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Weather Related Organizations => CWOP Forum => Topic started by: NiceBill on September 01, 2008, 04:24:22 PM

Title: Davis Vantage Pro 2 "Barometer" and CWOP
Post by: NiceBill on September 01, 2008, 04:24:22 PM

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.

 
 
 
Title: Re: Davis Vantage Pro 2 "Barometer" and CWOP
Post by: tinplate on September 01, 2008, 07:37:30 PM
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
Title: Re: Davis Vantage Pro 2 "Barometer" and CWOP
Post by: NiceBill on September 01, 2008, 08:57:12 PM

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.>>>>>>>>>>>>> :-)
Title: Re: Davis Vantage Pro 2 "Barometer" and CWOP
Post by: George Richardson on September 02, 2008, 08:39:57 AM
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
Title: Re: Davis Vantage Pro 2 "Barometer" and CWOP
Post by: NiceBill on September 02, 2008, 10:08:30 AM
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% #-o

Bill.>>>>>>>>>>>> :-)
Title: Re: Davis Vantage Pro 2 "Barometer" and CWOP
Post by: tinplate on September 02, 2008, 11:15:23 AM
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
Title: Re: Davis Vantage Pro 2 "Barometer" and CWOP
Post by: NiceBill on September 02, 2008, 11:50:24 AM

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.>>>>>>>>>>>>>>>
Title: Re: Davis Vantage Pro 2 "Barometer" and CWOP
Post by: tinplate on September 02, 2008, 12:11:19 PM
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
Title: Re: Davis Vantage Pro 2 "Barometer" and CWOP
Post by: NiceBill on September 02, 2008, 01:10:46 PM
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. :arrow:  http://ambientwxsupport.com/smf/index.php?topic=4955.msg35072;topicseen#msg35072
Title: Re: Davis Vantage Pro 2 "Barometer" and CWOP
Post by: NiceBill on September 03, 2008, 11:15:35 AM

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.>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>> :-)
Title: Re: Davis Vantage Pro 2 "Barometer" and CWOP
Post by: NiceBill on September 03, 2008, 12:02:46 PM
And that's that! The powers at TOP have spoken. It will never be right. #-o-) =D> =D> =D> =D> =D> =D> =D> =D> =D> =D>

Bill.>>>>>>>>>>>>>> :-)
Title: Re: Davis Vantage Pro 2 "Barometer" and CWOP
Post by: tinplate on September 03, 2008, 02:33:06 PM
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
Title: Re: Davis Vantage Pro 2 "Barometer" and CWOP
Post by: blackjack52 on September 03, 2008, 03:59:26 PM
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.
Title: Re: Davis Vantage Pro 2 "Barometer" and CWOP
Post by: blackjack52 on September 03, 2008, 04:04:54 PM
Also, there are a lot of barometer readings going into CWOP which are not accurate and throw off the entire system anyway.
Title: Re: Davis Vantage Pro 2 "Barometer" and CWOP
Post by: NiceBill on September 03, 2008, 04:21:41 PM

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" :grin:\

Bill.>>>>>>>>>>>>>>>>>>>>>>>>>>>>>> :-)
Title: Re: Davis Vantage Pro 2 "Barometer" and CWOP
Post by: NiceBill on September 03, 2008, 04:32:07 PM
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.>>>>>>>>>>>>>>>>>>> :-)
Title: Re: Davis Vantage Pro 2 "Barometer" and CWOP
Post by: George Richardson on September 03, 2008, 04:39:26 PM
Bill,
Just for curiosity, was your Davis that accurate out of the box or did you calibrate it?
George
Title: Re: Davis Vantage Pro 2 "Barometer" and CWOP
Post by: NiceBill on September 03, 2008, 04:57:47 PM
 
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.>>>>>>>>>>>>>>> :-)
Title: Re: Davis Vantage Pro 2 "Barometer" and CWOP
Post by: NiceBill on September 03, 2008, 05:17:08 PM

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. :grin:

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

Bill.>>>>>>>>>>>>>>> :-)
Title: Re: Davis Vantage Pro 2 "Barometer" and CWOP
Post by: Garth Bock on September 04, 2008, 02:30:01 PM

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. :grin:

