Problems with Barometric pressure values

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zefram47
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Re: Problems with Barometric pressure values

Post by zefram47 » Fri Sep 09, 2016 10:50 pm

With the new SmartHub firmware I have it set to Station Pressure. The corrections mentioned in post 20865 are still at play to get the WU and WFU numbers anywhere near accurate, i.e. making my station pressure read near correct with a pressure correction and an altitude correction to get the reported MSLP in the ballpark. It is abundantly clear that the mechanism that calculates MSLP for WU/WFU is not the same as reports to CWOP, as that is showing an MSLP that would be correct for that actual elevation I entered, where my station is actually quite a bit lower, but that elevation was used for WU/WFU accuracy (which still isn't quite right). If I were to put in my correct elevation, I believe CWOP would be correct, but WU/WFU would not. This is kind of ridiculous. Refer to the CWOP QC data for my station and station information. The MSLP data track reasonably with the analysis estimate, but obviously skewed high to to my need for WU/WFU to have the station report a higher elevation than actual. Here's the WU data for comparison. Just for further clarity, when I look in the Meteobridge interface under Live Data, my station pressure reads 24.08 inHg (815.5 mb) and the indoor pressure / MSLP reads 30.20 inHg (1022.6 mb), which is what it should be when comparing against station KBJC which is within 5 mi of my home and using the calculator referenced in the other post to get correct station pressure for the correction factors. Despite these readings being correct, CWOP is showing that I'm uploading data that is biassed higher by 8-10 mb.

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Re: Problems with Barometric pressure values

Post by joe002 » Fri Sep 09, 2016 11:44 pm

My CWOP numbers looked a lot like yours. I currently have my Pressure Correction set to -10.800 mbar/hPa and CWOP is now happy again.

My WU numbers look OK too. I'm reporting 29.86 and a nearby airport is 29.89.

I read somewhere, but can't find it now, that WU can/is performing some kind of adjustment to pressure numbers when they think they are off. Maybe you need to do what I did and adjust your numbers so CWOP is correct then wait a few days and see if WU comes back in line?

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Re: Problems with Barometric pressure values

Post by zefram47 » Sat Sep 10, 2016 5:26 am

I don't think that will work. For the moment, let's ignore any upload of weather data to one of the networks. If you only refer to the local display from Meteobridge, I can show that things aren't right. There is only one pressure reported via sensors, that of the SmartHub itself, reporting as station pressure in Meteobridge. I should mention that I do hold a degree in meteorology, so at least from a data and theory standpoint, I understand how to get from point A to point B.

Noting that I have a station, KBJC, located a few miles from my home, I can use their barometric pressure as a proxy for my own as Mean Sea Level Pressure, i.e. station pressure corrected to sea level given the station's elevation, in this case 5551 ft. My station is located at 6150 ft. Using this calculator you can enter your station's elevation in the first box. Enter the nearest station's MSLP in the box below that in either inHg or mb/hPa. Then hit calculate and you find in the boxes on the right, you get the proper station pressure in inHg and mb/hPa. That number is what we should see in Meteobridge as our station pressure...if it's not correct, we can use the pressure correction parameter in the config to adjust it. My station pressure, as reported in Meteobridge from the SmartHub is correct after using the pressure correction parameter.

Now, the seapress term, or MSLP calculation, uses the elevation config parameter to perform the correction to MSLP given a known equation. If that calculation is correct, then entering 6150 ft should result in the same MSLP that one entered in the box below the elevation in the calculator. If not, then there's a problem with either the calculator from the National Weather Service, or a problem with the code in Meteobridge. Using my station elevation of 6150 ft, I get 30.25 inHg, rather than the expected 30.43 inHg in the local display. Now, the CWOP reported value is 1028.4 mb or 30.37 inHg...much closer, but still not correct as it should be 1030.4 mb or again 30.43 inHg.

