Some tips to help sort out some of the mystery.
- The elevation of the METAR is not something to be concerned with when getting their QNH (sea level adjusted pressure).
- Relative pressure = Sea Level pressure
- Absolute pressure = your local station pressure
Therefore if you merely make it so that your Relative pressure is the same as the METAR QNH pressure you are done. But that is the easy way out and it is incomplete because you are then ignoring if your Absolute pressure is also correct or not. Also if you are uploading data to some online services like CWOP where Altimeter pressure needs to be calculated from a correctly set Absolute pressure then you are going to have problems if you ignore also setting the Absolute. Or even for your own knowledge even if you aren't uploading to CWOP, you may just want to be thorough and also get the Absolute dialed in.
To get both Relative and Absolute pressure dialed in you need to do this in two steps. First step is to figure out by how much your Relative and Absolute should differ by. This difference is constant for our weather stations. Technically speaking it isn't really constant but we need to figure out an average that works for all year so that you don't have to constantly be messing with calibration. Meaning that at some point the margin of error is acceptable because it will be plus/minus within some small range. I recommend using the Keisan Casio online calculator to figure out by how much that average difference should be between your Relative and your Absolute. This difference is directly related to your elevation. Therefore you need to use a good reference to find out at what total elevation your barometer is. I use Free Map Tool Elevation Finder to get your elevation. Just enter in your Latitude and Longitude and you'll have your elevation. But you must remember to add to that the elevation of your barometric sensor....which is not outside. Your barometric sensor is in your house either in your display console or in the indoor temp/humidity/barometric sensor depending on which model station you have. If your barometric sensor is on the first floor or second floor will make a bit of a difference so just add the appropriate height to the elevation you get from the Free Map Tools site. Then you'll just need to enter this total elevation into the Keisan Casio calculator and see what the resulting Atmospheric Pressure is. Do not change the default temperature and do not change the default Sea Level Pressure. The reason you do not change these default values is because we are looking for an average difference between Relative and Absolute for your elevation. We don't want to know what it is now...we need a difference that will work for all year...forever. Once you click on the Execute button all you have to do is subtract the 1013.25 hPa from the resulting Atmospheric pressure....and that is your difference. You'll then dial in that offset into your Relative pressure of your station. Then never again touch your Relative directly. That completes step 1. The next step is to adjust the Absolute until it affects the readout of the Relative so that the Relative equals the current METAR. That is it! You are now 100% calibrated.
Reference links:
https://www.freemaptools.com/elevation-finder.htmhttps://keisan.casio.com/exec/system/1224579725Long term hints: Keep an eye on your Relative after performing the above steps. You can do this initial calibration at any time and then fine tune it later. The best time to fine tune is when the pressure is stable and on a nice clear day when the METAR is close to 1013 hPa.
