Topic: Davis Vantage Pro 2 "Barometer" and CWOP

[tinplate]

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

[George Richardson]

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

[NiceBill]

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.>>>>>>>>>>>>>>>>>

[Carson Weather]

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






 

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.

[Mark / Ohio]

...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.   

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.   

[tinplate]

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

[mac]

This is indeed a very informative (and entertaining - I wanna slap that damn bug, NiceBill!  ) 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