by Weewx minus > > 944.5 read by sensor) PLUS use relevant section in weewx.conf: > > This is confusing, but if weewx is getting 1007.7 hPa and the weather > service says 1015.0 *AND* you are sure your elevation is right, then > this means there is a 7.3 hPa error in the station pressure sensor. > That's a lot, but this is not professional equipment. > > (Actually, I think the reduction is not linear, so this is strictly > slightly messier, but probably not relevant at the 0.1 hPa level.) > > > [StdCalibrate] > > [[Corrections]] > > barometer = barometer + 7.2 > > Almost. The problem is that pressure is wrong. Barometer is calculated > and given correct pressure and correct altitude will be correct. > > So instead: > > pressure = pressure + 7.2 > > to fix that instead. After you do this, look at your barometric > pressure readings and the official ones over time and see how it tracks. > You may decide to change to 7.3 or 7.0 or something. > > > I only need confirmation that I'm going in right direction - or opposite. > > We talked about this earlier, but given that 1 hPa is about 8.3m, if > your elevation is 8.3 m too high, or if your pressure sensor is 1 hPa > too low, the two situations look the same, and you basically can't tell > them apart. > > So on one hand, a 7 hPa error and a 58m elevation error are the same > thing, in terms of barometric pressure. But on the other hand, in one > case your station pressure is right, and in the other tit is not. > > > > It would be interesting to hear from others. My experience with Davis, > 2x BME280 and a BME680, a basic casio watch with pressure esnsor, and > one other not-very-expensive sensor is that most agree in a 1-2 hPa > range, they cannot in general be believed to better than 1 hPa and it is > rare, perhaps not in my data set, to be 3 hPa off. So 7 is odd. But I > do not have epxerience with Fine Offset stations. Still, I find it > surprising. > > I would love to hear from people with Fine Offset stations that have > made careful measuremennts of station pressure calibration. > -- 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/b788bbe4-c181-4698-8457-7133430d740en%40googlegroups.com.
