Topic: Davis Vantage Pro 2 "Barometer" and CWOP

[NiceBill]

Hi Gang,

  I am doing a study as to how my Davis baromiter and MADIS get along.  I started this study a bit late in the month of Aug., but I plan on going through the winter.  It should prove to be interesting.  The month of Aug. went quite well. I hope you can read this report, I will add a mo. by mo. update to this post.

Bill.>>>>>>>>>>>>>>>>> 

This is a study with 4 of the stations used by MADIS QA to reflect my % of accuracy, of  barometric pressures reported. The barometer in use is a Davis, enclosed in an Envoy.  The stations are located at airports.  Bloomington-KBMI 20 mi.. N.,   Champaign-KCMI 30 mi. E.,   Decatur-KDEC 20 mi. S.,  Lincoln-KAAA  20 mi. W.   ME-CW9625. No sensors have been or wil be changed or recalibrated during the study period. 8-2008    A=average  E=error  Mb.


DATE   Bloom.   Champ.   Dec.   Linc.   Me   3 Day %   A --- E
8-1   N/A                   
8-2   N/A                 
8-3   N/A                 
8-4   N/A                 
8-5   N/A                 
8-6   N/A                 
8-7   N/A                 
8-8   START 8-9                 
8-9     29.99   29.99   30.00   30.01   30.01   100    0.5~0.5
8-10   30.02   30.01   30.01   30.02   30.04   100    0.2~0.3
8-11   30.08   30.06   30.08   30.09   30.11   100    0.2~0.3
8-12   29.93   29.94   29.94   29.93   29.97   100    0.2~0.4
8-13   29.85   29.86   29.86   29.86   29.87   100   -0.3~0.4
8-14   29.88   29.87   29.89   29.90   29.92   100    0.1~0.5
8-15   30.14   30.13   30.13   30.13   30.15   100    0.4~0.3
8-16   30.14   30.14   30.15   30.14   30.14   100    0.1~0.4
8-17   30.12   30.14   30.14   30.14   30.14   100   -0.4~0.4
8-18   30.04   30.06   30.07   30.06   30.07   100   -0.5~0.4
8-19   30.09   30.11   30.11   30.10   30.12   100   -0.1~0.3
8-20   30.11   30.11   30.10   30.10   30.11   100   -0.0~0.2
8-21   30.03   30.06   30.03   30.02   30.02   100    0.1~0.4
8-22   30.03   30.05   30.04   30.04   30.04   100    0.2~0.5
8-23   30.09   30.12   30.09   30.10   30.09   100    0.7~0.3
8-24   30.02   30.00   30.01   30.03   30.01    99     0.8~0.4
8-25   30.01   29.99   29.97   30.00   30.02    99     0.6~0.7
8-26   29.95   29.93   29.93   29.94   29.96    99     0.6~0.6
8-27   29.88   29.88   29.88   29.88   29.88   100   -0.2~0.3
8-28   29.80   29.83   29.82   29.80   29.81   100    -0.2~04
8-29   29.97   29.96   29.97   29.99   29.97    99     6.2~0.5
8-30   30.18   30.15   30.15   30.16   30.18    99     0.3~0.6
8-31   30.19   30.17   30.14   30.14   30.15   100    0.2~0.6

All readings taken at noon.  W.H.S.

 
 
 

[tinplate]

It's very important that you compare apples to apples. Airports report different barometer values for different purposes. One value is Sea Level Pressure (SLP). The other is altimeter. The VP console supplies (and displays) only SLP. CWOP, however, wants altimeter, so a number of software programs that submit to CWOP now convert the VP's SLP into an altimeter value and send that. Unless you are at sea level elevation, SLP and altimeter will be different values for the same location. For a comparison like you're doing, it would be best to compare altimeter. One reason is that the formula the davis console uses to convert the raw sensor pressure into SLP is different than the formula used by the equipment at the airports. This is because the NWS formulas are more involved, and contain custom correction factors for each location based on many years of historical data.

Steve
SoftWx

[NiceBill]

Hi Steve, thank you for your reply.

I use the info. that is posted on my web page.

http://www.texasilweather.com/

That is SLP. However it gets to CWOP and converted to what ever they wish to do with it is fine with me.
I know one thing.  The NWS station 20 mi west of me, adjoins the airport property in Lincoln IL.  The NWS station has 3 barometers they use.  They take the average of the three and that's what the corrected SLP is at the time of the reading.  The pressure value at the NWS station is the same as the reading the airport reads.

I have checked the airports pressure and the NWS stations report many times and they are the same. I am the same at times and some times +- 0.01.   

I don't think my comparison is to far off.  I don't think CWOP is using SLP and altimeter readings at the same time to evaluate some ones station.
The graphs that are displayed on ther QA page are reflecting SLP.


