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

[Karl Uppiano]

I have had good results with mine once I shielded it from the light. I found 
that even indirect sunlight on a bright sunny day -- even inside a radiation 
shield -- would cause the sensor to get all flaky. I put a small piece of 
black heat shrink tubing over it, leaving openings at each end for the air 
to circulate (i.e., I did not shrink the tubing, I just allowed it to remain 
loose). This sensor has been in place for about 18 months, and the original 
one might have been OK too, if I had shielded it from the light.

The relative humidity around here is frequently 100%. I wrote my own 
proprietary software 
(http://mysite.verizon.net/Karl_Uppiano/wxservice.html), which re-reads the 
instrument until three consecutive values agree within a configurable 
tolerance, and returns the average of the three.

----- Original Message ----- 
From: "Steinar Midtskogen" <steinar at latinitas.org>
To: "List for 1 Wire Weather Stations and devices" <weather at buoy.com>
Sent: Monday, November 27, 2006 11:55 PM
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
>
>
>