Author Topic: CWOP pressure incorrect for VP2  (Read 1115 times)

openvista

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CWOP pressure incorrect for VP2
« on: June 27, 2017, 06:49:33 PM »
I know you are working on another bug report of mine regarding Synthetic Channel dropouts (thank you!), but I thought I'd pass along this one as it, potentially, affects more users.

Apparently, CWOP expects altimeter values for their "barometer" spec. Unfortunately, there is not a way to get this value out of Davis VP2 consoles as they only transmit mean sea level pressure (MSLP). For reference see: http://server.gladstonefamily.net/pipermail/wxqc/2006-July/004319.html. I found that link in CWOP's FAQs.

Because WeatherCat simply passes on MSLP to CWOP, sometimes I get data quality errors from MADIS, particularly during the cold months, because my pressure is too high.

Other software packages, like WeatherLink, are taking that MSLP value from the VP2 and working backward to get altimeter.

Note: this does not affect Vantage Vue users since the Vue console CAN be set to pass altimeter.

UPDATE 28 Jun: I found the Davis formula for MSLP (see pg 28-9): http://www.davisnet.com/product_documents/weather/app_notes/AN_28-derived-weather-variables.pdf
UPDATE 29 Jun: Found some open-source python code that converts various weather stations' pressure implementations to altimeter including the VP2!  The rub here is that to automate the conversion we would need a new tag 12HOURSAGO (similar to 24HOURSAGO) which currently doesn't exist in WeatherCat. I am in the process of mocking up a simplified PHP version of this script for the VP2 only (for use with the SQL driver to pull historical temps from db). For now, here's the original [attached].

I very much appreciate your efforts at continual improvement of WeatherCat!
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openvista

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Re: CWOP pressure incorrect for VP2
« Reply #1 on: June 27, 2017, 07:06:53 PM »
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Blicj11

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Re: CWOP pressure incorrect for VP2
« Reply #2 on: June 27, 2017, 07:42:57 PM »
Openvista makes a good point. My barometric data on CWOP consistently fails MADIS quality tests, even in the summer months for the reason he states. The only way I can get a pass from them is if I "calibrate" my console to make them happy. Unfortunately, that makes all of my other barometric reports incorrect. It would be very nice if WeatherCat could address that issue with its CWOP uploads.

Note: This issue isn't a WeatherCat bug and Edouard should probably move this thread to Feature Requests.
Blick


openvista

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Re: CWOP pressure incorrect for VP2
« Reply #3 on: June 27, 2017, 07:55:09 PM »
Note: This issue isn't a WeatherCat bug and Edouard should probably move this thread to Feature Requests.

Yes, I wasn't quite sure where to put it. However, I will say that if you accept the premise that altimeter is a "requirement" of passing pressure to CWOP, then it would be incorrect to pass MSLP. That being said, Davis doesn't make it easy to comply and puts the onus on software developers to correct their lack of configuration options in the VP2 console.
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elagache

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Moved to feature requests (Re: CWOP pressure incorrect for VP2)
« Reply #4 on: June 27, 2017, 11:45:40 PM »
Dear OpenVista, Blick, and WeatherCat "let's keep things tidy" types,

Note: This issue isn't a WeatherCat bug and Edouard should probably move this thread to Feature Requests.

DONE!
  :)

Cheers, Edouard  [cheers1]

Blicj11

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Re: CWOP pressure incorrect for VP2
« Reply #5 on: June 28, 2017, 03:32:47 AM »
Davis doesn't make it easy to comply and puts the onus on software developers to correct their lack of configuration options in the VP2 console.

100% in agreement.
Blick


xairbusdriver

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Re: CWOP pressure incorrect for VP2
« Reply #6 on: June 28, 2017, 04:02:44 PM »
Perhaps the fact that my station is within <100 feet altitude of the main airport as well as the NWS station allows my baro data be "close enough" for CWOP? OTOH, I'm not sure whether they are grading my data in any way. I don't see any 'stars' or warnings at my CWOP page.

