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cs
12-06-2010, 03:37 PM
Okay maybe I don't know much about this, well no maybe about it, but I have a question. Say you have on of those cars that tell you what the outside temperature is. Okay I understand that and I know that thermometers use mercury to measure and I'm sure modern day electronics has a way to measure air temperature, probably by some sensor that expends and contracts and the expansion/contraction is measure and convereted to temperature.

Okay, how? And I know that there is a difference between temperature and wind chill, but how does the thermometer know the difference? When you blow on something it cools it down, right? So if you have some way of measure the outside temperature in your car, does not the wind effect it? If so why not, if not so why?

Chad

Paul Pless
12-06-2010, 03:40 PM
the thermometer* is protected from the wind

probably not really a thermometer but some type of electronic thermister is what is used in the cars

Peerie Maa
12-06-2010, 03:40 PM
Perhaps the temperature sensor is in a pocket of dead air somewhere under the bonnet, where there is no wind to speak of?

cs
12-06-2010, 03:43 PM
But if that is the case wouldn't the heat of engine have at least a minimal effect on the outside air temp?

Chad

JBreeze
12-06-2010, 03:48 PM
Wet bulb vs dry bulb thermometer:

http://en.wikipedia.org/wiki/Wet-bulb_temperature

BarnacleGrim
12-06-2010, 03:50 PM
Okay, how? And I know that there is a difference between temperature and wind chill, but how does the thermometer know the difference? When you blow on something it cools it down, right? So if you have some way of measure the outside temperature in your car, does not the wind effect it? If so why not, if not so why?

Chad
An electronic thermometer sensor (a thermistor) changes its resistance depending on its temperature. It's unaffected by wind chill.

The human body loses more heat in high winds. The wind chill is the temperature at zero wind where you would lose as much heat as in high wind. This of course depends on clothing, body weight, etc.

Canoez
12-06-2010, 03:50 PM
Carefully placed RTD or Thermistor.

cs
12-06-2010, 04:03 PM
I see several different ideas here and have come to the conclussion that it is above my limited understanding of temperature. I will keep studying this to try and understand more.

Chad

bob winter
12-06-2010, 04:08 PM
The one in our car seems to be reasonably accurate. I could do without the warning about possible slippery conditions when it is -7C though, Who would have thought

Dan McCosh
12-06-2010, 05:00 PM
"wind chill" depends on the effect of evaporation to cool the object subjected to the wind. A dry object will not be cooled by the wind below the ambient temperature, although it will reach this temperature faster if it starts out warm--which is how an air-cooled engine works.

seanz
12-06-2010, 05:03 PM
I see several different ideas here and have come to the conclussion that it is above my limited understanding of temperature. I will keep studying this to try and understand more.

Chad

We drive in a variety of conditions here Chad.......Sea-level, Alpine pass etc. and we've found the temp read-out on the dash to be fairly reliable....matches fairly well with the weather report temp.

They're fun to play with when you're going into the mountains......push the button every five minutes and see the temp drop as you climb.

Of course, I'm very easily amused.
:)

htom
12-06-2010, 05:24 PM
I think they're a better warning for ice (air temp under 34 F) than the flashing sensor, which frequently seems to flash -after- I've felt that little jink of one tire losing traction.

cs
12-06-2010, 07:51 PM
We drive in a variety of conditions here Chad.......Sea-level, Alpine pass etc. and we've found the temp read-out on the dash to be fairly reliable....matches fairly well with the weather report temp.

They're fun to play with when you're going into the mountains......push the button every five minutes and see the temp drop as you climb.

Of course, I'm very easily amused.
:)

Friday I was in a Chevy HHR (government car) and it gives you a constant reading. As I was going up and over Mont Eagle I noticed the outside air temp drop as I went up and climb as I went down. It was only 3 degrees, but there was a difference between top and bottom.

Chad

BTW I did notice this morning when I went out to re-fuel the HHR that when I turned on the ignition the dash said "possible ice" and the temp was 29 outside.

Keith Wilson
12-06-2010, 08:20 PM
When you blow on something it cools it down, right?If something is warmer than the air that's blowing on it, (like a human body in winter) it will cool down faster in the wind. It's the rate of heat transfer that changes; heat's transferred from the warm thing to the air.

Wind chill normally has nothing to do with evaporation in winter, unless you like get wet somehow. I don't recommend it.

