So how many times do you get in your car in the summer and the gauge on your dashboard tells you its 113 degrees outside. You know that's not possible, but have your ever wondered why it's wrong so much?
Few industries integrate technology into their products faster than do automobile companies. One of those features, now ubiquitous in most cars, is the dashboard thermometer display. But the temperature reading on your vehicle’s dashboard is often misleading and not representative of the actual temperature.
In its most basic definition, temperature is a measurement of how fast gas molecules are moving or its average kinetic energy. Heat excites molecules, causing them to bounce around much more frequently, resulting in a higher temperature value. The most common way to measure this reaction is with a mercury thermometer, where the liquid mercury will physically expand and rise to a particular value when heat is added.
Your car is equipped not with a thermometer but with a thermistor. Thermistors work in a similar manner to thermometers, but rather than using a liquid like mercury, thermistors measure the change in electrical current as a result of heat added or taken away. Thermistors are quite convenient, since they are small, cheap to make and for the most part, accurate. And really, the problem is not that your car uses a thermistor but rather where that thermistor is placed.
Most vehicles have their thermistors on the front of the car, located behind the grill. This location makes the instrument’s measurements sensitive to reradiated heat from the road surface. If you’ve ever walked barefoot across sand or concrete that’s been exposed to direct sunlight, then you’ve experienced reradiated heat directly.
Surfaces such as roadways are great absorbers of the sun’s incoming radiation. And consequently, they heat up very quickly, creating a localized hot spot right at the surface. As you can imagine, the temperature of a road surface in the middle of a hot summer day is not a true representation of the air temperature. But the heat generated is real, and it’s this “extra” heat that is picked up by a car’s thermistor, artificially inflating the reading you see on your dashboard.
So in effect, your car’s thermistor is measuring the temperature of a very localized environment and not what we would consider the air temperature. The average or ambient temperature that we associate with your daily highs and lows is measured in a sheltered, controlled environment, to limit the effects of heating from nearby surfaces as much as possible.