Topic: Setting Correct Barometer reading with VP2 and Weather Envoy

[CW2274]

The WL Console User guide (7395.402_F_WeatherLink_Console_User_Guide) does actually outline an accurate Elevation is important for accurate barometric pressure settings (page 17). Barometric Pressure on page 22 also outlines the relationship with "altitude to determine the offset value to consistently translate atmospheric pressure into barometric pressure". Not sure why there continues to be a misunderstanding with this?

The other document that some should read the Barometric Pressure section of is AN_28-derived-weather-variables (Application Note 28) which outlines the Derived Variables in Davis Weather Products. Even in 2006 the two methods were clearly being clarified so not really sure what the problem is?

This has nothing to do with the price of tea in China. Tells us nothing of what pressure reduction algorithm is being used. Big difference between SLP and altimeter, especially when you get into higher elevations and the differences become much more apparent. Makes just a tiny difference on ones setup. Not really sure what the problem is with understanding this.

BTW, I knew my concern was justified. See post #35. I had completely forgotten I spoke to Davis on this subject.

https://www.wxforum.net/index.php?topic=45032.25

[Mattk]

Who has not read this?

https://support.davisinstruments.com/article/igpcv664kz-app-notes-derived-variables-in-davis-weather-products

[CW2274]

Who has not read this?

Me, for one. Never saw this before. That definitively answers my simple question. Obviously, the Davis employee I spoke to in the other thread had no idea what he/she was talking about. Shocking.

Thanks for the info.

[Swede]

So, to reaffirm the reason for my original post, I am using a Davis Weather Envoy + Data Logger to connect to my computer.  That Envoy uses the Standard/Barometric Pressure system instead of the Altimeter method used on the 6313 Console.  I am trying to get the Envoy to track and report Pressure readings close to what the local airport is using since I upload to CWOP.

The Davis Envoy/Data Logger does a good job of matching the swings in pressure up and down, but I'm just trying to fine tune the readings so they track closely.  I am over 300 feet higher in altitude, and I guess for the meteorological community, my station elevation of 5818 feet (1773.33 meters) is considered high for this science.  For this part of the United States, this is actually quite normal.  Perhaps I am being obsessive-compulsive, but just trying to report good data.  With the WeatherLink 6.0.5 software, you input both the correct elevation of the station,  AND the pressure reading from the local reference source.  That hasn't provided me good results.

[johnd]

...I guess for the meteorological community, my station elevation of 5818 feet (1773.33 meters) is considered high for this science...

It's not really that it's considered a high elevation for the science, but purely and simply a consequence of the fact that in order to get a sense of atmospheric pressure patterns on a large scale then pressure readings are everywhere adjusted to what the equivalent sea-level pressure might be. But the necessary correction to sea level pressure (SLP) is imperfect because a number of ultimately unknowable assumptions have to be made. It's also imperfect because there are both simpler but less accurate (Altimeter) and more accurate but more complicated (Standard) ways of calculating the correction. And the greater your altitude then the greater the impact of the correction; the greater the uncertainty involved; and the more the choice of assumptions in the correcting calculation becomes important.

Your airport reading is almost certainly using the simple Altimeter correction. So your best way of mimicking that correction is to use the same correction, which in the Davis stations is only easily** available from the Vue console.

**Actually IIRC the absolute pressure is available from the LOOP2 data record that most modern Davis stations with legacy consoles can also provide, but AFAIK (but please correct me if I'm wrong here) most software doesn't access that data item and also doesn't implement the necessary Altimeter correction. So I guess that software like CumulusMX and Weather Display could in principle generate both pressure values. But Weatherlink for Windows will presumably never do so because it is no longer being developed.)

[Mattk]

Sounds like instead of simply accepting the lowest common denominator, for those that want to be obsessive-compulsive somebody should take it up with CWOP and get them to fix their simple and obviously inadequate sh$t instead of trying to fit this square peg in a round hole.

[mcrossley]

**Actually IIRC the absolute pressure is available from the LOOP2 data record that most modern Davis stations with legacy consoles can also provide, but AFAIK (but please correct me if I'm wrong here) most software doesn't access that data item and also doesn't implement the necessary Altimeter correction. So I guess that software like CumulusMX and Weather Display could in principle generate both pressure values. But Weatherlink for Windows will presumably never do so because it is no longer being developed.)

If LOOP2 is enabled then Cumulus MX will supply Altimeter and Station pressures in addition to the SLP - but only via web tags, it uses SLP as its standard pressure value.

[Ahill]

Just a bit of information.  I have had the Vantage Pro 2 since February and transmit simultaneously to both a new 6313 Console, and a Weather Envoy.  The Console does a good job of tracking my local barometer reference, but the Envoy barometer consistently reads low and responds slower to pressure changes.  The console is used mainly for casual readings in the house.  The Envoy is connected with the Davis Data logger to a computer with Weatherlink software uploading to Weatherlink.com.  There my pro package sends the data to CWOP, WU, etc.

I have read the 6313 Console uses the Altimeter method for tracking the barometric pressure, but the Envoy uses the "Standard" method.  I cannot get the Envoy to work and match the reference data.

The reference data is a local regional airport and it sits at 5505 foot altitude and is 4.16 US Miles from my Envoy and logger.  The Envoy sits at 5818 foot altitude.  So it's 317 feet higher.  As an example just today, my current reading of the reference altitude is 30.12 inches and my Envoy is reading 30.00.  It reads consistently low.   Just for comparison, the 6313 Console is reading 30.13, similar to the airport reading.  Also the 6313 Console and the Weather Envoy are only 25 feet apart in the same house.

So how do I fix this?  CWOP is hating my barometric pressure reading via the Davis Weather Envoy, and I don't know how to fix this.

There is a member “gszlag” who is a pro on here when it comes to setting up pressure readings to send to cwop. He helped setup my ambient system to correctly send pressure to cwop now i have a vp2 and wondering if i need to figure out and do the same thing.
So the vp2 does not send the correct “altimeter” pressure that cwop is requiring?
Thanks

[Swede]

Here's an update to my original issue.  In the WeatherLink 6.0.5 software and Barometer setting, I removed my elevation from the box and entered 0 (Zero), then just put the local reading from the airport.  Without my elevation number, the readings track closer (not perfect) to the reference reading.  I have no idea why this would make a difference.