[Weather] 1-wire humidity sensor for outdoor use?

[Nafis, Christopher A (GE, Research)]

Does anyone one know what the higher end weatherstations (ie. The Davis
Vantage Pro2) use for humidity sensors?
Maybe we could learn something? 

-----Original Message-----
From: Steinar Midtskogen [mailto:steinar at latinitas.org] 
Sent: Tuesday, November 28, 2006 2:56 AM
To: List for 1 Wire Weather Stations and devices
Subject: Re: [Weather] 1-wire humidity sensor for outdoor use?

[Nafis, Christopher A (GE, Research)]

> I've been using the AAG TAI8540 Humidity sensor mounted in a Davis 
> Radiation shield for an outside humidity/temperature sensor (AAG 
> states "designed for indoor use"). I've coated the circuit board with 
> liquid tape, but the honeywell sensor still seems to "die" after a
year or so.
> Hobby Boards also sells the HT3-R1-A Humidity / Temp sensor with a 
> moisture resistant coating.

My Honeywell sensors usually don't last longer than half a year.
After a while, I get 0% readings, or readings stuck at a certain %, or
they indicate more or less random humidity.  Even new sensors using
factory calibration are a problem; they sort of work, but without
further calibration they may report more than 115% in heavy fog.

I've tried a lot of things: different shieldings, different boars (AAG
indoor, AAG outdoor, Springbok), coating and heating.  The idea with the
heating was to keep humidity well below 90% at all times and calculate
the true rh based on the assumptation that the dew point in the heated
box and in the ambient air is the same.  Perhaps the sensor would last
longer, but I didn't get a useable accuracy.  Right now I'm trying to
wrap a nylon sock (from old stockings) around the sensor (either the AAG
enclosure or the sensor itself), which is suggested in the Weather Toys
book.  So far it doesn't look too promising as it seems to make the
sensor very slow and less accurate (though it seems to fix the problem
with readings above 115%).

My conclusion is that the Honeywell sensor is fine for indoor use or in
climates where humidity is always between 30 and 80%.  For coastal or
mountain use, it's a constant calibration and maintenance nightmare.  I
just had 10 days of continous fog and rain here (i.e. rh in the high
90's all the time and about 130 mm of rain/sleet and windy at times) and
two newly mounted sensors that were matching eachother perfectly, were
off eachother by 10 to 15% when the fog lifted and things dried up.

If anyone has any other ideas how to fix the honeywell sensor, I'm ready
to try anything.  Or if anyone can think of a design using another
sensor, I would happily switch to those if they're likely to need less
maintenance, even if they're $200 each.  I can live with bad accuracy
above 90%, if the sensor goes back to normal accuracy below 90%.

--
Steinar