Author Topic: Different Barometric Readings on Davis Console vs What is posted on CWOP  (Read 2674 times)

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Offline WheatonRon

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I have a Davis Vantage Pro 2. My station data is uploaded to CWOP every 5 minutes using Davis' WeatherLink software. At the same time, such data is uploaded using the same software to Weather Underground. Barometric pressure on the Vantage Pro 2 is taken from the console not the outside ISS. I just noted that my barometric pressure on my console, after uploading to CWOP is different than what is displayed and shown on CWOP. I was on the  phone with Davis support and they could not explain why, particularly, since Weather Underground has the correct reading! Can any one explain this rather odd situation!? My CWOP station is CW5020 and my WU station is KILWHEAT17.
« Last Edit: March 03, 2016, 04:15:11 PM by WheatonRon »
Davis VP2 with Daytime FARS, SHT31 (2 complete systems-1 for uploading to the internet, the other system for test and play); Rainwise 111; CWOP--CW5020; WU--KILWHEAT17; CoCoRaHS--IL-DP-132

Offline 92merc

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First off, the ISS doesn't have a barometer in it.  It only resides in the console.

Second, its likely your software isn't sending the same "type" of barometric reading.  I forget the differences.  Someone with with more Weatherlink experience could explain it better.  But there are different methods for calculating pressure.  Some don't take into consideration changes above sea level.  But I'm sure what ever WL is sending, isn't the same as CWOP is expecting.
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Offline ValentineWeather

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WeatherLink is uploading altimeter which CWOP and most others require and use. The VP2 console is showing MSLP which few use other than possibly meteorologist.

The VUE console actually has an option for selecting altimeter to show on console unlike the older VP2 console. Because of this I use the VUE for my website. I've seen as much as .20 hundredths inches of mercury difference. The higher your elevation the larger the likely difference. Those below 1000' see less difference except during very cold weather.
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Offline ValentineWeather

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Here is an example with high elevation how messed up it can be and look. Flagstaff AZ. 7000' elevation: SLP 1019mb (30.09) inches vs altimeter (30.25).
Difference is (.16 inches)
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Offline WheatonRon

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Different Barometric Readings on Davis Console vs What is posted on CWOP
« Reply #4 on: March 03, 2016, 05:56:33 PM »
Why does CWOP report a different reading than what WU, my console and Weatherlink show? The latter three are the same. CWOP is different. Does someone have a contact at CWOP now that Russ Chadwick has retired?
« Last Edit: March 03, 2016, 05:58:40 PM by WheatonRon »
Davis VP2 with Daytime FARS, SHT31 (2 complete systems-1 for uploading to the internet, the other system for test and play); Rainwise 111; CWOP--CW5020; WU--KILWHEAT17; CoCoRaHS--IL-DP-132

Offline ValentineWeather

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Why does CWOP report a different reading than what WU, my console and Weatherlink show? The latter three are the same. CWOP is different. Does someone have a contact at CWOP now that Russ Chadwick has retired?

Maybe I wasn't clear. Weatherlink uploads altimeter to CWOP. The rest are getting SLP which is what you see on the console.

From Davis: CWOP prefers to receive barometer data in the form of altimeter. Altimeter is the CWOP
pressure standard because it is the simplest pressure reduction format that most CWOP
members can reliably deliver.
WeatherLink versions 5.7 and later automatically convert Vantage Pro and Vantage Pro2
barometer data to altimeter.
Randy, the Aviator is my father in 1963 with his Indian bike

Offline WheatonRon

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Different Barometric Readings on Davis Console vs What is posted on CWOP
« Reply #6 on: March 03, 2016, 07:11:14 PM »
Sorry, but I am a retired CPA, so "weathering" is only a hobby that sometimes I struggle with. When you say uploading altimeter, what does that mean? I am using Weatherlink 6.0.3. I am surprised that Davis support did not identify that as the cause. I spent over one hour on the phone with John from that group, and he could not explain it--he was looking at my data on CWOP and WU and I was telling him what was on my console. He was "perplexed." As you know, CWOP "grades" our weather data and tells us where our data might be off. If I correct my barometer readings (using nearby large airports here in Chicago) that get uploaded to CWOP to get closer to their "grade," the display on my console will be wrong and my data on WU will be off so I guess I will accept CWOP as being close but not precise.
« Last Edit: March 03, 2016, 07:15:10 PM by WheatonRon »
Davis VP2 with Daytime FARS, SHT31 (2 complete systems-1 for uploading to the internet, the other system for test and play); Rainwise 111; CWOP--CW5020; WU--KILWHEAT17; CoCoRaHS--IL-DP-132

Offline ValentineWeather

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No problem, What you could do is compromise, go into console and set elevation to 0'. Set the barometer on console to match nearby airport altimeter this will make all readings the same and you will be close to correct altimeter.

