WeeWx Calculations

[Andrea Di Saverio]

If, as stated here https://github.com/weewx/weewx/wiki/Barometer,-pressure,-and-altimeter, the `pressure` is the raw value measured by the station, why is it in the `Calculations` section in the configuration file (https://www.weewx.com/docs/usersguide.htm#[[Calculations]])?

I expected it cannot be calculated, rather it is the starting point to calculate the `altimeter` and `barometer` values.

Could anybody clarify?

Thanks

[Greg Troxel]

There is a long chain of processing.

Somewhere in the weather station (you didn't say which one) there is a
pressure sensor, and almost always these are measuring station pressure
(with some offest that may require calibration).

Then, various weather stations process that pressure to form barometric
pressure or altimeter pressure, because that's what ~everybody wants to
see.

The weather station communicates some kind of pressure information to
weewx, and that depends on at least the station type, and perhaps on how
it is connected.

weewx then does some combination of:
- use values that are reported ("hardware" is the code word for this)
- calculate values, in the case that
* they are not reported
* config, or defaults, say that's better than using the reported value

The following behavior would not be surprising at all:

station has station_pressure sensor

station applies some reduction to barometric pressure

station UI displays only barometric pressure

station reports only barometric pressure to weewx

weewx calculates station pressure by essentially inverting the
station's reduction to barometric, using configured elevation and
temperature

weewx calculates altimeter pressure from station pressure and
configured elevation


See:
https://www.weewx.com/docs/usersguide.htm#pressure_calibration

[Tom Keffer]

Greg has it exactly right.

Until the introduction of LOOP2 packets about 8 or 9 years ago, the Vantage stations behaved this way. They only published pressure reduced to sea-level (SLP), so station pressure (and altimeter pressure) had to be back calculated.

-tk

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[Andrea Di Saverio]

Thank you for your precious answers and for the historical insight, always useful to get the reason behind a choice