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Originally Posted by SumitBahl I have yet not seen the correct temperature on DZire.
Its mostly off by 2-4 degrees, depending on the time of the day.
Its mostly accurate at nights. |
Quote:
Originally Posted by rajismine I had noticed that in my safari...a temp diff of 2-4 degrees...and the TASS guy told me the temperature reads the temperature of a place inside the body/cabin where the termostat is located...hence the location of the thermostat makes the difference...our cars do not have uniform cooling all around u see... |
It is really difficult to have a temperature sensor that accurately measures the exterior ambient temperature. Even on some very expensive cars, you will see similar errors.
There are several reasons: The outside temperature varies from point to point (sunny, shadowy, windy etc.) and also with wind speed. Ignoring wind-chill effects, we want to measure the "steady state temperature" of the surrounding air. How would we measure it? If we place a sensor on the exterior part of the car body, then when the car is in motion, this sensor will get severely affected by the fast air moving on it (artificial wind-chill effect) and measure a lower temperature. On the other hand, if we place the sensor somewhere inside the car body (e.g. in the engine compartment), then it will show artificially higher temperature because the engine bay has a good degree of thermal insulation from the surrounding.
Basically we want a sensor in a moving car that will be in "thermal equilibrium" with the surrounding, and this is not a trivial task!
Further, If I am not mistaken, in many European and American cars, the purpose of these sensors is to measure the "road temperature" and not just the "ambient" temperature. The motivation is that this reading could warn the driver when the road is close to freezing temperature and there is possibility of ice formation on road. In such cases, the sensor is commonly located under the front bumper, as close to the road as possible. Now this also has a drawback: in summer days (or in general when it is sunny), the road surface will be at a higher temperature than the surrounding air, and these sensors then show an artificial high number. Also, when the car is moving, these show an artificially lower number (because these are located on the exterior, though under the bumper, and are again subject to wind-chill effects), and will show a "icy road condition" warning much before it actually is true. But since this error is on the conservative side, it is better to tolerate this error than the other way round (sensor showing higher temperatures when the road is close to freezing point). In my Audi A4 in the US, I had noticed that I used to get the "icy roads" warning on the MID when the temperatures were actually close to 35F (instead of exact 32F, the freezing point*). These errors were on the conservative sides and made sense.
In summary, this is a difficult problem unless some exotic and expensive technique is used.
Thanks,
-AD
* I am ignoring here the fact that the freezing point for the water on road will change due to the pressure applied when the tyre of a vehicle moves on it. Due to this, it is not theoretically correct to say that 32F or 0C is the freezing point for the water on the road.