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

[Paul Davis]

There is a potential alternative in the Humirel HTM1735. It's spec'ed at
0-100% RH operating range. And they say "HTM1735 is specified for accurate
measurements within 10% to 95%" and "Excursion out of this range (<10% or
>95%) does not affect the reliability of HTM1735 characteristics." That
sounds like that's about as good as it's going to get.

This is a linear voltage output sensor, like the HIH-3610/HIH4000. I'm sure
the slope is different, so you'd need to adjust in software.

DigiKey has them for $37.61, Farnell doesn't appear to carry them. The
Humirel web page (under samples) quotes a 5 pack at 110 Euro with a two week
lead time.

I've attached the datasheet to this message, but I don't know if the list
software will strip it out. A link to the datasheet is here:
<http://www.humirel.com/product/fichier/HTM1735%20RevG%20.pdf>

Haven't got one yet, but it's on the (rather long) list of things to do this
winter. If anyone actually tries these, please let us know how it goes. Hope
this helps.

Paul

On 11/28/06 2:55 AM, "Steinar Midtskogen" <steinar at latinitas.org> wrote:

> [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%.

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