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
Title: Re: Davis Vantage Pro 2 "Barometer" and CWOP
Post by: NiceBill on September 05, 2008, 02:27:18 PM

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. :grin:

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. :grin:

Bill.>>>>>>>>>>>>>>>>> :-)
Title: Re: Davis Vantage Pro 2 "Barometer" and CWOP
Post by: SLOweather on September 05, 2008, 03:19:58 PM

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. :grin:

Bill.>>>>>>>>>>>>>>>>> :-)

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

(http://www.sloweather.com/blog/2008/080805vws.jpg)
Title: Re: Davis Vantage Pro 2 "Barometer" and CWOP
Post by: NiceBill on September 05, 2008, 03:36:37 PM

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. :grin:

Bill.>>>>>>>>>>>>>>>>> :-)

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

(http://www.sloweather.com/blog/2008/080805vws.jpg)


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.>>>>>>>>>>>>>>> :-)
Title: Re: Davis Vantage Pro 2 "Barometer" and CWOP
Post by: W Thomas on September 07, 2008, 01:52:38 AM
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
Title: Re: Davis Vantage Pro 2 "Barometer" and CWOP
Post by: tomcj2 on October 22, 2008, 03:43:02 PM
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?
Title: Re: Davis Vantage Pro 2 "Barometer" and CWOP
Post by: tinplate on October 24, 2008, 11:40:22 PM
Originally, VPLive was a test program while I developed the barometer formulas for the VP. There are a number of different versions of the various pressure formulas. I probably ought to take the multiple versions out  :-)

The ASOS altimeter equation is the one I found in the ASOS system documentation. I don't think this is available directly online anymore, but there is a copy of the relevant part here: http://wahiduddin.net/calc/refs/ASOS_Pressure.htm . The MADIS equation was found on the MADIS website: http://madis.noaa.gov/madis_api.html in software associated with the MADIS system. Since the data sent through CWOP ends up at MADIS, I chose to use that formula since I assume MADIS reverses the altimeter calculation to derive the other pressure values it calculates from the altimeter it receives.

MADIS and ASOS should be pretty close to each other. In a well calibrated system, the VP console display may not exactly match airport SLP or Altimeter. The console displays SLP, so we should not expect it to match airport altimeter values. And the console's SLP calculation differes from the NWS, so the console may not match the airport's SLP exactly.

What I do to calibrate my VP console is to look at recent recorded data at a specific time for both my station's altimeter as sent to CWOP, and for a nearby, reliable NWS site. Mesowest http://www.met.utah.edu/mesowest/ is a good source for that data. So, if at 1pm yesterday, my station reported altimeter of 29.92, and the airport reported 29.85 at about the same time, then I note that my barometer is reading .07 inHg too high. I then go to my console, display pressure in inHg, push the buttons that put the console into adjust barometer mode, and then lower the pressure by .07 inHg. It's not really important what the console happens to be showing at the time. All that's important is that I lower it by .07 inHg. Doing it this way, I get my pressure right on.

Steve
SoftWx
Title: Re: Davis Vantage Pro 2 "Barometer" and CWOP
Post by: George Richardson on October 25, 2008, 12:00:03 PM
I would like to express an opinion.

If, in fact, CWOP accepts barometric pressure as altimeter, I would like to see that posted prominently on their graphs.

I would also like CWOP to adopt a waiting period before new members' data is incorporated in the comparison pool. I am convinced that you could go the local K-Mart, buy the cheapest "weather station" that links to a computer, and immediately send inaccurate "station pressure" to CWOP and it would be published AND compared equally with area stations.

A final conundrum is, if VWS only sends SLP to CWOP and CWOP does not adjust the data it receives, how can any barometric data from any PWS transmitted to CWOP via VWS be accepted.

FWIW, I have a Davis VP2 station and use VWS software. I do however, use Steve's VPLive to send my data to CWOP. I really do not think very well in all of these Barometric scales but I do know that if I had a thermometer that read temperature in Fahrenheit and someone asked the temperature in Celsius, if I just slide the F scale to give the "right" answer, the next time I looked at the scale, it would give the wrong temperature.

