Breakdown: Why your car thermometer is usually inaccurate

Car Temperature Wrong Breakdown
Car Temperature Wrong Breakdown(NOAA)
Published: Dec. 3, 2018 at 10:15 AM CST
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MEMPHIS, TN (WMC) - Let’s face it, we are all guilty at checking the outside temperature on our car’s “thermometer," but what if I told you that the temperature reading wasn’t coming from a thermometer at all and it isn’t as accurate as you may think?

In this episode of the breakdown, we explain why you might not want to always trust the temperature reading that your car displays on its dash.

If you are truly looking at an accurate reading of the outside temperature, pay no attention to the car’s thermometer! It might be better to look at the WMC First Alert Weather App, as long as you are parked and not driving.

Usually on hot summer days, the temperature reading from your car is significantly higher than the actual air temperature outside, and there are numerous reasons why that is the case.

Once reason is the car does not have an actual thermometer, while the temperature reading might be right on the dash, that reading comes from a thermistor NOT a thermometer.

In most cases, a temperature reading is measured with a mercury thermometer. The liquid mercury in the thermometer expands and it will then rise to a certain value when heat is added, the mercury contracts and will fall to a lower value when the heat is removed.

A thermistor measures the change in electrical current as a result of heat added or removed. The problem does not lie with the car’s thermistor itself, in fact they are usually accurate, and small and cheap to make.

The big problem with a thermistor, is where it is located on the car. Most manufacturers place this part on the front of the car usually behind the grill. This location exposes the instrument’s readings to re-radiated heat from the road surface.

Roadways and blacktop are great absorber of incoming solar radiation, this just means they heat up quickly, raising the temperature right at the surface, so temperatures over the blacktop are higher than over grassy or shaded surfaces.

As a result, the car’s thermistor is recording the warmer temperature over the roadway, which can be 10-plus degrees warmer than the actual air temperature, which is measured 6 and a half feet above grassy surfaces, typically at an airport location.

At night, the accuracy of the thermistor is better, because it’s not picking up as much heat from the ground, this is also trying on a cloudy day and when the sun is not heated the ground as much, or when the car is traveling very fast.

During the winter the car thermometer is not helpful. This is because it isn’t accurate to always distinguish between above and below freezing temperatures meaning it can’t help you figure out if the road may be coated with black ice.

With the various limitations of the car’s temperature reading, it may be a fun toy or a gimmicky feature, but it is not as accurate as you might think it to be when traveling along the roadways.

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