From: Spehro Pefhany
Subject: Re: Humidity sensor - peculiar behavior
References: <email@example.com> <firstname.lastname@example.org>
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Date: Wed, 08 Jan 2003 04:39:02 GMT
NNTP-Posting-Date: Tue, 07 Jan 2003 23:39:02 EST
On Wed, 08 Jan 2003 04:31:32 GMT, the renowned Steve Turner
>>Spehro Pefhany wrote:
>>>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.
>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. 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.
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.
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