From: Steve Turner
Subject: Re: Humidity sensor - peculiar behavior
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Date: Wed, 08 Jan 2003 04:11:33 GMT
NNTP-Posting-Date: Wed, 08 Jan 2003 04:11:33 GMT
Organization: AT&T Worldnet
Spehro Pefhany wrote:
>>sensor. The working theory is that the sensor was being heated by the
>>power dissipation (all of about 30 mW!!!) of the onboard voltage
>>regulator (which takes 12v down to 5v at a tad over 4 mA). The
>>following four experiments all point to this conclusion:
>Are you counting the regulator bias current in this? If not, double
>it approximately, as a 78L05 draws about 4mA with no output.
The 4 mA figure that I mentioned was an acutal measurement. The
current consumption of the entire sensor board was about 4.6 mA,
nearly all of which was presumably dissipated in the regulator.
Consistent with what you mention above.
>Yes, if the air isn't stirred. Your salts should act as a buffer to
>maintain the RH fixed if it is stirred, right?
Yes, they do. But this equilibration is much slower, typically taking
several hours for the drift in output reading to stop. I usually
allow overnight for re-equilibration after opening a jar. This really
slows experimentation, as you can imagine.
>The temperature effect on the actual sensor output should be MUCH
>less, zero to 10% for a change of 85'C in Ta, according to the
>data at: http://www.aag.com.mx/PDF%20Docs/hih3610.pdf
>I have not dragged out Liptak and the psychrometric chart to see
>what happens when you heat a sample of air by a few degrees, but
>perhaps you should.
Undoubtedly. Ummm.... what's Liptak?
You seem to be doubting that sensor microenvironment heating is
responsible for the effect. I find it hard to believe myself, but I
can't come up with any other explanation that makes any sense. If you
can, please feel free to enlighten us! :)
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