Re: WiFi range ...

c186282 <[email protected]> wrote:
> 
>   Now you CAN make a wet/dry-bulb unit using two DS
>   18b20 temperature sensors - one kinda wrapped in a
>   light wet cloth. Suggest dipping both sensors in epoxy
>   to enhance water resistance. DO adjust the reported
>   temperatures, dry, to be the same. The Dallas 1-wire
>   devices are GOOD, but no two of them will read
>   EXACTLY the same. Calibrate at the bench, using a
>   scientific conventional thermometer as reference.
> 
>   Now the wet bulb, how the hell do you KEEP it wet ?
>   This can be a PROBLEM depending on environment. Where
>   I was, there was surely a little rain every 7-10 days.
>   A cheepo reservoir was easy to rig. It's also possible
>   to lace the wet cloth with a chemical, ammonium nitrate
>   for example, which is extremely hydroscopic - but the
>   chosen salt WILL affect the reading and compensation
>   will be needed.
> 
>   Anyway, wet+dry will give you a very real figure, but
>   at added complexity.
> 
>   Do you really NEED super-accurate humidity readings ?
>   For like a big commercial greenhouse, YES. For yer
>   back-yard, close-enough may be close-enough.
> 
Back when I was an student in the 80's, one mechanical engineering
professor described doing wet/dry bulb measurements.  The thermometers
were tied to lanyards, and you would whirl them around in the air at
a certain rate to get more air flow over the bulbs during the
measurement.  It increased the difference between the wet and dry
readings for better accuracy, and the tables were adjusted for that
affect.  My old thermodynamics textbook has a Psychrometric Chart in
the back that you could use for calculating the absolute humidity from
the temperatures.  It doesn't say whether the bulbs should be whirled
around or not, maybe that was common knowledge back then.

Cheers,
Rob Komar