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- Path: sparky!uunet!cs.utexas.edu!zaphod.mps.ohio-state.edu!wupost!usc!randvax!edhall
- From: edhall@rand.org (Ed Hall)
- Newsgroups: sci.electronics
- Subject: Re: Where to mount an outside temp probe on a car?
- Message-ID: <4145@randvax.rand.org>
- Date: 5 Jan 93 02:26:40 GMT
- References: <1993Jan4.134028.27684@phx.mcd.mot.com> <86134@ut-emx.uucp> <1993Jan4.223650.9171@phx.mcd.mot.com>
- Sender: news@randvax.rand.org
- Organization: RAND
- Lines: 52
- Nntp-Posting-Host: ives.rand.org
-
- In article <1993Jan4.223650.9171@phx.mcd.mot.com> schuch@phx.mcd.mot.com (John Schuch) writes:
- >Excuse me but if you cool the sensor by placing it in an airstream, your
- >not reading the actual temperature of the air.
-
- Say what? Assuming the sensor doesn't generate heat, how does moving
- air past it do anything but speed up its equilibrium with the air
- temperature?
-
- >Reading a sensor cooled
- >by an airstream, by itself, tells you nothing. If you measure one sensor
- >in an airstream and another one nearby but shielded from the air stream,
- >you can determine the speed of the airstream.
-
- But that's only if the sensor generates heat! Otherwise (ignoring venturi
- and friction effects for the moment) the two sensors will reach exactly
- the same equilibrium temperature, except the one in the airflow will do so
- more rapidly, and will respond faster to changes.
-
- >Other than that, I don't
- >know why you would want to cool the sensor.
-
- Neither do I.
-
- >You will note that weather stations keep the thermometer in a little
- >louvered box to break-up the wind. Even high-end indoor/outdoor
- >thermometers include a little shell to place over the external
- >sensor.
-
- Yup. Helps isolate the sensor from the sun and other sources of
- radiant heat. It isn't to protect it from the wind; in fact, you might
- well find a circulating fan in that little louvered box to help lower
- the sensor's time constant and reduce the effects of IR absorption by
- the enclosure.
-
- But back to the original subject: placement can make a difference,
- especially at higher speeds, due to the venturi effect. (Air friction
- is probably less of a problem at normal automotive speeds.) This could
- lead to either a high or low reading, depending upon whether the air
- is under local compression or rarification.
-
- Any physics types want to estimate the magnitude of this effect?
-
- Proximity to local heat sources (engine, pavement, tires, or surfaces
- subject to solar radiation) can have an effect; air movement might well be
- a good way to minimize this, however. Shielding would be a good idea, but
- the idea is to minimize IR absorption, not shield from wind. Perhaps a
- thin insulating tube, painted outside with reflective paint and oriented
- /parallel/ to the airflow, with a small sensor mounted inside (such that
- the airflow is obstructed by it as little as possible).
-
- -Ed Hall
- edhall@rand.org
-