I would use O'Hare its your closest source.
 
http://mesowest.utah.edu/cgi-bin/droman/meso_base_dyn.cgi?stn=KORD&unit=0&timetype=LOCAL
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Offline WheatonRon

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Great suggestion. Thanks.
Davis VP2 with Daytime FARS, SHT31 (2 complete systems-1 for uploading to the internet, the other system for test and play); Rainwise 111; CWOP--CW5020; WU--KILWHEAT17; CoCoRaHS--IL-DP-132

Offline C5250

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When you say uploading altimeter, what does that mean?

A VP2 Console displays Sea Level Reduced Pressure which tries to adjust the pressure for the temperature of the column of air above it; CWOP expects altimeter pressure which is only corrected for altitude above sea level. The difference is minor if you are at a lower altitude, but becomes greater at higher altitudes.

Neither is "wrong", they are simply calculated differently and therefor the result is different.

Years ago, people reporting to CWOP with Davis stations got dinged for this on their barometer quality, but a few years back Davis changed things so that VP2 station could report altimeter pressure to CWOP. Before then, setting your altitude to 0 and setting an offset for the barometer was a common procedure to avoid this issue, some software also "undid" the SLRP calculation to derive altimeter.
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Offline WheatonRon

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Different Barometric Readings on Davis Console vs What is posted on CWOP
« Reply #10 on: March 03, 2016, 11:29:27 PM »
This forum is great. The above guidance was not known by Davis when I talked with them for 1 hour today. Separately, I bought a new ISS for my Vantage Pro 2 with purchasing help from other forum users who suggested buying from Scaled Instruments, a company that offers great products at great prices! Thanks to all who add such meaningful content to this forum!
« Last Edit: March 03, 2016, 11:35:39 PM by WheatonRon »
Davis VP2 with Daytime FARS, SHT31 (2 complete systems-1 for uploading to the internet, the other system for test and play); Rainwise 111; CWOP--CW5020; WU--KILWHEAT17; CoCoRaHS--IL-DP-132

Offline ValentineWeather

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Something else about the Madis CWOP QC check , you have a very bad station nearby that could affect your numbers. Many have complained about this and why I hate half ass stations reporting to CWOP. Altimeter is a major player so take the QC graphs with a grain of salt.

 This is a bad station very close by. http://weather.gladstonefamily.net/site/D9474
« Last Edit: March 06, 2016, 08:47:59 PM by ValentineWeather »
Randy, the Aviator is my father in 1963 with his Indian bike

Offline WheatonRon

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Different Barometric Readings on Davis Console vs What is posted on CWOP
« Reply #12 on: March 04, 2016, 07:52:09 AM »
Yes, you are right. This station (DW9474) should be removed from CWOP as its weather readings are clearly not in line with surrounding stations or "reality."
« Last Edit: March 04, 2016, 07:57:49 AM by WheatonRon »
Davis VP2 with Daytime FARS, SHT31 (2 complete systems-1 for uploading to the internet, the other system for test and play); Rainwise 111; CWOP--CW5020; WU--KILWHEAT17; CoCoRaHS--IL-DP-132


Offline ValentineWeather

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Later versions from WeatherLink 5.7 on, already automatically convert Vantage Pro2 pressure uploads to CWOP over to altimeter.
 
Original question was concerning why WU & console showed same pressure but CWOP was different. 

There is no absolute fix getting all 3 to show altimeter, compromise is setting station to 0' elevation and matching nearby accurate altimeter. This gets you in the ballpark but when pressure goes very high or low it will still be off slightly.
 
The VUE has the fix and works very well, unfortunately Davis hasn't added this feature to the VP2 console. And from what I can tell their front-line help seems rather oblivious to the issue.

