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driving concerns

How can the external thermometer on my VW be accurate? Wouldn't the metal on the car affect the reading — or the wind while driving? — D., Halifax.

Speed shouldn't blow off that outside temperature reading on your dashboard, but a short run to the store might.

"The system displays the last known temperature when starting the vehicle," says Centennial College automotive professor Garrett Nalepka. "It may take some time and mileage to update to an accurate current reading."

After you start the car, you have to be driving more than 64.3 km/h (40 mph) for up to five minutes before the temperature gets updated, Nalepka says.

"This strategy was intended to eliminate wildly fluctuating readings," Nalepka says. "It may cause some customer confusion leading to unnecessary diagnostics and parts replacements."

Sensor sensibility

You car uses a temperature sensor to determine the temperature. In fact, it might use a few.

"Most sensors for the in dash readout found in most cars are placed somewhere safe where they can't be damaged or influenced by engine heat," says Volkswagen Canada spokesman Thomas Tetzlaff in an e-mail. "A common location is inside one of the side mirror housings, but others are located inside the fenders or ahead of the vehicle's radiator."

It really depends on the vehicle, Centennial's Nalepka says.

"Basically, there is no set place,"Nalpeka says. "Front of the vehicle somewhere, in the airflow."

There are other sensors that measure the temperature of outside air. Why does your car need to know the temperature?

Sensors in the HVAC system keep the A/C from turning fully on and getting damaged when it's freezing out, Tetzlaff says. On modern cars, the condenser turns on to dehumidify the air.

And, some automatic climate control systems use the outside temperature, says Centennial College automotive professor Stephen Leroux.

"The sensor typically sends data to a module that interprets the information, performs some calculations and then commands a device in the vehicle's (HVAC) system to turn on, speed up, slow down or turn off," Leroux says.

Err flow?

If most car's sensors are located around the airflow, won't the wind at highway speeds throw off the reading? You might feel wind chill, but your car doesn't, Tetzlaff says.

"Wind chill is something that affects living creatures and not inanimate objects like temperature probes," says Tetzlaff. "Wind chill is caused by the flow of air around a body that disrupts the insulating layer of warm air close to the surface, causing the person to feel colder than the actual outside temperature."

For example, if you were sticking your head out the sunroof while your car's going 100 km/h on a -20C day, it would feel like -40C.

If the car's thermometer were not protected from the wind, it could actually read slightly warmer than the actual outside temperature at highway speeds. That's because moving objects collide with more air molecules, says Rachel Chang, Canada Research Chair in Atmospheric Science at Dalhousie University .

"If the thermometer were sticking out in front of the car and you're driving at 100 km/h, that could affect the measured temperature but by less than one degree Celsius," Chang says in an e-mail. "But if it's shielded, for example behind the licence plate, then the effect of the car moving or wind would be less."

As long as there is enough air exchange between the air surrounding the thermometer and the outside, then it shouldn't matter, Chang says.

"I've actually always wondered how the heat from the engine doesn't affect the temperature, but being up in the front would help," she says.

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