George
Title: Re: Davis Vantage Pro 2 "Barometer" and CWOP
Post by: NiceBill on October 25, 2008, 12:48:13 PM

Hear is some more room for thought.  I do not know about any of the other PWS,  My interest is in the Davis equipment.

The Davis barometers that are incorporated in the consoles and the Envoys are NOT temperature compensated.  So depending on where your unit is located and if the temperature is not maintained the same each time the data is read and up loaded, your barometric pressure sent to CWOP may or may not be 100% correct.
If you maintain the exact temperature in the area of your Console (and I would think, dont have the back light on some times and off others) or your Envoy, the data should be some place in the ball park.

I don't know how much of an effect this can have, just another reason I do not use CWOP as a place to get me upset.

The information, that the units are NOT temperature compensated cane from the Davis technical dept.

Bill.>>>>>>>>>>>>>>>>> :-)
Title: Re: Davis Vantage Pro 2 "Barometer" and CWOP
Post by: Carson Weather on October 25, 2008, 01:36:59 PM
The information, that the units are NOT temperature compensated cane from the Davis technical dept.

That would be a *good* thing, based on our previous discussion based on indoor temp sensors. ](*,)

I may be misinterpreting, but Davis' own website says that the pressure IS temperature compensated (using outdoor temp, to boot).

I wonder who is right? Davis or Davis?

I'm just glad that I'm not manic about pressure, only indoor temperature :)






 
Quote
BAROMETRIC PRESSURE
What is it:
The weight of the air that makes up our atmosphere exerts a pressure on the surface of the
earth. This pressure is known as atmospheric pressure. Generally, the more air above an area,
the higher the atmospheric pressure, this, in turn, means that atmospheric pressure changes
with altitude. For example, atmospheric pressure is greater at sea-level than on a mountaintop.
To compensate for this difference and facilitate comparison between locations with different
altitudes, atmospheric pressure is generally adjusted to the equivalent sea-level pressure. This
adjusted pressure is known as barometric pressure. In reality, the Vantage Pro and Vantage
Pro2 measures atmospheric pressure. When entering the location’s altitude in Setup Mode, the
Vantage Pro and Vantage Pro2 calculates the necessary correction factor to consistently
translate atmospheric pressure into barometric pressure.
Barometric pressure also changes with local weather conditions, making barometric pressure an
extremely important and useful weather forecasting tool. High pressure zones are generally
associated with fair weather while low pressure zones are generally associated with poor
weather. For forecasting purposes, however, the absolute barometric pressure value is
generally less important than the change in barometric pressure. In general, rising pressure
indicates improving weather conditions while falling pressure indicates deteriorating weather
conditions.
The following section applies to Vantage Pro and Vantage Pro2 systems only:
Parameters Used: Outside Air Temperature, Outside Humidity, Elevation, Atmospheric Pressure
Formula:
Simply,
PSL = PS * (R),
where PSL is sea level pressure, PS is the unadjusted reading sensed by the Davis barometer,
and R is the reduction ratio, which is determined as follows:
First, Tv (virtual temperature in the “fictitious column of air” extending down to sea-level) can be
determined as follows. The result is in degrees Rankine, which is similar to Kelvin except it
uses a Fahrenheit scale divisions rather than Celsius scale divisions:
Tv = T + 460 + L + C,
where T is the average between the current outdoor temperature and the temperature 12 hours
ago (in Fahrenheit) in whole degrees. L is the typical lapse rate, or decrease in temperature
with height (of the “fictitious column of air”), as calculated by:
L = 11 Z/8000,
where L is a constant value with units in °F. Z is elevation, which must be entered in feet.
The current dewpoint value and the station elevation are necessary to compute C. C is the
correction for the humidity in the “fictitious column of air”. It is determined from a lookup table
(provided in the attached table). The table consists of dewpoints in °F every 4°F and elevations
28 - 10 Rev A 5/11/06
in feet every 1500 feet. Linear interpolation is performed to obtain the correct reduced pressure
value. For dewpoints below –76°F, C = 0; for dewpoints above 92°F, a dewpoint of 92°F is
assumed.
Now, Tv can be determined. From this, the following can be computed:
Exponent = [Z/(122.8943111*Tv)]
Once this exponent is computed, R can be computed from the following:
R = 10^[Exponent].
Thus, PSL = PS * (R) can be calculated. Pressure can be in any units (R is dimensionless) and
still yield the correct value.
This procedure is designed to produce the correct reduced sea-level pressure as displayed.
This requires the user to know their elevation to at least ±10 feet to be accurate to every .01” Hg
or ±3 feet to be accurate to every 0.1 mb/hPa.
This is a simplified version of the official U.S. version in place now. The accepted method is to
use lookup tables of ratio reduction values keyed to station temperature. These are based on
station climatology. These values are unavailable for every possible location where a Davis
user may have a station, thus this approach is not suitable.
It should be noted that if a sensor’s pressure readings require adjustment, the user can adjust
either the uncorrected or the final reading to match the user's reference, as appropriate. If the
user chooses to measure uncorrected atmospheric pressure or use another reduction method,
they should set their elevation to zero. Subsequently, output data using the VantageLink can be
read by or exported to another application and converted as desired.
The calibration of the sensor is a separate one time function performed on the unit during the
manufacturing process. It is a completely independent operation from the calculation the
Vantage Pro and Vantage Pro2 console makes to display a reading corrected to sea-level. The
calibration is done to ensure the sensor reads uncorrected or raw atmospheric pressure (not
barometric pressure) properly. Any properly functioning unit will read the uncorrected
atmospheric pressure within specifications. However, limits in the displayable range of the bar
value may prevent the user from setting an incorrect elevation for their location. That is, a user
at sea-level, may see a dashed reading if they set their unit to 5000' elevation or vice-versa.
So, the best way to tell if a unit is functioning properly, is:
• use a reference that has been adjusted to indicate sea-level pressure and setting the
Vantage Pro and Vantage Pro2 console to the proper elevation or
• use a reference that is reading the raw, uncorrected atmospheric pressure and set the
Vantage Pro and Vantage Pro2 console elevation to zero
and verify that these readings are comparable.
Title: Re: Davis Vantage Pro 2 "Barometer" and CWOP
Post by: Mark / Ohio on October 25, 2008, 10:14:51 PM
...I may be misinterpreting, but Davis' own website says that the pressure IS temperature compensated (using outdoor temp, to boot).....

