[wxqc] VP2 Hygrothermometer

[Evan Bookbinder]

Does anyone know how the Davis Vantage Pro 2 measures the outdoor dwpt/humidity?

I just purchased a fan-aspirated VP2 and have it mounted on a 4x4 pressure treated wood post in the backyard, correctly sited about 2m above the ground.

Temperatures have been spot on with my sling psychrometer and the Lee's Summit ASOS (about 3 miles away). The dewpoint however seems to be running a couple F too high. In fact, after the rain stopped this afternoon, the dwpt read 75-76F for a time, and granted it was humid, but this was some 4-5F too high.

I was a bit surprised to see in the console manual under the troubleshooting section that for the dewpoint it says "Check calibration numbers for temperature. Remember, dew point depends on temperature and outside humidity". Maybe I'm reading this wrong, but I'm curious to see how this instrument actually measures ambient moisture.

I would hate to calibrate an instrument right out of the box -- this is something the manufacturer *should* be doing prior to shipment. Furthermore, calibrating the humidity seems like the wrong variable to be calibrating, considering it's dependent on 3 other variables. This would lead me to believe that the dewpoint is being calculated indirectly instead of directly -- and I'm curious to know as to what formula Davis is using here.

Thanks,
Evan
Senior Meteorologist
NWS Kansas City/Pleasant Hill
-------------- next part --------------
An HTML attachment was scrubbed...
URL: http://pond1.gladstonefamily.net:8080/pipermail/wxqc/attachments/20060710/84fd7d44/attachment.html