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

[Paul Davis]

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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