🚀 go-pugleaf

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