-> how can one set up their own "shadetree mechanic" type of
instruments like t 
-> wet bulb thermometer? is it just a matter of having two thermometers the sam 
-> and mounting out with a cloth that is soaked with water? 
  
The wet bulb has to be very well "ventilated", to allow easy exchange 
of both water-vapour and heat with the atmosphere. At the very least, 
you should have some sort of fan blowing over it. Also, you have to 
make sure that impurities don't build up in the water impregnating the 
cloth. Obviously, if you use tap water, with some impurities, their 
concentrations will build up as the water evaporates, which will reduce 
the vapour-pressure of the water, and therefore the lowering of the 
bulb's temperature. So you should use distilled water and, even so, you 
should rinse out the cloth quite frequently (using distilled water) to 
get rid of the impurities. 
  
And you must make sure that virtually no heat gets *conducted* to the 
wet bulb from the surroundings, which will be warmer. 
  
It's not a very easy instrument to make. Do you *really* need to know 
the atmospheric humidity more accurately than would be shown by a cheap 
hygrometer using something like a hair? 
  
                           dow 
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