If something is already at pretty much the same temperature as the air, (like the car thermometer) blowing on it has no significant effect. There's no heat transfer between things at the same temperature. You can't make something that's at 20 degrees any colder by blowing 20 degree air across it. In fact, you'd probably get a more accurate reading by putting the thermometer out in the air stream; it would be less affected by the heat of the engine.

The thermometer in my car always reads a couple of degrees high, probably due to engine heat, or maybe it's just calibrated wrong.

PaulC
12-06-2010, 10:35 PM
Shhhh...............Look inside of the outside rear view mirror.

clancy
12-06-2010, 11:22 PM
I just read that by the 2012 model year all US cars will have to be equipped with rear view cameras. When I learned to drive we didn't have right side mirrors.

Ethan
12-07-2010, 01:59 AM
I just read that by the 2012 model year all US cars will have to be equipped with rear view cameras. When I learned to drive we didn't have right side mirrors.

That's 'cause you're old ;)

Phillip Allen
12-07-2010, 03:21 AM
you GOT to be kidding me!

wind chill isnot and has never been a measure of actual tempature...it is a "effect". If you're standing outside in 40 degree weather (farenhite) and the wind comes up...you may feel colder but no amount of wind is gonna lower that 40 degree temp at all...unless the wind itself is cooler than the previously static air....if it could, cars would freeze solid as they speed down various hiways

I suppose next someone will tell me that if the wind comes up on a summer day and I put a glass of water out in it, it will freeze...?

Ron Williamson
12-07-2010, 06:37 AM
Phillip
That water will cool from the effects of it evaporating.
The same reason you sweat and why you feel cooler on a hot dry day than a hot humid day.
A different thing from the other kind of windchill,(windcheel,winsheel,or windshield around here,I dunno why.)
R

Paul Pless
12-07-2010, 07:15 AM
I just read that by the 2012 model year all US cars will have to be equipped with rear view cameras. seems a bit ridiculous, does a smartcar really need a backup camera???

John Turpin
12-07-2010, 09:00 AM
I bought an aftermarket thermometer kit once for a pickup I owned. I didn't really need it, but my previous ride had one and I got used to seeing the outside temp at a glance. Anyway, I followed the directions and found a nice protected place for the sensing unit as instructed. The amazing thing--besides its inaccuracy--was that the display gave me the temp with a tenth of a degree measurement (ie, 92.3). I thought that this was very cool at first; knowing the temperature that precisely. Soon though, I discovered that the displayed temp was innacurate to a +- 10 degree range. So, 92.3 on the dash might actually be anywhere between 82.3 to 102.3 outside. I never took the trouble to remove it but was somewhat amused by the highly erroneous temperature reading it was giving me; wrong, but to a tenth of a degree.

Milo Christensen
12-07-2010, 09:04 AM
seems a bit ridiculous, does a smartcar really need a backup camera???

Ever seen someone as old and stiff as I am try to twist around enough to see where I'm going when I'm backing up my gargantuan SUV? All of us sexagenerians are going to become these shrunken up little tiny white topped septuagenarians who can't see over the top of the seat. We need those backup cameras. Actually, since I'm off to the body shop today, I could have also used a camera showing how close my right front bumper was to that immovable pole. Not to mention hooking up the trailer.

cs
12-07-2010, 09:10 AM
My uncle used to be a truck driver and he always made me back up using the mirrors. You know truck drivers can't look over their shoulder unless they want to look in the sleeper. So I keep my mirrors adjusted, glance over my shoulder to check the blind spots and use the mirrors to back up. I've never hit anything backing up. Situational awareness is all it takes.

Chad

Phillip Allen
12-07-2010, 09:34 AM
Phillip
That water will cool from the effects of it evaporating.
The same reason you sweat and why you feel cooler on a hot dry day than a hot humid day.
A different thing from the other kind of windchill,(windcheel,winsheel,or windshield around here,I dunno why.)
R

I was aware...it is neglagible in the example

htom
12-07-2010, 09:53 AM
My wife's in love with the backup camera in the new Forester. She can now back down the driveway without hitting an edge, which is great. I tried to show her how to use the mirrors, but she either couldn't or didn't. I wish there was a switch on the camera so that when I crept out of a parking place, it would show the traffic going up and down the parking lot isle, looking at right angles to the direction of travel.