From: John Popelish
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Subject: Re: Humidity sensor - peculiar behavior
Date: Sat, 04 Jan 2003 06:17:06 GMT
NNTP-Posting-Date: Sat, 04 Jan 2003 01:17:06 EST
Steve Turner wrote:
> I am interested in adding a humidity sensor to a computerized weather
> station of my own design. The sensor is a Hycal HIH-3610 003 unit,
> which is a three-lead device with V+, ground, and Vout. The output
> voltage is [supposed to be] a linear function of humidity, varying as:
> Vout = 0.0314 * RH + 0.841 (values dependent on sensor specimen)
> I have wired my sensor into a small PC board containing a 5v voltage
> regulator (78L05) and a LMC 6062 op amp follower for sensor output
> buffering. I then attempted to verify the slope and intercept values
> in the equation above by equilibrating the sensor board overnight in a
> constant humidity chamber and then measuring the output voltage. When
> I do this, I find that the *initial value* of the output, on first
> powering the sensor board, is exactly where it should be. This is
> true at three different humidities (30%, 75%, 93%). However, the
> initial voltage output then decays in a nonlinear fashion (quickly at
> first, then more slowly), eventually stabilizing at a new value which
> is 10-30% lower!!
> I thought this type of behavior may be due to sensor self-heating, but
> the entire board (sensor, vreg, and op amp) only consumes about 4 mA @
> 12v. The sensor itself is only supposed to consume 200 uA @ 5v. The
> output of the regulator does not change. There is no detectable
> oscillation of the voltage follower.
> Any ideas what is causing this, and what the solution is??
> Many thanks.
> Steve Turner
> Real address contains worldnet instead of spamnet
Are you keeping the sensor in the dark during this test? It is light
sensitive, but I have no idea if this sensitivity shows up as the
symptom you describe.