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/iframe></center><br><br> <br><br> </center> </td></tr> </table> <center> <table> <tr> <td width="600"> <center><hr width="400"><font size="2"><b>The <a href="http://www.cyber-spy.com/">Cyber-Spy.Com</a> Usenet Archive Feeds Directly<br>From The Open And Publicly Available Newsgroup<br>Sci.Electronics.Design<br><br>This Group And Thousands Of Others Are Available<br>On Most IS NNTP News Servers On Port 119.<br><br><a href="http://www.cyber-spy.com/">Cyber-Spy.Com</a> Is NOT Responsible For Any Topic,<br>Opinions Or Content Posted To This Or Any Other<br>Newsgroup. This Web Archive Of The Newsgroup And<br>Posts Are For Informational Purposes Only.<br><br></b></font><hr width="400"></center> <center><font size="2"><pre><b> From: Steve Turner <srturner1@spamnet.att.net> Newsgroups: sci.electronics.basics,sci.electronics.misc,sci.electronics.design Subject: Re: Humidity sensor - peculiar behavior Message-ID: <lndk1vce922cem10s372hni4igue54i57p@4ax.com> References: <7trc1v41b04ltr5l2h9m5uphqa4opq1dfc@4ax.com> X-Newsreader: Forte Agent 1.92/32.572 Date: Tue, 07 Jan 2003 03:34:21 GMT NNTP-Posting-Host: 12.87.100.144 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 sensor; 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 today! Steve Turner Real address contains worldnet instead of spamnet </b></pre></font><hr width="650"><a href="http://www.cyber-spy.com/electo-index.html"><font size="3">Go Back To The <a href="http://www.cyber-spy.com/">Cyber-Spy.Com</a><br>Usenet Web Archive Index Of<br>The sci.electronics.design Newsgroup</font></a><br><br></center></td><img src="http://cyber-spy.com/cgi-bin/livecntr.pl?0+1+unetsci" width="1" height="1" alt="The Cyber-Spy Toolkit - powerful internet spy software - live police scanners - usenet fbi secrets archive - newsgroup spy electronics archive - search engine spy tools - download TCST today!"> </tr> </table> </center> <center> <br><center><hr size="3"></center> <table width="100%" cellpadding="0" border="0" cellspacing="0"> <tr> <td align="left" valign="top"><small><b><i><a href="http://www.cyber-spy.com/index.html">Cyber-Spy.Com Live Resources</a></i></b></small></td> <td align="right" valign="top"><small> Copyright Alpha Omega Delta,<br> All rights reserved. </small></td> </tr> </table> <center><hr size="3"></center> <br><br> <br><br></center> </body></html>