Bill.>>>>>>>>>>>>>

[George Richardson]

Bill,
You ought to get VPLive from SoftWx to evaluate while you are doing a barometric pressure test. The price is right and it has 13 values reported relative to barometric pressure. I went through this barometric pressure morass when I first installed my VP2. Steve knows what he is talking about.
FWIW
George

[NiceBill]

Thanks for the note George, I do understand.

I have two Calibrated Aneroid Barometers, at a cost of $400.00 each that I also use.  The Davis barometer is dead nuts on with them. My interest is in why MADIS will vary so much at times.  I have had my evaluation as low as 68%, so now I am watching it.

What I find very interesting, although I am not recording it, is the fact that I can receive a 98% evaluation for 7 days, however for the 3 day evaluation during the 7 day period I get a 100% evaluation.

edit:  Maybe I should note:  for 3; 3 day periods in a row I get a 100% report, during the same 7 day period at 98%

Bill.>>>>>>>>>>>>

[tinplate]

Here's an example of what I was talking about. Take the following line in your data
8-29   29.97   29.96   29.97   29.99   29.97    99     6.2~0.5

At noon, you have Bloomington at 29.97 and you at 29.97

The actual data for Bloominton at their nearest observation to noon is
11:56 SLP 29.97, altimeter 30.01
so from this, we can see that the values you're collecting are SLP for this airport.

The actual data MADIS got from your station was
11:56 SLP 29.94 altimeter 29.97 (they got the altimeter value, or what they expected was altimeter, and SLP is calculated from that)

From this, it appears that you are sending SLP to CWOP. CWOP is expecting altimeter, so when it takes your pressure value and converts it to raw and SLP, the result skews the data for your station. From your viewpoint your pressure perfectly matches Bloomington airport, and that is in fact the case. But because you're sending SLP to CWOP/MADIS, youre forcing it to compare apples and oranges. From MADIS's viewpoint your data is .04 inHg different than Bloomington. If your software had calculated altimeter from your console SLP reading of 29.97, it would have sent an altimeter of around 30.01.

The drift you describe, where sometimes your data matches, and sometimes it doesn't, perfectly describes the symptoms of sending SLP to CWOP/MADIS. This is because the algorithm for calculating altimeter from raw sensor pressure uses only elevation and pressure. The SLP calculation however, uses elevation, pressure, temperature, and humidity. So, at some temperature/humidity your SLP will equal your altimeter. But as your mean temperature goes above and below that, your SLP will diverge from your (and nearby stations') altimeter. You'll notice this most in the short term when fronts pass through, causing rapid temperature differences from one day to the next. You'll also notice a larger seasonal drift from winter to summer. The best solution is to send altimeter to CWOP, since that's what it expects. Another solution is to watch the QC of your station's pressure drift relative to the airports, and play a never-ending game of adjusting your console barometer to match the airport's as your seasonal drift grows. The fact that you're having this issue made me guess you're using VWS, and looking at your raw CWOP data, that is in fact the case. Most of the other weather programs now correctly calculate and send altimeter for the VP stations.

bloomington link
http://www.met.utah.edu/cgi-bin/droman/meso_base.cgi?product=&stn=KBMI&unit=0&time=LOCAL&day1=30&month1=08&year1=2008&hour1=0
cw96255 link
http://www.met.utah.edu/cgi-bin/droman/meso_base.cgi?product=&stn=c9625&unit=0&time=LOCAL&day1=30&month1=08&year1=2008&hour1=0

[NiceBill]

Very interesting Steve, thank you.

I was not aware of the VWS issue.  Maybe a good place to start, would be with getting the VWS program in line with the others??

So I guess what you are pointing out, is that, if you are using VWS, the CWOP QC % report may not be what it is.  I did not want to say,  inaccurate??

Something to look into.

Bill.>>>>>>>>>>>>>>>

[tinplate]

I know in the past, VWS just sent SLP to CWOP, which is the wrong thing to send. Prior to about two years ago, ALL the weather programs sent the VP's SLP to CWOP, and so there was a lot of wrong data being sent (over half the CWOP stations are Vantage Pro 1 or 2, if I remember correctly). It was a real problem, especially for those like me above 1000 feet, where the difference between altimeter and SLP can be quite significant.

A push was started to fix this issue, and most of the weather programs have since addressed this. You may want to check if there is a newer version of VWS that has this fix. Otherwise, you'll just have to accept that the QC is going to be unhappy with your data from time to time since it's not getting the right values from you. It's kind of like if you were sending temperature in Celsius instead of Fahrenheit as it expects. Your sensor may be perfectly calibrated, but if you and CWOP aren't on the same page as to what it is you're sending and it's expecting, then there will be a problem.

