thanks for the comments.  I will be digesting all of the replies and
CWOP data and coming up with a cal plan.

"[email protected]" <[email protected]> writes:

> For a Davis VP2, SLP is already calculated for you in the console so all 
> you need is the elevation of your sensor. Hwever you still need to check if 
> your barometric sensor is accurate or not,

Agreed.

>  The BMP280, BME680 are accurate 
> to 1 hPa and so is the VP2 barometric sensor. The newer Bosch barometric 
> sensor (BMP390) is accrate to 0.5 hPa.

My data so far indicates that a BME280 and a BME680.
BME680 is +2.4 hPa from VP2.
BME280 is +1.4 hPa from VP2
Brunton ADC is -0.1 hPa from VP2.

So I don't believe those claimed accuracies.   I am tentatively
believing that VP2 and Brunton ADC are correct until I can take the ADC
to visit other barometers.

> You could calibrate your VP2 with the higher spec BMP390. Just match the 
> pressures and you are done! or alternatively you can match Altimeter with a 
> close-by METAR.

I am going to try to compare my barometric and altimeter pressures with
NWS stations on a day with a low rate of change.

> So no, we will not be achieving 0.05 to .10 hPa accuracy WMO standards with 
> our consumer grade personal weather stations!

VP2 seems pretty stable so I think 0.2 hPa is not crazy.

> Keep in mind that different countries use different pressures. Most 
> countries don't use SLP or Altimeter - they use QNH in whole integer 
> amounts which makes matching difficult as you would have to match 1013, 
> 1014, 1015, etc. If you luck out, some countries publish decimal QNH like 
> the BOM in Australia.

I thought QNH was Altimeter.  The US uses that in aviation and SLP or
barometric pressure for weather.  But yes, roudning to whole hPa makes
it hard.

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