In order for my local Meteobridge display to show the correct MSLP given a correctly adjusted station pressure, I have to use 6550 ft station elevation, or ~500 ft higher than actual. Prior to updating to the SmartHub firmware, I required 6735 ft elevation to get reasonable numbers in the local display. My point being that the local display, and as a result WU/WFU/PWS upload, is not correct for MSLP given a correct station pressure correction and correct station elevation. With an incorrect local display of MSLP, CWOP upload is very close to the correct MSLP value and different from that on the local display and uploaded to other weather networks, not just WU.

If nothing else, I'd like to see the developer address the discrepancy between the CWOP uploaded data (raw APRS packets) and that of the local display, which is used to upload to all other weather networks. I don't care if WU modifies the uploaded data, though they shouldn't...the local display should show the correct conversion and the CWOP APRS packets should show the same data as that in the local display.

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Re: Problems with Barometric pressure values

Post by joe002 » Sat Sep 10, 2016 7:22 am

Like you, ignoring the data sent to the networks and the reporting of the stations near me, on my meteobridge display I'm currently seeing (with my correction of -10.800 mbar/hPa:

1008.7 (966.9) - 29.79 (28.55)

For my elevation of 1230 ft. I think your calculator is saying I should see:

1008.7 (964.7) - 29.79 (28.49)

Is that correct? One thing you are asking is why aren't the numbers on the meteobridge display the same as the calculator you linked, correct?


The other question/issue is that it appears the data that is transmitted to WU and CWOP don't seem to be the same as what you see on the display, correct?

I can't say I checked them before, but looking now I can say the pressure on my meteobridge is 29.80 (28.56), WU is showing 29.89, MyAcuRite is 28.88 WeatherBug is showing 29.80, local airport #1 is 29.91, and local airport #2 is 29.88 (the airports are about the same elevation as my station and I'm using the numbers reported by WU). I don't know how to see the current CWOP numbers as their posted data always seems to have a good sized delay (they are delayed 30 minutes for me right now). How do you see the current CWOP numbers? In any case, I don't know what meteobridge transmits to each service and what each service does to the numbers they receive.

When I first setup my meteobridge I actually set my elevation to zero and used the adjustment until CWOP said I was good to go. With this firmware upgrade I changed to supplying meteobridge with my actual elevation and made the additional adjustment to get CWOP happy again.

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Re: Problems with Barometric pressure values

Post by admin » Sat Sep 17, 2016 2:03 pm

1) WU expects sea level pressure, CWOP expects altimeter pressure. Both are slightly different.
2) In high elevations depending on used formulas sea level and altimeter might not be identical between the solutions you use. Davis computes different from Meteobridge (which should be rather close) and what Acurite is doing is up to them...
3) When using the aculink bridge with new firmware setting elevation to zero there is the way to go. MB computes altimeter and sea level pressures based on altitude given in MB

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Re: Problems with Barometric pressure values

Post by joe002 » Sat Sep 17, 2016 4:47 pm

I don't think we can change the AcuRite elevation anymore because it seems that it picks the elevation based on our location. I suppose you could say you are at some location that has an elevation of zero...

In any case, selecting "Station Pressure" will make it so the new firmware will not adjust the pressure that's coming out of the bridge - which should allow meteobridge to adjust the reported value based on the elevation we report to meteobridge.

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Re: Problems with Barometric pressure values

Post by admin » Sun Sep 18, 2016 9:23 am

Yes, you are right, telling Acurite bridge to stick at "Station Pressure" is the way to go.

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Re: Problems with Barometric pressure values

Post by woodmedic » Tue May 02, 2017 6:20 pm

So can someone put in laymans terms what I would need to do to get my pressures accurate for WU and CWOP when using a Davis vantage pro setup?

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Re: Problems with Barometric pressure values

Post by admin » Sat May 06, 2017 5:39 pm

Just set altitude correctly on Meteobridge "station" tab and you are done. WU will then receive SLP and CWOP altimeter, as they should.

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