Steve
SoftWx

[NiceBill]

Thanks Steve,

I will look into what you have noted with the VWS program.  As to how the info. - data, using the VWS program is presented to CWOP at this time is a concern.  I do know what my station reflects on the web pages I present on the internet to the public, I have, is what it is at my station. To me that is most important.

Two TV stations use the information as well as the local radio station.

Thanks again, Bill.>>>>>>>>>>>>>>>

edit:  I have presented my question over at TOP.   http://ambientwxsupport.com/smf/index.php?topic=4955.msg35072;topicseen#msg35072

[NiceBill]

I have re-presented my question over at TOP.  I don't know if this one will last very long, but it's up.

I guess if you have a VP and use VWS to send barometric pressure data to CWOP, at this time it will never be correct.

http://ambientwxsupport.com/smf/index.php?topic=5154.msg35096;topicseen#msg35096

Has any one seen the cricket??

Bill.>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>

[NiceBill]

And that's that! The powers at TOP have spoken. It will never be right. -)

Bill.>>>>>>>>>>>>>>

[tinplate]

You have four choices for addressing the CWOP/VP barometer issue, each with pros and cons:

1. Leave it the way you have it, i.e. you console has the properly set elevation, so it calculates and displays SLP. Let VWS send SLP.
  Pros: easy, because that's how you already have it; console displays SLP
  Cons: CWOP QC will drift in and out of compliance as because of the issue I described in earlier posts

2. Set your console elevation to zero, then set you console to match a nearby airport's altimeter. Setting elevation to zero turns off the SLP calculation since no conversion is neccessary if you're at sea level. Adjusting your barometer to match the airport's altimeter causes the console to apply a fixed offset which it adds to the raw pressure, and this raw pressure + offset is what the console will display. The result is that your console will from then on display something similar to an altimeter value. It won't exactly be a true altimeter value, because the offset applied to the raw pressure that results from an actual altimeter formula is not linear over the range of pressures you'll see over time. But it will be close most of the time.
  Pros: you can continue using VWS for CWOP submissions, because now your console is displaying and passing on to VWS something close to an altimeter value; Not as simple as #1, but still fairly easy to do. Data sent to CWOP will be better than option #1.
  Cons: Your console now displays psuedo-altimeter, not SLP. Some other calculations (EvapoTranspiration, if I remember correctly) in the console use the elevation, so they will be not be quite right since the console elevation says 0, but your true elevation is something else.

3. Use VirtualVP so you can run VPLive (or WeatherLink, or WeatherDisplay, or some other program that calculates and sends altimeter to CWOP) and VWS simultaneously.
  Pros: You just turn off the CWOP sends in VWS, but all the other stuff VWS does for you still gets done as before; CWOP gets the true altimeter; once you have VirtualVP set up, you can take advantage of other things, such as running WeatherLink for creating what I think is the best file archive of your weather data of any of the other weather programs.
  Cons: $30 for VirtualVP (VPLive is free, and you should already have WeatherLink), more complicated setup (but not too complicated

4. Stop using VWS and use some other weather program that does altimeter right for CWOP.
  Pros: CWOP gets the true altimeter.
  Cons: You have to completely re-do your weather setup; you probably have to buy another weather program; you have to fiddle around with importing all your historical VWS data into a new program (and hope that works).

My opinion:
#3 (VirtualVP) best solution (I'm biased - it's my program)
#2 (zero elevation) an ok solution
#1 (status quo) not a great solution
#4 (stop using VWS) worst solution (I'm assuming you like what VWS gives you)

Steve
SoftWx

[blackjack52]

Why not just leave things as you have it, Bill?  If I was still flying then I would be concerned with having the correct pressure for my pressure altimeter; which is set at 29.95.  The aerodrome doesn't have updated pressures anyway; maybe on the hour.  The biggest concern is knowing when the pressure is changing.

[blackjack52]

Also, there are a lot of barometer readings going into CWOP which are not accurate and throw off the entire system anyway.

[NiceBill]

I must say thanks again Steve.

This exercise has been a great learning curve.  I now understand why my CWOP evaluations were so erratic at times.
The barometer I use in the Davis Envoy to upload data to the internet remains spot on with the aneroid barometers I have located about the station, as well as being consistent with the barometer values at the local NWS station, and the four surrounding airports.

My main interest with my station, is that, what it is hear at the station is uploaded and presented to the public as accurate as it can be.  If the pressure is in fact 29.94, I want it to reflect 29.94 on any internet weather web page that my information is shown on.

I know I have and maintain enough calibrated equipment to maintain my station to that degree.

What and how it gets to CWOP, I can not really be concerned with at this point, my question has been answered.  I do wish that some day others may correct that and realize it may benefit others and it is not always the correct way.

