Barometer/Altimeter Discrepency

[NorCal Dan]

My altimeter reading is much higher than the CWOP MADIS analysis line.
http://weather.gladstonefamily.net/cgi-bin/wxqchart.pl?site=D1695

I have spent days researching the problem.  The closest ASOS station to me is reporting similar altimeter readings.  But all other ASOS stations are reporting much lower altimeter readings.  Unfortunately, none of them are at or near my altitude.
http://weather.gladstonefamily.net/site/D1695?asosonly=1;tile=10;days=7#Data  
All the ASOS stations are listed at the bottom of that page.

Another problem I have found is that when WD is set to send altimeter data to CWOP it is NOT adjustable.  WD uses the raw station pressure (SLP) to calculate the altimeter pressure.  The only way I can think of to make any adjustment is to change the altitude setting in the Vue console as none of the offsets in WD will affect the altimeter reading.

My conclusions -- Either my Vue console barometer needs to be adjusted (not sure its possible or how to do it), or, all the stations I am being compared with are NOT sending true altimeter pressure readings.  I am NOT an expert, so if anyone has an idea I am all ears.


Add: WUnderground stations and barometer readings::

Condon Park - 29.80  (2500')
Banner Mtn. - 29.85  (3280')
Forest Knolls - 29.78  (3196')
Cedar Ridge - 27.20  (2800')
Cape Horn Colfax - 29.83  (2850')

Assuming these are NOAA barometer readings then my station showing 29.81 at 2428' is not out of line.

[KeithW]

Personally, I do not pay any attention to the Gladstonefamily.net analysis; I think he is
way off or there is some error in the calculations. For example, my temperature and baro
readings nearly exactly track his "analysis", yet my station is flagged with his big red X,
while I get two thumbs up from MADIS.

I just don't bother trying to adjust my station data to match his "analysis". FWIW and YMMV,
of course.

-Keith

[NorCal Dan]

Don't get me wrong...I am NOT adjusting my data to match the analysis.  AFAIK, my data is correct and everybody else is wrong  

Besides the closest ASOS airport matching my data I did find one other station that is close to my readings.  I emailed a bunch of the closest stations and received 4 replies so far...all but 1 claim to be sending altimeter pressure...and 1 guy is running the original VP under Lightsoft Weather Center software on a Mac computer and all he set was his altitude on the console, he's not sure the software is sending altimeter pressure.

I am going to contact DAVIS and see how much it would cost to NIST my console and sensors, or swap out my station for a NIST station, not sure how they do it or if that is even an option with the Vue.

[niko]

Just to put this in perspective, if I read the spec correctly the NIST calibration will get it within +/- 0.03 inch Hg, which I calculate as +/- 1 millibar, and the discrepancy in CWOP is approx + 2 millibar.

[niko]

I checked the quality reports for the 10 nearby sites listed on your CWOP analysis:

These 5 are worth something:

AR335, the only station that matches the analysis plot.
CW5488 the plots are OK, station readings low.
CW0713 the data has some problems, but the trend is that the station readings are high.
CW1789 plots are OK, station readings low.
KAUN plots are OK, station readings are high - but that's Auburn airport, and if that doesn't match the analysis

The other 5 are (IMHO) not worth much:

AS856 the analysis curve is ridiculous.
DS2319 the analysis curve isn't much better.
C1397 the analysis and data plots are bad.
AT999 the data is bad.
CW6842 analysis plot is bad.

I understand your frustration with this, but I guess I wouldn't lose too much sleep over it myself 

[NorCal Dan]

Stations I emailed include: AS856-D2319-C1397-AT999-C5488-AR335-C0713

I heard back from C1397, AR335, C5488, and C1789 (my old station) and all report to be sending altimeter data except C5488.  Philip Gladstone seems to think KAUN is suspect and needing calibration.  Go figure...

Yea, NIST probably isn't worth what ever Davis is charging nowadays...

[NorCal Dan]

I heard back from Davis today:

1)       Is there such a thing as a NIST certified Vue console (barometer only)?  If so, can I trade my console for a NIST model and what is the cost?

Yes and the cost is $95.00

http://www.davisnet.com/weather/products/weather_product.asp?pnum=06030

 
2)      Is the Vue barometer field adjustable?  If I determine my barometer is wrong, can I calibrate it myself, either raw or SLP

Yes.  I would first confirm the elevation is set correctly within the setup of the console.  Press & release the 2nd key.  Press & release the <done> key.  Press the <Bar> key nine times to display the current Elevation.  Use the +/- , </> to set.  Press and hold <done> for two seconds to exit setup.  To calibrate your Barometer press the <bar> key.  Press and release the <2nd> key.  Press & hold the <graph> (set) key and after the 4th flash of the current barometer reading release the key and you then manually set the barometer.  When finished press and release the <done> key to get back to the main weather screen. 