« Last Edit: March 04, 2016, 12:39:29 PM by ValentineWeather »
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Offline CW2274

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There is no absolute fix getting all 3 to show altimeter, compromise is setting station to 0' elevation and matching nearby accurate altimeter. This gets you in the ballpark but when pressure goes very high or low it will still be off slightly.
After trial and error, I found that this is the best solution. I'm at 2400', and have the elevation set for zero. The most I'm ever off is .03", and that's in really high or low pressure. In any case, try to set the altimeter in relatively benign WX conditions. (ie little wind, no fropa, ect) and when the reading is around 29.92", which is standard atmospheric pressure.
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Offline WheatonRon

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Terrific idea. Will try this. I sent a copy of my question and a link to this Forum to Davis support. They should be thankful, I think, of the "work arounds" folks are using to address our frustrations as amateur weather folks! Thanks again to this forum for providing great discussion topics and resolutions!
Davis VP2 with Daytime FARS, SHT31 (2 complete systems-1 for uploading to the internet, the other system for test and play); Rainwise 111; CWOP--CW5020; WU--KILWHEAT17; CoCoRaHS--IL-DP-132

Offline WheatonRon

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The suggestion to set my Davis Vantage Pro 2 elevation to zero instead of actual, then adjust my console's barometric pressure to match a known quantity (O'hare airport in my case) solved my issue. Thanks to all for the suggestion. Question for the forum group. Will this "tweak" affect my other readings posted on WU or CWOP, like dew point, outside temperature or humidity? I believe not as such readings come from the ISS not the console, but any views would be appreciated!
Davis VP2 with Daytime FARS, SHT31 (2 complete systems-1 for uploading to the internet, the other system for test and play); Rainwise 111; CWOP--CW5020; WU--KILWHEAT17; CoCoRaHS--IL-DP-132

Offline CW2274

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The suggestion to set my Davis Vantage Pro 2 elevation to zero instead of actual, then adjust my console's barometric pressure to match a known quantity (O'hare airport in my case) solved my issue. Thanks to all for the suggestion. Question for the forum group. Will this "tweak" affect my other readings posted on WU or CWOP, like dew point, outside temperature or humidity? I believe not as such readings come from the ISS not the console, but any views would be appreciated!
None whatsoever.
Davis Wireless VP2 SHT31 24hr 67CFM FARS
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Offline WheatonRon

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Then Davis would do everybody a favor, since they don't want to create a Vantage Pro 3, to have users do exactly what I did as suggested by this great and helpful forum rather than its directions in the console manual.
Davis VP2 with Daytime FARS, SHT31 (2 complete systems-1 for uploading to the internet, the other system for test and play); Rainwise 111; CWOP--CW5020; WU--KILWHEAT17; CoCoRaHS--IL-DP-132

Offline ValentineWeather

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The altimeter will still be slightly off doing it this way (.03-.04) when pressure gets real high or low. Its just a compromise. We still need the actual conversion already on the Vue.
Randy, the Aviator is my father in 1963 with his Indian bike

Offline WheatonRon

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I assume you are talking about an error in inches rather than mb?
Davis VP2 with Daytime FARS, SHT31 (2 complete systems-1 for uploading to the internet, the other system for test and play); Rainwise 111; CWOP--CW5020; WU--KILWHEAT17; CoCoRaHS--IL-DP-132

Offline ValentineWeather

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Yes inches, something I noticed over the years of using the VP2.
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Offline CW2274

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Then Davis would do everybody a favor, since they don't want to create a Vantage Pro 3, to have users do exactly what I did as suggested by this great and helpful forum rather than its directions in the console manual.
Well, the VP2 console is not intended for the altimeter setting, per se, it's designed for MSLP, or whatever. I've used the altimeter setting all my life for pressure readings, as does basically the whole country. So that being said, the manual is not wrong. If they wanted to add a "addendum" saying this is best way for the altimeter, then so be it. The obvious and correct solution is to make the software available to the VP2 console as it does the Vue, especially since the VP2 is their flagship.
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Offline C5250

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Yes inches, something I noticed over the years of using the VP2.

Yup, it'll drift a few hundreds of an inch long term, far better than how much the drift will be using SLRP. I did such soon after joining CWOP in 2006, haven't touched it since  and rarely see a bad barometer QC. Currently my QC page says my average error is 1.6 mb with a "worst" error of 2.9 mb (+/- 3 mb is excellent as far as MADIS is concerned).
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