[TriLUG] OT: thermodynamics of A/C question

[Joseph Mack NA3T]

On Sat, 23 Jun 2012, Matt Flyer wrote:

> For example here is a graph showing total campus usage for 
> the last 24 hours relative to dry bulb and wet bulb 
> temperature. 
> http://chwreports.unc.edu/pic2.php?time=1340450882&bldg[]=2654&bldg[]=2653&bldg[]=2533&timedate=2012/06/23%2007:26:26&interval=900&period=86400&dateForm=YYYY/MM/DD&source=tomkins
>
> You can see how even as the dry bulb temperature went down, the cooling
> requirements went up as the wet bulb temperature changed.

thanks. I didn't know people were making this sort of data 
available.

This is a difficult graph to read. The temperature scales 
for the wet and dry bulb are offset and the spacing for the 
two scales is different; it's 1deg/div for wet and 2deg/div 
for dry. My immediate reaction is that the graph has been 
plotted specifically to obscur something. There's no way to 
correlate any of the data without replotting.

Here's similar data, where the wet and dry bulb are plotted 
on the same scale. Here you can see that the wet bulb 
doesn't change significantly through the day compared to 
the dry bulb.

http://www.austintek.com/belhaven_weather-index.gif

With little change in the wet bulb temperature and large 
changes in the dry, it would be difficult to find a 
correlation between power usage and wetbulb. I would expect 
that most of the power usage correlates with dry bulb. If 
the case is otherwise, the unc graph obscurs it.

Joe

-- 
Joseph Mack NA3T EME(B,D), FM05lw North Carolina
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