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

[Thomas V Thomas via TriLUG]

https://www.engineeringtoolbox.com/dry-wet-bulb-dew-point-air-d_682.html


> On Dec 21, 2017, at 11:50 AM, Joseph Mack NA3T via TriLUG <trilug at trilug.org> wrote:
> 
> 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
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