WC reported Pressure: 30.03 inHg [-0.01 change in last report] (10:06 am CDT)
MEM METAR Altimeter: 30.08 inHg (14:54Z)
MEM METAR Sea Level Pressure: 1018.1 mb (14:54Z)

openvista

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Re: CWOP pressure incorrect for VP2
« Reply #7 on: June 28, 2017, 05:43:09 PM »
Perhaps the fact that my station is within <100 feet altitude of the main airport as well as the NWS station allows my baro data be "close enough" for CWOP? OTOH, I'm not sure whether they are grading my data in any way. I don't see any 'stars' or warnings at my CWOP page.

WC reported Pressure: 30.03 inHg [-0.01 change in last report] (10:06 am CDT)
MEM METAR Altimeter: 30.08 inHg (14:54Z)
MEM METAR Sea Level Pressure: 1018.1 mb (14:54Z)

Quality metrics aren't reported on the conditions page for your station on CWOP. Here's the link for your station to the MADIS quality report: http://weather.gladstonefamily.net/site/search?site=E7115;Get%20information=Get%20information.

Yes, at warm temperatures like you would experience (given your location) there probably won't be a large difference between altimeter and MSLP most of the year. But for those of us in colder regions, during the winter months MSLP really cranks up the pressure because unlike altimeter, it factors in temperature (colder air = denser).
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Felix

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Re: CWOP pressure incorrect for VP2
« Reply #8 on: June 28, 2017, 07:14:09 PM »
Click on those green thumbs, X-Air, for a percentage rating.


I don't pay much attention to Gladstone. I usually run at 100% QC but then occasionally a Red X will pop up on RH. And then a few days later, I'll be back in the green despite no changes whatsoever.


I get a note that it might be a siting problem but I doubt that since the location meets NWS standards. I'm adjacent to watershed protection lands on one side and a lake on the other, so we have a peculiar little microclime which Gladstone can't account for.

openvista

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Re: CWOP pressure incorrect for VP2
« Reply #9 on: June 28, 2017, 07:44:26 PM »
Yes, I don't concern myself with overall quality ratings either as they are largely dependent on nearby PWS's (many of whom have poorly sited or calibrated stations; garbage in, garbage out).  The only reason I mention it is because this is where you can most easily see the effect of sending bad pressure data consistently to CWOP. In particular, the comparison to ASOS stations -- in the "Neighboring Stations" section -- really shines a light on it.

After having calibrated my console barometer the other day, I decided to stop sending pressure data to CWOP. But I'd like to be able to turn that back on once I can feed them altimeter and use the MADIS data to compare my pressure readings to a couple nearby airports to check for accuracy (drift) going forward.
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openvista

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Re: CWOP pressure incorrect for VP2
« Reply #10 on: June 28, 2017, 08:49:19 PM »
Just edited the original post of this thread to include a link to Davis' MSLP formula.
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xairbusdriver

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Re: CWOP pressure incorrect for VP2
« Reply #11 on: June 28, 2017, 10:14:32 PM »
Thanks for the link! I probably have it some where in my 2500+ others! Need to prune/arrange those!! [computer]

HA! Obviously FAKE NEWS! Anyone observing my agricultural 'skills' would scoff at the implication that I have two green thumbs!!

Felix

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Re: CWOP pressure incorrect for VP2
« Reply #12 on: June 29, 2017, 03:26:32 AM »
Thanks for the link to that "Derived Variables in Davis Weather Products" document, openvista

That's a pretty dated document (5/11/06). Wonder if Davis has updated firmware since that was published? Anyone remember off hand the correct sequence of button pushes to get the console version?

Case in point regarding Wind Chill. Now the NWS wind chill calculator only works for temps at/below 50°F and wind speeds above 3 mph using the following formula: 






http://www.weather.gov/media/epz/wxcalc/windChill.pdf


Oh, and Davis forgot to list the current VP2 console version number (a/o 5/11/2006) in the Wind Chill section.


"An even newer version of this formula is available in WeatherLink version 5.6 or later and Vantage Pro2 console firmware version (not listed) and later."

openvista

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Re: CWOP pressure incorrect for VP2
« Reply #13 on: June 29, 2017, 11:04:25 PM »
Updated original post to include file with conversion code, including VP2 conversion to altimeter.
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WCDev

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Re: CWOP pressure incorrect for VP2
« Reply #14 on: June 30, 2017, 09:22:29 PM »
Great request, thanks - added to the feature request list  [tup]