# Can't Fix WeeWX Pressure Difference

This seems to come up periodically, but I’ve been scouring these forums and weewx discussion group, and can’t find a good solution. I’ve recently installed my Tempest and I’m using WeeWX to upload to PWS Weather. I understand the difference between station pressure and sea-level pressure, but I can’t get the WeeWX and PSW Weather pressure (reported as barometer/sea-level pressure) to match what is shown in the WeatherFlow app and web page (and sent from WF to Weather Underground).

The WeeWX/PSW Weather barometer value is always about 0.1-0.2 inHg lower than the WF Pressure (sea level) value. For example:
WF “Pressure”: 30.003 inHg
Weather Underground: 30.00 inHg
WeeWx “Barometer”: 29.925 inHg
PSW Weather “Pressure (MSLP)”: 29.84 inHg
WF “Station Pressure”: 28.972 inHg

At first, I thought this might be an elevation vs. elevation+AGL issue. My station is 957 ft above sea level, and 8 ft above ground level. At first, in WeeWX and PSW Weather, I reported the elevation as 957, but then changed it to 965 (957+8). Still, no luck.

Is this just a basic difference in how the services are calculating SLP, even given the same station pressure and elevation (and temp, humidity)? If not, what should I fix?

What is the right answer for what to report as ‘elevation’ in WeeWX and PWS Weather? The WF ‘Elevation’ or elevation plus height above ground?

You want to use your actual elevation of the sensor, but if you are +/- 10 feet it’s not going to make much of a measurable difference. Just be consistent in whatever you use in WF and weewx.

If you want to do the math, the WF published formula for correction is at https://weatherflow.github.io/SmartWeather/api/derived-metric-formulas.html#sea-level-pressure

There’s a bit of a weewx discussion at https://github.com/weewx/weewx/wiki/Barometer,-pressure,-and-altimeter as well.

But short answer is weewx saves SLP based on station measurement and the altitude you put in weewx.conf there. The WF gear seems to report station pressure in the UDP. The math lined up for me here when I compared the two a lot of months ago.

But I can’t speak to what WF’s app/site or PWS does (or does not do) after it gets the data from your station.

Thanks for those thoughts — I tried the math, but it still doesn’t work out. WF actually reports both station pressure and sea-level pressure, and it seems to be using that math to adapt.

It looks like the WeeWx driver uses the station pressure, and WeeWx must be converting it on its own to sea-level pressure, as the number it reports doesn’t match either WF station pressure or sea-level pressure.

Anyone know how (or if) I should re-do the config so that it uses the same calculation?

At the bottom of the weewx config you can choose to use station hardware or software for the various readings - might help your issue. Mind you i use software and make sure the station higher / location is the same setting as in the WF app.

Andy

When I first setup WeeWX I saw exactly the same behaviour.

This is what I pinned it down too. There are multiple ways of calculating sea level pressure from station pressure. From what I can tell, WeeWX uses observed station pressure and corrects for altitude, temperature, and (frequently) humidity. In contrast, the WF algorithm uses observed station pressure but only corrects for altitude (it assumes an atmospheric temperature profile). Given these differences, I find that SLP reported in the WF app doesn’t match SLP reported in WeeWX (the `barometer` field), especially in warmer temperatures.

To get a close match, you can use the `altimeter` field in WeeWX instead of `barometer`. By default this gives a much closer value to SLP reported in the WF app. To make it even closer, I changed the `altimeter` algorithm in my `weewx.conf` file to ‘aaWOB’ by adding the following to the `[StdWXCalculate]` stanza:

``````    [[Algorithms]]
altimeter = aaWOB
``````

I used the WF elevation plus height above ground

Thanks so much! That is really helpful detail. Will just stick with things as they are then.

I appreciate your thorough response! I’ll give that Altimeter config change a shot, too.

Thanks again.