Where in/on the car do they place the sensor which measures the outside temperature? How do they ensure that the temperature of the car body won’t affect the reading?
On my car, it must be somewhere at the bottom of the windscreen in all those ducts that make up the cabin air intake and its drainage system. I know this because I pored hot water over my windscreen the other day to defrost it and the ambient temperature reading rose to quite a way above freezing, for a while.
I don’t think any of my hot water will have actually immersed the sensor, but it certainly will have warmed the air inside the drainage ducts.
My old Citroen has the ambient temperature sensor underneath the left-hand wing mirror. There was no way I’d have found that myself - I had to ask someone who knew.
Indeed the placement will affect the indicated temperature - I can tell you that my wing mirror reaches about 45 degrees Celcius in the sun. A constant airflow (when the car is moving) will give a more accurate reading, though again rain and humidity will affect the reading slightly.
The air temperature gauge on your car is only approximate, which is why the frost warning kicks in at about 4 degrees Celcius instead of 0. All the other gauges are pretty approximate too - oil level meters, water temperature gauge, fuel gauge, even the speedometer. The latter is reasonably accurate, but will generally always read a little fast. In the UK the legal speedometer tolerances are 0% to +10%. Reading too slow is illegal.
They don’t always. The owner’s manual for my Bonneville told me that if the engine was hot, and I returned to car, the temperature would be inaccurate until I got moving again. I never really saw a big shift in temperature, though; it always seemed to be pretty accurate.
In my current car, there’s no warning in the owner’s manual, and it seems to be pretty okay, too.
I’ve been wondering where the sensor is on my car, too. It’s normally quite accurate, but very occasionally it reads inexplicably high - like 22ºC when the temperature is only 9º or 10º - when I first set off. I know it’s not due to direct sunlight - mainly because there hasn’t been any recently in this joke of a winter we’re having in the UK*, so I put it down to some quirk in the electronics.
- Like mild, overcast, average min 6ºC and max 10ºC, unchanging for 2 weeks now. I feel like I’m living at the bottom of a washing-up bowl.
Even sunlight on an overcast day is enough to heat your car up above the ambient air temperature, and any temperature sensor attached to your car bodywork is going to respond to this. Your car will lose some of the heat it absorbs from the sun by radiation and convection, which is what stops it from ultimately melting. If there’s a constant airflow over the car (for instance, if you’re moving) then your car will lose more heat to the surroundings due to increased convection, and the car’s temperature will tend towards the ambient air temperature. That’s why your sensor reads high when the car’s been sat outside in the day, but drops when you drive off.
Of course the engine is another heat source. If the ambient temperature sensor is too near the engine then you’ll see the temperature rise if the car stops, even (temporarily) after the engine is turned off.
If a moving car loses heat due to convection, wouldn’t it the sensor end up displaying the ambient temperature + wind chill factor?
Wind chill only chills:
1/ dry things down to air temperature (which is of course a good thing if that’s what you’re trying to measure)
2/ wet things below that, due to heat loss through evaporation. So unless the sensor’s wet, this isn’t a problem.
:eek:
My stepmom tried that once and put a huge crack in her windshield. I hope you had better luck.