Topic: How to Calibrate Pressure for Meteobridge and (GW1000 or ObserverIP)

[galfert]

Problem: If you have a Meteobridge and a GW1000 or an ObserverIP you may be wondering why the Sea Level Pressure does not match on both devices.

Quick Answer: The Meteobridge only pulls Absolute Pressure from your GW1000 or ObserverIP. That is by design as the Meteobridge uses your altitude to then calculate its own Sea Level Pressure. If you configure everything else correctly (on both devices) this problem is a non-issue.

Explanation: The GW1000 or the ObserverIP do not have a place to enter in your altitude. Your altitude is actually indirectly entered by the difference between your Absolute pressure and your Sea Level Pressure on your console (GW1000 or ObserverIP). You need to have the correct offset between Absolute Pressure and Sea Level Pressure to match your elevation.

The proper way to figure out the offset between Absolute Pressure and Sea Level Pressure is to calculate it yourself. I recommend using the following Keisan Casio online calculator (must use metric units):
https://keisan.casio.com/exec/system/1224579725

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If you are at 672 feet (204.826 meters) and you know the Sea Level Pressure to be 1016.0 hPa from your local METAR then your proper Absolute pressure should be 991.67 hPa or rounded to tenths is 991.7 hPa.
EDIT UPDATE: This advice is not the best in regards to comparing pressure to METAR. You only need to figure out the offset. It doesn't matter what the current sea level pressure is. What you want is an average offset for all year....so you leave the pressure on this calculator set to 1013.25 and you leave the temperature at 15°C. This will then give you the best offset for all year use.


You need to be sure that your GW1000 or ObserverIP have the correct Absolute Pressure (also called Station Pressure). This is important because this is the only pressure that the Meteobridge uses to then calculate Sea Level Pressure, as it is a derived value not a read value. The last thing you need to do is confirm that your Meteobridge has the correct altitude for your location under the Weather Station tab.

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Realize that you can double check that you have properly configured your Absolute Pressure. If you enter in to this other Keisan Casio calculator your Absolute Pressure and your Sea Level Pressure (Relative Pressure) it should give you your altitude. What this basically means is that between the 3 variables (Absolute, Relative, and Altitude) given any two of those you can figure out and calculate the remaining one.
https://keisan.casio.com/exec/system/1224585971

The other method that I don't recommend you do to figure this out without using the online calculator or other such tool is by trial and error and then you just keep changing your console Absolute pressure (that is the GW1000 or the ObserverIP) till the Meteobridge shows you the correct Sea Level Pressure. That is provided that you had already previously entered in your correct altitude in the Meteobridge under the Weather Station tab. Once you arrive at this proper Absolute Pressure on your console (GW1000 or ObserverIP) then you can fix the Sea Level Pressure on the console. So it is a two step process and this isn't a good method. I just stated this so that it would help to make sense of what is happening and how everything relates.

There is another method that may seem like a solution that I also do not recommend. You can zero out your altitude so that Absolute Pressure and Relative Pressure are the same (equal to each other). And you would do this on both devices. This has several other adverse consequences for proper tracking of pressure that I won't get into. Again I only bring this up because some people may think of this as a solution and I'm advising against it. If you've already done this on your station and it works for you, great. You are entitled to your opinion, and I to mine.

Tip: For an even more precise way to use you local METAR take a look at this other tutorial that I wrote. I called it Barometric Pressure 201 and it teaches you how to use isobars to increase the effectiveness of when to and not to use that local METAR and it also allows you to use a further METAR if a close one is not available.
https://www.wxforum.net/index.php?topic=36579.0

[galfert]

You may be wondering how to calibrate or fine tune the barometric pressure your console (GW1000 or ObserverIP). The answer is that after you have dialed in the the proper offset between Absolute and Relative pressure that corresponds to your altitude, never again touch the Relative Pressure offset difference between it and Relative pressure - which confusingly is actually something that would be change if you affected change directly to Relative pressure. Because doing so, is like changing your altitude, and you don't want that. What you do instead to cause Relative to go up and down indirectly is that you adjust accordingly your Absolute pressure up or down by how much off you are in the Relative Pressure reading. If you just move Absolute pressure up or down you'll notice that Relative also goes up or down by the same amount.

[shdwkeeper]

Thank you. How do you edit elevation on weather underground, I don't see it anywhere. Mine is way off.

[galfert]

Thank you. How do you edit elevation on weather underground, I don't see it anywhere. Mine is way off.

This is not possible. You have to create a new WU station ID. But then you lose your history, because it is a new station ID. Well you don't really lose it as you can have as many station IDs as you want, but it won't be contiguous information and it won't show up for others (unless they remember or saved the old station ID) as the old station will be offline but you can get to the data from the old station ID if you want.

Also WU has changed how new station IDs are created. It used to be that you could enter in your altitude. Now it is done automatically based on the map location you select. If this is your case then creating a new station wont help. Some people have reported that this new method of automatic altitude is way off.