Davis has a publication *Calculations of Derived Variables
<https://www.davisinstruments.com/product_documents/weather/app_notes/AN_28-derived-weather-variables.pdf>*,
that shows the formula they use. It's the same formula used by WeeWX. From
the publication, it appears that they do not apply the 3 mph and 50F
limits. Nevertheless, if you go to the NOAA source
<https://www.weather.gov/safety/cold-wind-chill-chart>, those restrictions
do apply. So, we apply them when calculating windchill in software.
But, that's not your question.
The VP2 emits windchill only in its LOOP packets, not the archive records
coming off the logger. So, when it comes time to put something in the
database, WeeWX has to synthesize something. By default, it uses the
average windchill seen during the archive interval. This is why you see
something different from what is on the console. So, it's coming from
hardware, just not the way you think it does.
Alternatively, you can have WeeWX save only the last value seen in the
archive interval by adding the following to weewx.conf:
[Accumulator]
[[windchill]]
extractor = last
That may bring it closer to what you're seeing on the console.
-tk
On Fri, Jul 24, 2020 at 4:37 PM brisguy foo <iamst...@gmail.com> wrote:
> Thanks for the reply. I did check the weewx.conf file and and found that
> wind chill should be coming from the VP2. See below. I believe your comment
> about when wind chill applies is only for the case where weewx calculates
> the result. The VP2 does not have the restriction of being below 50deg.
> Note that if I use the software supplied by Davis (WeatherLink), it will
> display the wind chill data that matches what is displayed on the console.
> Is there something else I may be missing?
>
> [StdWXCalculate]
>
> [[Calculations]]
> # How to calculate derived quantities. Possible values are:
> # hardware - use the value provided by hardware
> # software - use the value calculated by weewx
> # prefer_hardware - use value provide by hardware if available,
> # otherwise use value calculated by weewx
>
> pressure = prefer_hardware
> barometer = prefer_hardware
> altimeter = prefer_hardware
> windchill = prefer_hardware
> heatindex = prefer_hardware
> dewpoint = prefer_hardware
> inDewpoint = prefer_hardware
> rainRate = prefer_hardware
>
> On Friday, July 24, 2020 at 11:03:45 AM UTC-7, brisguy foo wrote:
>>
>> Hello all,
>>
>> I have been using weewx (3.9.2.1 with standard skin) successfully for
>> several months now and just noticed that the wind chill graph does not
>> reflect the values reported by my VP2. It always matches the outside temp
>> values. I have seen that weewx can calculate wind chill on its own, but
>> since the VP2 provides this data, I was expecting weewx to use it. Does it?
>>
>> Thanks
>>
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