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From: Spehro Pefhany
Subject: Re: Humidity sensor - peculiar behavior
X-Newsreader: Forte Agent 1.92/32.572
Date: Wed, 08 Jan 2003 00:31:15 GMT
NNTP-Posting-Date: Tue, 07 Jan 2003 19:31:15 EST
On Tue, 07 Jan 2003 03:34:21 GMT, the renowned Steve Turner
>Okay, this should be titled "Eureka, I've found it!!" (I think).
>This has been one bugger of a problem to diagnose. I am posting the
>apparent solution for those who are interested. Sorry for the length,
>but I find this interesting and wanted to be clear.
>I kept coming back the time-dependent, exponential-like decay of
>voltage output which occurred only AFTER the sensor board was powered
>up. This would not be expected with components or PCB material which
>was itself humidity sensing. The effect was highly reminiscent of a
>heating effect. I now believe that this was the cause of the
>anomalous results I was seeing, but it wasn't self-heating of the
>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.
>1) Feeding the board with a smaller voltage (7v, just enough to
>maintain regulation of the onboard 78L05) results in a much smaller
>voltage decay after powerup of the board. Interpretation: smaller
>power dissipation in the regulator = smaller thermal effect on sensor;
>2) Running the sensor board in a cardboard box with a small fan
>blowing air across it continuously results in a reading with some
>minor random variation but NO systematic decay of output after
>powerup. Interpretation: moving air across sensor prevents heating of
>3) Adding a 1k resistor near the sensor, which could be powered on and
>off at will with the sensor in the humidity chamber, had a dramatic
>effect on output. This small (~140 mW) heater had an effect which was
>qualitatively similar but much larger in magnitude than what I had
>been seeing. Interpretation: heating of the sensor microenvironment
>does cause a large negative shift in sensor output;
>4) Bypassing the onboard voltage regulator and feeding the board with
>5v from a bench supply gave an output which decayed only slightly
>after powerup. Interpretation: most heating was due to the voltage
>regulator, as expected. Some small drift could be due to op amp
>heating or self-heating of the sensor. In fact, results obtained with
>this configuration approach the results with the naked sensor (which
>also drifted slightly, apparently due to self-heating).
>For what it's worth, this hypersensitivity to thermal effects may be
>exactly as anticipated; I haven't done the calculations. If the
>amount of moisture in a given sample of air remains the same, heating
>the air will lower the RH.
Yes, if the air isn't stirred. Your salts should act as a buffer to
maintain the RH fixed if it is stirred, right?
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.
"it's the network..." "The Journey is the reward"
firstname.lastname@example.org Info for manufacturers: http://www.trexon.com
Embedded software/hardware/analog Info for designers: http://www.speff.com
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