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

[Joseph Mack NA3T via TriLUG]

On Thu, 21 Dec 2017, Matt Flyer via TriLUG wrote:

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

I had expected discontinuities at 32F too, but if you look at a psychrometric 
chart, there are no discontinuities at 32F. The vapour pressure -v- temp is 
smooth through freezing, the dew point temperature goes smoothly through 
freezing as a function of the mass of water/mass of air, the wet bulb lines are 
straight as they cross freezing. From looking at a psychrometric chart, you 
can't spot a phase change at 32F.

At low humidity (eg presumably Atacama Desert), frost won't form till well below 
freezing.

Here on the east coast, we're used to frost forming at freezing, because a short 
time previously the air had been above freezing with dew point above freezing. 
As the temperature dropped, the air would have become saturated before the 
temperature dropped to freezing. The condensation would initially come out as 
water, which turned to clear ice as the temperature reached freezing. As the 
temperature dropped further, the water vapour would have condensed directly as 
microscopic icicles on top of the clear ice. We expect frost at freezing, but 
it's only because there's a lot of moisture in the air.

I can't measure dew point directly here at home, but in the recent mornings, the 
wet bulb has been below the dry bulb, when it's been below freezing. So the wet 
bulb keeps working below freezing.

Another familiar scenario: The freezing point of CO2 is -78.5C at 1Atm (about 
1bar) (see phase diagram at

http://www.chemicalogic.com/Pages/DownloadPhaseDiagrams.aspx

)

However CO2 is 0.04% of the atmosphere, so the partial pressure of CO2 in the 
air is not 1Atm but 1/2500 Atm. To freeze CO2 out of the atmosphere, you need 
temperatures way below -100C.

(for further discussion see 
https://wattsupwiththat.com/2009/06/09/co2-condensation-in-antarctica-at-113f/

The temperature to form CO2 snow on Mars is similarly affected by the low 
pressure of CO2 on Mars.)

It turns out that they don't make bricks of dry ice directly from air, they make 
it from liquid CO2 which is poured into molds and then allowed to evaporate till 
it freezes.

https://www.continentalcarbonic.com/how-is-dry-ice-made.html

They don't say where the liquid CO2 comes from.

It seems it doesn't come from the atmosphere, which has almost CO2. It comes 
from a place that you get 100% CO2, either exhaust gas, or fermentation (or 
sometimes CO2 wells).

from 
https://www.google.com/search?q=where+does+liquid+CO2+come+from&ie=utf-8&oe=utf-8&aq=t&rls=org.mozilla:en-US:official&client=firefox-a

High-purity liquefied carbon dioxide gas plants separate this CO2 from the 
exhaust gas and refine it, and then compress it to a liquid state for storage. 
The high-purity liquefied carbon dioxide gas produced in such plants can be used 
in dry ice, or to produce bubbles in carbonated beverages.

also see

http://www.pbnation.com/showthread.php?t=1082541
https://www.ascoco2.com/us/co2-production-and-co2-recovery-plants/general-information-about-co2/from-liquid-co2-into-dry-ice/
http://www.hitachi.com/businesses/infrastructure/product_site/ip/products/carbon_dioxide.html

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
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