I am using the BMP388 sensor that measures Air pressure (since my console is deceased) and find it pretty accurate having done some tests. I use some Python code to convert the Absolute Pressure given to Relative pressure using the following formula from the referenced web page.

pressureR = float(pressure) + ((float(pressure) * 9.80665 * altitude_amsl)/(287 * (273 + temperature + (altitude_amsl/400))))

See this site for description:

https://gist.github.com/cubapp/23dd4e91814a995b8ff06f406679abcf

What has suprised me is that quite a few sites have Absolute air pressure greater than Relative pressure. (This is from Ecowitt.net sites) Ok a few may be below sea level!!

bmp.pressure, bmp.temperature, a2ts

978.24 26.99 1016.58

Does that seem right for 350M ASL?

So much testing, formatting, etc, etc, etc fun never ends.

At 350m ASL your Altimeter reading will always be 41.3 hPa higher than the current BMP pressure so you are out about 3 hPa.(too low).

To correct for that, you will have to add a correction (we call that an offset) in order to match the Altimeter reading at a close-by airport. You will have to

add 3 hPa to compensate.

Note that this is not necessarily an error correction - you are correcting for elevation. Mentally, you are adding pressure as you are approaching sea level - air pressure is higher at sea level than it is at say, the top of Mt. Everest.

Basically, when you add pressure to correct for elevation above sea level, you are pretending that your BMP388 is located exactly at mean sea level elevation ( 0 meters) even though it is currently sitting at 350m.

Take a look at this table in order to visualize the concept:

[ You are not allowed to view attachments ]

The above table is a theoretical calculation (an approximation of Altimeter) under what's called "standard atmosphere" conditions.

Calculating SLP ( on a METAR report) is a far more complex calculation as it not only corrects for your elevation but temperature too. To calculate SLP you would need to know your 12 hour average temperature or minimally, your current temperature. WeeWX does all these pressure calculations dynamically in real time. In WeeWX all you have to do is put in your BMP388 elevation ASL and you are done assuming that your BMP388 is perfectly calibrated and accurate. But that is another topic.

Update: I just realized you are using Mike's algorithm after wondering what the heck is A2TS? That's what the dev named his github script.

The algorithm uses temperature as an input variable so it must be calculating SLP rather than Altimeter. Altimeter ignores temperature.

If QNH/Altimeter < QFF/SLP) then outside temperatures are lower than your ISA temp of 12.7C for your elevation of 350m. However, a 3 hPa difference seems to be a pretty big difference this time of year. Your temps would have to be about -8C?

For some comparisons, use this calculator and see what you get:

https://www.starpath.com/barometers/baro_cal.php. Note: Make sure your elevation is as accurate as possible.

Bottom line: Verify the accuracy of your barometric sensor by matching your airport's METAR Altimeter(setting) reading first and then use WeeWX to calculate "Barometer/MSLP" on the default Seasons skin or use another algorithm of your own choosing.