Tomasz Lewicki <[email protected]> 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.
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