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

[Matt Flyer via TriLUG]

I am not entirely certain what it is you're asking, but I will try to
answer based on what I think you're asking.

As you know, the dry bulb is what we think of in terms of the
temperature.  It reflects mostly sensible (dry) heat.  The wet bulb
temperature takes into account the effect of evaporative cooling and it
and is a measure of how much moisture is in the air as latent heat.  As
the wet bulb goes up for a particular dry bulb, there is more moisture
in the air.  If you have a day with the same dry bulb temperature, but
a higher wet bulb temperature it will feel much more humid.  Perceived
comfort and AC loads are pretty much direct functions of wet bulb
rather than dry bulb.  

As I mentioned above, there are two forms of heat energy, sensible and
latent.  The latent heat is the amount of energy required to make the
state transition change from liquid to vapor.  The wet bulb temperature
takes this into account.  Take for example a cooling tower.  Water is
sprayed and run down a surface with counter flowing air.  Some of the
water evaporates which removes heat from the water.  The wet bulb
temperature is the asymptote that you can approach by the evaporative
cooling.  

The dew point or saturation point is the temperature at which the air
will saturate and it can no longer hold any more moisture.  When the
air is completely saturated, 100% relative humidity the wet bulb and
the dew point are equal.  

Obviously the dew point stops at 32F as below that water will freeze.  

I hope that helps, and if not, I will try to clarify.

On Thu, 2017-12-21 at 18:50 +0000, 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
> map generator at http://www.wm7d.net/azproj.shtml
> Homepage http://www.austintek.com/ It's GNU/Linux!