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How much air should be moved across a wet bulb ??
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Author:  Gregg C [ Mon Apr 12, 2010 9:46 am ]
Post subject:  How much air should be moved across a wet bulb ??
How much air should be moved across a wet bulb ??
I ask this because I have experimented with my small 4" fan at 12", 24" & 36"
from my web bulb test assembly.
It seems that ,of course, the more air moved across the lower the wet thermometer
reading.
#1...Is there a "standard" amount of air that should be moved ??
#2...Or should the air being moved always consistant ??
I'm thinking the latter.
Author:  Alexandru Marian [ Mon Apr 12, 2010 10:23 am ]
Post subject:  Re: How much air should be moved across a wet bulb ??
I am using a 2 L plastic bottle with the ends cut, and at one end fitted with a computer 80mm fan blowing in, powered from the computer :) The thermometer is inserted vertically through a hole in the center of the pipe. It takes a few minutes for the temperature to stabilize. It is easy to notice when it is not going done anymore and then you can use the reading. I tried a few other methods before but it never worked, always had too small temperature differences between dry and wet. This sealed tunnel thing works fine for me now.
Author:  vpelleri [ Mon Apr 12, 2010 11:10 am ]
Post subject:  Re: How much air should be moved across a wet bulb ??
As Alexandru stated, there is no "required" amount of air. You want to move the air across the bulb until you get a stable reading.
Author:  CWLiu [ Mon Apr 12, 2010 12:22 pm ]
Post subject:  Re: How much air should be moved across a wet bulb ??
I've read that the wind speed must be over 4 m/sec, IIRC.
Author:  Gregg C [ Mon Apr 12, 2010 1:34 pm ]
Post subject:  Re: How much air should be moved across a wet bulb ??
4 m/sec = 9 mph
Author:  Darryl Young [ Tue Apr 13, 2010 8:23 pm ]
Post subject:  Re: How much air should be moved across a wet bulb ??
I think it would be better to suction air across the thermometer rather than blow it. A fan can raise the temperature of the air when it's compressed by the fan blade.

Wasn't it Mario that used a vaacum cleaner to suction air across his wet/dry bulb thermometers? That sounds like a nice idea......pull air across the thermometers that hasn't been near your body.
Author:  Hesh [ Wed Apr 14, 2010 1:41 am ]
Post subject:  Re: How much air should be moved across a wet bulb ??
A couple of things:

First I don't think that it matters how much air is moving either in as much as the stable reading as stated above is the goal. You get to a stable reading by air flow and time and the combination of these two essential things can vary accordingly. Less air movement requires more time to stabilize and vice versa.

I draw air over the bulbs with a vac too - it works fine. You do have to be mindful that RH is dynamic and even my vac being on for several minutes exhausting hot air can skew results.

Also be sure that your air movement plan has the wet bulb down stream of the dry bulb so the wet bulb's evaporating moisture does not influence the dry bulb.
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