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Subject: Re: Humidity sensor - peculiar behavior
References: <firstname.lastname@example.org> <email@example.com>
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NNTP-Posting-Date: Sat, 11 Jan 2003 14:53:40 GMT
Organization: Insight Broadband
Date: Sat, 11 Jan 2003 14:53:40 GMT
On Thu, 09 Jan 2003 02:47:48 GMT, Steve Turner
wrote in message :
>Spehro Pefhany wrote:
>>>I should mention one other thing -- this kind of calculation is quite
>>>beyond me. I know the power dissipation in the circuit -- about 30
>>>mW. If I knew the volume of air that this was heating then I could
>>>figure the heat rise, but I haven't the faintest idea what the
>>>"effective" volume of air being heated is. It is certainly not the
>>>entire volume of the chamber. This is very much a non-equilibrium
>>>situation. I would guess that there is convective streaming upwards
>>>from the voltage regulator, and the effect on output will be greatest
>>>if the humidity sensor sits in this stream. But what its temperature
>>>is -- sheesh, I don't know.
>>Right, but my inclination, as an engineer, is to try to find some
>>way to ball-park it, perhaps by working backwards and finding out
>>what the temperature delta would have to be to make that much change,
>>and seeing if that appears to be reasonable.
>I understand, and I agree it would be advisable to do this.
>This "small" addition to my weather station has soaked up far more
>time than originally anticipated. I enjoy learning and experimenting,
>and it's unfortunate that I also have to earn a living -!
>> There does not seem
>>to be much left that it could be- the RH in your chamber, in the
>>vicinity of the sensor, simply is not what it should be, probably due
>>to the heating, would be my conclusion at this point, but I'd
>>look for a "sanity check" on the conclusion before accepting it.
>Agreed. And I am still NOT 100% convinced that thermal effects are
>wholly responsible for what I'm seeing. There is STILL some drift
>even when the sensor is completely isolated thermally from other
>circuitry (self heating from 200 uA of power consumption!!???). The
>explanation that I posted is just the one which seems to me to fit
>[most of] the data. That doesn't make it correct.
>>Liptak ( _Process Control_ and _Process Measurement_ ) is one
>>of the bibles. A good bunch of info on humidity can be sourced
>>from ASHRAE and in the commercial vendors' info such as General
>>Eastern and Vaisala.
>Okay, thanks for the tip. It would seem to me, though I have yet to
>do the calculations, that the only empirical data needed would be a
>table [or graph] of moisture content of water-saturated air (i.e.
>100% RH) as a function of temperature.
>Real address contains worldnet instead of spamnet
If the effects you measure ( up to 40% change) are due to temperature changes
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 literature.
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 fa ciliates
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 four
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
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