[TriLUG] OT: the difference between web bulb and dew point temperatures

[Joseph Mack NA3T via TriLUG]

I have just recently found that wb != dp. (I had assumed they were the same.) 
The definitions are inscrutable and presumably only for those who already 
understand the difference.

The HVAC people know all about this, but when you go to their forums, whenever 
anyone asks for an explanation, they are directed to the definitions. Well if 
the definitions had explained the difference, the person wouldn't have needed to 
ask.

I've plumbed the depths of the people I know and haven't found an answer. I know 
I'm not supposed to ask about non FOSS topics here, but I don't know where else 
to go.

I have a reasonable understanding of how you measure both

o wb - you put a wet sock over a thermometer and blow air over it. The 
temperature of the water drops till (I think) the water vapour pressure of the 
cooled water is the same as the water vapour pressure in the air. So the air is 
moving.

o dew point. you chill the thermometer till water condenses out of the air onto 
the thermometer. I assume the air has to be still for this to work.

I can look up the psychrometric charts to see that the two temperatures are 
different and that dewpoint < wb, but I can't think of a thermodynamic way of 
moving from one to another, like you do when moving around a Carnot cycle 
diagram, which shows the difference between the two situations and that shows 
that dewpoint < wb.

Does anyone know?

(we now return to our regular program of FOSS)

Thanks
Joe

-- 
Joseph Mack NA3T EME(B,D), FM05lw North Carolina
jmack (at) austintek (dot) com - azimuthal equidistant
map generator at http://www.wm7d.net/azproj.shtml
Homepage http://www.austintek.com/ It's GNU/Linux!