I use a system just like this in a humidity chamber controller.  It has to
control a room at 90% RH +/- 2%.  The wet bulb, of course, has to remain
wet, and we use de-ionized water to prevent salts from building up on the
sock as it dries.  The sock around the wet bulb (actually it is a stainless
steel thermocouple) tends to get a little nasty, if there is any dirt it
will grow a nice crop of mold.  DI water really helps this, though.

Wet/dry bulb systems are low tech, cheap, and pretty reliable, as long as
your wet bulb sensor is made of stainless.  We hang them all in front of a
small plastic centrifugal fan, you could use a muffin fan as well.

The math to convert wet bulb/dry bulb to RH is kinda hairy, especially for a
memory starved PIC programmed in 8 bit math/assembly.  In C, with 2K or 4K
of elbow room and 16 bit math, it would be very doable.

My program is running on a decicated PC and is written in a control language
called TestPoint.  It would not be portable to a micro.  Somewhere I might
have notes on the math involved, basically look up an ASHRAE Fundamentals
handbook and go throught he formulaes in the psychrometrics chapter.

--Lawrence Lile

----- Original Message -----
From: "juan manuel garofalo" <animacion3d@HOTPOP.COM>
To: <PICLIST@MITVMA.MIT.EDU>
Sent: Wednesday, September 26, 2001 6:08 PM
Subject: [EE]:humidity sensors


> Sorry this has been discussed a couple of days ago...
>
> What about using two temperature sensors in a wet/dry bulb configuration ?
A
> psycrometer  is it called ? (I'm sure i got the spelling wrong...)
>
> It's cheap, for one thing. I don't know about its drawbacks, though. I'd
> like to hear some comments.
>
>
> thanks.
> jm
>
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