W dniu 3.09.2023 o 14:02, gszla...@gmail.com pisze:
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.
First of all, *huge* thanks for taking your time and claryfing whole
process in detail. In the meantime, before you wrote your instructions,
I used your previous tips
(https://www.wxforum.net/index.php?topic=45601) and done calibration
process. Of course I can't be sure I got now "real" values but - as you
have noticed - my console doesn't allow entering the offset (which in my
opinion is much more instructive way than just entering REL and ABS
values). I have to check once more ABS value with certified barometer.
By the way: I'd be grateful if someone could explain the following
abbreviations - SLP and ISA :)
--
Tomasz Lewicki
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