[wxqc] VP2 Hygrothermometer

[spamfree at pensom.org]

Hi,

The formula Davis says it uses is here http://www.exploratorium.edu/weather/dewpoint.html . This information is verbatim from a Davis document explaining all the forumulas they use, but I don't have a link to that handy. Although they don't say so, I would not be surprised if the VP console uses a table lookup approximation of that formula for the dew point display on the console. It's important to note that a dew point value is not included in either the live data packet nor the archive data packet that is provided via the serial interface. So dew point must be calculated by whatever weather software is being used to connect to the console, using the temperature and humidity values. I would imagine that WeatherLink is using the full calculation described in the link I gave. VPLive uses the same formula as that described in the Davis docs. Other weather programs likely use the same or similar formula, the only differences probably being which of the many vapor pressure formulas they chose to use. But the difference between them should be small.

Steve
  
======= At 2006-07-10, 19:27:05 you wrote: =======

>Most of these, that I've played with, use a gold foil RH sensor and then 
>calculate dew point via either a simplified implementation of the 
>partial differential equation, or via a modified table lookup.
>
>A long time ago, when computational power was expensive, findu made the 
>decision to do a detailed table lookup for T, RH, Pb -> Td rather than 
>the complete calculation.
>
>I'll see if I can get the real answer.  I suspect Davis is using a 
>simplified form of the more complex equation.  I suspect I would.
>
>gerry
>