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

[norwoodm via TriLUG]

The only thing I have to add about this is if you go to the WRAL booth at 
NC State Fair and Greg Fischell is there, he will explain dew point in 
tremendous detail if you ask him about it.

Mike

On Thu, 21 Dec 2017, Joseph Mack NA3T via TriLUG 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
> jmack (at) austintek (dot) com - azimuthal equidistant
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