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Subject: Re: Humidity sensor - peculiar behavior
References: <email@example.com> <firstname.lastname@example.org>
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NNTP-Posting-Date: Sat, 11 Jan 2003 18:50:34 GMT
Organization: Insight Broadband
Date: Sat, 11 Jan 2003 18:50:34 GMT
On Sat, 11 Jan 2003 14:53:40 GMT, Marc_F_Hult
wrote in message :
>If the effects you measure ( up to 40% change) are due to temperature
>in the air caused by heat dissipated by electronics as you postulate, the
>sensor is badly out of spec. See the last graph on the spec sheet.
>Note that Honeywell suggests a way to measure air at near 100% relative
>humidity (which is difficult to do) by locally reducing the RH to about 90%.
>This involves setting the sensor on top of a 120W resistor. Nuff said.
>I have a bunch of HIH-3610 including several that have never been out of
>factory sealed packaging. Send me your snail-mail and I'll loan you a couple
>to test/examine. I paid about $18 each when they first began shipping from
>the Mexico factory so these are from early production. Dunno who might be
>charging the $40 you paid. If you decide that you need to replace yours, you
>can have one for what I paid.
>Other technical information of interest including humidity equations,
>constants, and concepts:
>Curious that none of the responders mentioned reading the product
>FWIW, I typically mount the HIH-3610 (and Panasonic PNA4603H for light and
>thermistor/LM34 for temperature ) on one side of a small pcb and a quad op
>amp and voltage references/regulators on the other side. This [facilitates]
>mounting the pcb in a case with the sensors in appropriate position with
>respect to case openings and would isolate the HIH-3610 from the thermal
>effects you postulate.
>I drive a HIH-3610 from a LM4040 voltage ref buffered by one section of a
>LM324. The three outputs (humidity, light, and temperature) are buffered by
>the other three LM324 sections. Fancy-antsy op amps aren't needed at room
>temperature to get adequate 0-5vdc (or 0-10vdc) signals . CAT-5 wire has
>twisted pairs for the 13.8vdc supply (from a sealed lead battery with a
>"float" regulator/supply) and the three DC signals. You can gild the lily by
>regulating the 13.8 vdc at the pcb with a 12vdc Low Drop Out TO-92 package
>regulator. Sprinkle with capacitors "to taste".
>HTH ... Marc
I should add that the HIH-3610 does not take kindly to being subjected to
100% (condensing)RH. IIRC, Honeywell recommends drying the sensor out for 10
hours at <10 RH. The sensor may not recover completely, but should recover to
within 3% or so. If you started your calibration with 100% humidity, this may
be part or all of your problems.
HTH ... Marc
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