Davis Vantage Pro 2

[STHirsch]

After having a WS2080 for a few years I just upgraded to a Davis Vantage Pro 2 and it is awesome and I love it. Just a quick question, is the Barametric Pressure relative or absolute??

[steve]

The simple answer is "relative", but it's not really either, in the same sense as the Fine Offset. The VP2 corrects pressure to sea-level using a combination of factors (temperature etc), rather than it just being a simple fixed difference as it is with the Fine Offset. It's this corrected sea-level pressure that it displays and supplies to Cumulus.

[STHirsch]

Thanks Steve, my next step is getting the data logger and connect to Cumulus!!

[gemini06720]

Steve, not that I want to contradict your statement about the Vantage Pro2 correct type of pressure.

It is my understanding, after reading the VP2 documentation (page 43), that the Vantage Pro2 console measures the atmospheric pressure (the pressure on the surface of the earth ... or on top of the Vantage Pro2 console). By entering the weather station's elevation, the Vantage Pro2 console translates the atmospheric pressure into the barometric pressure (the sea level pressure).

There is even a warning in the documentation that "before calibrating the barometric pressure, be sure the station is set to the correct elevation".

[steve]

I don't think you're contradicting me at all. Elevation is one of the "combination of factors" that I mentioned. The full list is: Outside temperature, outside humidity, and elevation. It uses those to convert the measured station pressure into mean sea-level pressure. Unlike the Fine Offset stations, which simply apply a fixed amount to the measured station pressure, so they are effectively supplying "altimeter" pressure, i.e. only taking elevation into account.

[oman4eva]

Hi Steve,

Can you confirm that cumulus simply reads the pressure off the Davis VP2+ and logs/displays it, irregardless of what you've set the Cumulus altitude to?

Our station is at 174 m altitude, however for our needs we leave the altitude at 0 m on Davis's WeatherLink as we want the pressure logged that is recorded beside our other instruments (and not corrected to sea level pressure). However i noticed that if i set the station altitude to 174 m in Cumulus it doesn't correct the pressure reading to sea level. I would like to set the altitude to 174 m in cumulus so that the web page displays the correct altitude. I could do this through editing the template i suppose but would rather leave the template as is.

thanks
oman

[steve]

Can you confirm that cumulus simply reads the pressure off the Davis VP2+ and logs/displays it, irregardless of what you've set the Cumulus altitude to?

Yes, that's right.

Our station is at 174 m altitude, however for our needs we leave the altitude at 0 m on Davis's WeatherLink as we want the pressure logged that is recorded beside our other instruments (and not corrected to sea level pressure). However i noticed that if i set the station altitude to 174 m in Cumulus it doesn't correct the pressure reading to sea level. I would like to set the altitude to 174 m in cumulus so that the web page displays the correct altitude. I could do this through editing the template i suppose but would rather leave the template as is.

If Cumulus did have a sea-level calculation based on altitude, it would just apply a fixed offset using whatever the usual formula is. So I guess you could just apply that offset yourself on the Cumulus calibration screen. It's not really intended for that purpose (doing sea-level correction), but it probably works.

[steve]

It occurs to me that you might have problems with the pressure sent to CWOP, if you use that.