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From: Steve Turner
Subject: Re: Humidity sensor - peculiar behavior
X-Newsreader: Forte Agent 1.92/32.572
Date: Tue, 07 Jan 2003 03:34:21 GMT
NNTP-Posting-Date: Tue, 07 Jan 2003 03:34:21 GMT
Organization: AT&T Worldnet
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:
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. This is exactly what I have seen!
However, I am absolutely amazed at the magnitude of the effect, or, to
put it another way, that 28 mW of heating in a nearby low-power
voltage regulator is so easily detected!
Bottom line: when installing these sensors, they need to be kept
thermally isolated from any associated circuitry.
Many thanks again to all who responded. Boy did I learn something
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