So that means the pressure outside the house is slightly different then inside the house.   #-o

Although I guess we already know that because any powered vent or even a good drafting chimney could pull the house to a slight vacuum.   Or a fresh air inlet to the HVAC system could add a little pressure inside the house not to even mention the small pressure wave caused by the neighbor's cars going by the house.  More reasons not to get too extreme worrying about exact readings.    ;)
Title: Re: Davis Vantage Pro 2 "Barometer" and CWOP
Post by: tinplate on October 26, 2008, 12:01:31 AM
I think the "temperature compensated" has to do with the operation of the sensor itself in generating the raw pressure value. It has to do with how a sensor's temperature affects its reading of pressure.

The console takes that raw pressure and applies a Sea Level reduction algorithm to normalize the pressure to a value that can be compared among stations at differing altitudes. The reduction algorithm does use temperature in the calculation, but this is for a different purpose, i.e. to estimate what the pressure would be if you extended the atmosphere around you down to sea level and moved your barometric device down to that elevation. This is a different use of temperature.

In the first case, you want the temperature of the sensor. In the second case, you want the temperature of the atmosphere outside. Since most VP consoles live indoors with a fairly narrow range of temperature, I think the lack of temperature compensation likely has a negligible effect.

Steve
SoftWx
Title: Re: Davis Vantage Pro 2 "Barometer" and CWOP
Post by: mac on October 30, 2008, 05:13:53 PM
This is indeed a very informative (and entertaining - I wanna slap that damn bug, NiceBill!  :-P) thread.  I was having the very same problem but using WeatherDisplay.  I saw a setting for sending altimeter pressure but wasn't sure what CWOP wanted and hadn't gotten a chance to dig through all the info on the web yet.

All my other stations (old lacross 2315, an oregon scientific wireless, and a kestral handheld) were all showing the same (altimeter) pressure values and I couldn't figure out why my shiny, new $600 VP2 was always "wrong" and couldn't seem to be calibrated.

Turns out it wasn't wrong at all.  Thank goodness!

Thanks for a very informative post.
MAC

DW1379