If some one asks why there barometric pressure values are showing a poor percentage at CWOP, I may just say, it's because "VWS has it there way" \

Bill.>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>

[NiceBill]

Why not just leave things as you have it, Bill?  If I was still flying then I would be concerned with having the correct pressure for my pressure altimeter; which is set at 29.95.  The aerodrome doesn't have updated pressures anyway; maybe on the hour.  The biggest concern is knowing when the pressure is changing.

Ua Know, that's what I am going to do.  I just wanted to know why, and why I couldn't get a simple answer to my question over at TOP.  I understand about the other stations that may be also used in the evaluation being a bit funky.

Hay, I fly once a week or more.  I have a flight simulator in the computer and pipe it to the 60" plasma screen in the family room.  I have about 50 planes to choose from, quite a show.  I can set up everything, but have yet to find the barometric pressure.

Just love the thing,

Bill.>>>>>>>>>>>>>>>>>>>

[George Richardson]

Bill,
Just for curiosity, was your Davis that accurate out of the box or did you calibrate it?
George

[NiceBill]

 
I did set it up one click when I first set it up back in Jan. of this year.  The one in use to the net is the one in the Envoy and the temperature in the area is quite stable.
I have a console that resides in the fam. room and never changed it and it reflects the same as the Envoy.

Very satisfied with the consistency of each.

Bill.>>>>>>>>>>>>>>>

[NiceBill]

Looks like I PO the Queen Of The Day over at TOP.  Probably not the first and I know not the last.

He pulled a post I had about maybe Davis could put something in there product that would allow the barometric pressure values sent to CWOP to be of a altimeter value, then they would be correct, as the VWS program was not going to change and fix it so the folks using the same would have a real feeling as to how there station was working.  Whaaaa.

I like it hear, it's where one gets honest answers.

Bill.>>>>>>>>>>>>>>>

[Garth Bock]

Looks like I PO the Queen Of The Day over at TOP.  Probably not the first and I know not the last.

He pulled a post I had about maybe Davis could put something in there product that would allow the barometric pressure values sent to CWOP to be of a altimeter value, then they would be correct, as the VWS program was not going to change and fix it so the folks using the same would have a real feeling as to how there station was working.  Whaaaa.

I like it hear, it's where one gets honest answers.

Bill.>>>>>>>>>>>>>>>

Hey Bill which pressure is wrong now ??? Your VPro 2 + .... or your Blood pressure....hehehe

[NiceBill]

Looks like I PO the Queen Of The Day over at TOP.  Probably not the first and I know not the last.

He pulled a post I had about maybe Davis could put something in there product that would allow the barometric pressure values sent to CWOP to be of a altimeter value, then they would be correct, as the VWS program was not going to change and fix it so the folks using the same would have a real feeling as to how there station was working.  Whaaaa.

I like it hear, it's where one gets honest answers.

Bill.>>>>>>>>>>>>>>>

Hey Bill which pressure is wrong now ??? Your VPro 2 + .... or your Blood pressure....hehehe

Since ASME and ANSI have been setting the the regulations for pressure vessels and power piping, the incidences of boiler explosions have been reduced dramatically.

Bill.>>>>>>>>>>>>>>>>>

[SLOweather]

Since ASME and ANSI have been setting the the regulations for pressure vessels and power piping, the incidences of boiler explosions have been reduced dramatically.

Bill.>>>>>>>>>>>>>>>>>

I guess WXWP was running an unregulated boiler. It looks like it exploded this morning...

[NiceBill]

Since ASME and ANSI have been setting the the regulations for pressure vessels and power piping, the incidences of boiler explosions have been reduced dramatically.

Bill.>>>>>>>>>>>>>>>>>

I guess WXWP was running an unregulated boiler. It looks like it exploded this morning...

No explosion at all.  It's just like a Nuclear Power Plant,  they are operated in compliance with the codes, for all of the public, or they get shut down so they dont cause an explosion.

Bill.>>>>>>>>>>>>>>>

[W Thomas]

Also, there are a lot of barometer readings going into CWOP which are not accurate and throw off the entire system anyway.

I can say I know that from experience and am guilty of that for several hours one day due to a problem I couldn't  get to or into via modem to fix.
One thing I have learned...A little bad data goes an awful long way as to screwing up the MADIS Quality Checks!!!


On the same topic of baro data..How about a fix or tweak for other station types such as OS ?
That's what I am currently running but I am hoping to update as soon as financially possible.
I do want to send accurate data as I possibly can given the equipment limitations. If there is a way to switch that from SLP to Altimeter in an OS station
I'd sure take that into consideration


Wayne

www.smythweather.net
cw8217

[tomcj2]

I have downloaded VP Live and have 1 question so far.

What is the difference MADIS Altimeter, and ASOS Altimeter in the Live Calculated Data section?

Would I be correct to assume that My console should read the same (or very close to) as the ASOS Altimeter, and the METAR report from the local, state owned airport....
but the MADIS Altimeter is the data sent to ARPS?