From the Davis site:

NIST Traceable: Barometer

Vantage Pro2/Vantage Vue: ± 0.03" Hg (± 0.8 mm Hg, 1.0 mb, or 1.0 hPa)

[NorCal Dan]

Is anybody here using the WeatherLink-IP to send data to CWOP?  As you know, I have contacted the stations local to me as I work to find the problem with my altimeter pressure readings.  One of the stations is using the WL-IP combo and his data appears to be flip-flopping between sending altimeter and SLP pressure.  Here's a snapshot showing his data (AT999), my data (D1695), and the gladstone analysis line:



I do not know what version of WL he is running, and from what I've read online, only the latest version correctly sends altimeter pressure.  But with the IP logger doesn't the data go to Davis first and they send it out to CWOP?

Note the negative spike in my data is when I rebooted my machine after doing a Mac software upgrade.  The first data send to CWOP was sent as SLP and not altimeter.

[niko]

I do not know what version of WL he is running, and from what I've read online, only the latest version correctly sends altimeter pressure.  But with the IP logger doesn't the data go to Davis first and they send it out to CWOP?

If he is actually running WL (maybe not continuously) with a WLIP, I wonder if the spiking is his baro going to CWOP two ways.

WLIP -> weatherlink.com server -> CWOP
WLIP -> weatherlink software -> CWOP

[NorCal Dan]

It's possible but I do not know his setup.  I'm now hearing on the wxqc mailing list that WL-IP -> weatherlink.com -> CWOP always sends SLP.  Apparently the Davis site is the broken piece in the puzzle.

[NorCal Dan]

He emailed me today and it appears that he may have 2 data sends to CWOP, one from weatherlink.com (from his IP datalogger) and the other from his local copy of Weatherlink.  I should know more tomorrow and hope I can get at least his station and my station calibrated correctly and reporting altimeter to CWOP.

FWIW, none of the other station owners have replied to my last email, but I am noticing slight changes in a couple stations altimeter data.  I suspect they are adjusting their barometers to match that stupid analysis line.

I've been contemplating if I should adjust my console barometer, or try to find $95 and send my console in to get NIST certified, barometer only...I hate not knowing if my data is correct...

[NorCal Dan]

I decided to save my money.  I am going to try and match up with the KAUN ASOS station by lowering my console barometer.  The problem I have in trying to re-calibrate my console is finding the correct SLP.  I decided to use only NWS for my calibration.  The Air Force base (KBAB) is showing SLP of 1007.5, and they are at about 100' above sea level.  The only other showing on the NWS map is Blue Canyon (KBLU) at 1008.0 at 5200' above sea level.  The bad news is my station, at 2428' above sea level, is showing SLP of 1006.9, so in theory I should be adjusting my station barometer even higher.  Obviously that is NOT going to help my altimeter reading that CWOP says is already too high.

So for now I have adjusted my console barometer -1.0 hPa mb and will monitor to see how it affects my readings on CWOP.

[niko]

How far away is KAUN? Maybe take a quick drive there with your console 

[NorCal Dan]

According to CWOP, 17.3 miles.  I could do that, maybe on day they are open, and talk to the field manager.  I still wonder why CWOP is not showing more faith in the ASOS readings, at least over all the other personal weather stations.  I have no idea how gladstone accounts for the differences in altitude of all the stations.  The analysis line is tracking the stations in the valley very closely, and I suspect everybody up here in the hills has adjusted their stations to meet that analysis, hence they are all reading low.  And without someone at gladstone taking a hard look at the data I doubt the situation will ever change.  Obviously one lone complainer is not going to get anything done 

[lddaly]

Note the negative spike in my data is when I rebooted my machine after doing a Mac software upgrade.  The first data send to CWOP was sent as SLP and not altimeter.

Hmmm... you are on the Mac version? Windows here, but I upgraded to v10.37Q-55 from build 51 this evening and my first data sent to CWOP was SLP and not altimeter. I have not noticed this before.
APRS,TCPXX*,qAX,CWOP-2:@100233z3306.25N/09645.10W_146/007g010t093r000p000P000b10068h39L0000.WD 31
APRS,TCPXX*,qAX,CWOP-2:@100243z3306.25N/09645.10W_164/007g013t092r000p000P000b10086h40L0000.WD 31