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

[Karl Uppiano]

I am using the TAI8540A 1-Wire Humidity Module from AAG. It runs on 
parasitic power. I upgraded the original sensor to an HIH4000 about a year 
ago, thinking that the sensor had gone bad. I think it was a light 
sensitivity problem, because the new sensor began doing exactly the same 
thing within minutes of installation. Shielding it from indirect sunlight 
did the trick.

I have had numerous problems with communication on the 1-Wire MicroLAN, 
which show up in my software logs (having strong opinions about what 
features I want in my weather station, I wrote my own software 
http://mysite.verizon.net/Karl_Uppiano/wxservice.html). If your software 
does not log 1-Wire errors, you might be blaming the sensor for a problem it 
is not causing.

Here is an excerpt from a recent log, showing the kind of things I routinely 
see on my 1-Wire network:

    Nov 28, 2006 8:56:45 AM wxservice.WindVaneTask$ReadTask run
    WARNING: OneWireContainer20 - Failure during conversion - Bad CRC
    Nov 28, 2006 10:45:24 AM wxservice.HygrometerTask$ReadTask run
    WARNING: OneWireContainer26-Bad CRC during read.99
    Nov 28, 2006 11:14:45 AM wxservice.WindVaneTask$ReadTask run
    WARNING: OneWireContainer20 - Failure during conversion - Bad CRC
    Nov 28, 2006 12:47:45 PM wxservice.HygrometerTask$ReadTask run
    WARNING: OneWireContainer26-Bad CRC during read.99

The software is fault-tolerant, so it just retries the operation, but the 
warnings give me an indication of the health of my 1-Wire MicroLAN.

----- Original Message ----- 
From: "Paul Davis" <pdavis at mrv.com>
To: "List for 1 Wire Weather Stations and devices" <weather at buoy.com>
Sent: Tuesday, November 28, 2006 9:33 AM
Subject: Re: [Weather] 1-wire humidity sensor for outdoor use?


I hadn't really considered parasitic power. I think this sensor presents
too low an impedance to power from the one wire. You would need to use
separate power like the barometer. I should think the accuracy would
improve anyway with a regulated supply. Since I added a hub a while ago,
powering sensors is pretty easy for me.

The Honeywell doesn't *require* temperature compensation, but since we
have the temperature from the DS2438, it's easy to add the compensation
and gain a little accuracy. The temperature coefficient of the HTM1735
is specified -0.05 to -0.1 %RH per degree C, so not much error, and
would appear to be less than half that of the HIH3610.

Once we get some in use we'll get a better understanding of the details.

Paul

-----Original Message-----
From: weather-bounces+pdavis=mrv.com at buoy.com
[mailto:weather-bounces+pdavis=mrv.com at buoy.com] On Behalf Of Steinar
Midtskogen
Sent: Tuesday, November 28, 2006 10:02 AM
To: List for 1 Wire Weather Stations and devices
Subject: Re: [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.

Would it be possible to run it on parasite power?  The datasheet says
that it needs 5V and 2.8 mA (4 max).

> 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.

I think the Honeywell needs temperature correction in software, while
the Humirel doesn't seem to require that, so the software should be
simpler.  And it should work fine in Oww as of today.  Simply say that
it's a barometer with a suitable slope and offset.

-- 
Steinar

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