Recently I was having my annual vehicle inspection that is required in most states. In the process, the mechanic and I were talking about the extremely low temperatures that we have had this winter and the problems with broken pipes, cars not starting, and the increased sale of antifreeze for car radiators. And then he mentioned that he has had many calls about the tire pressure monitoring systems, (TPMS) indicating low pressure, being a problem. The solution is usually just adding air and increasing the pressure so that the on-board computer, (OBC) will reset. All vehicles manufactured since 2008 are required to have these sensors, and the law was mandated the National Highway Safety Administration. There are two types of sensor systems, direct, where the tire pressure is measured in the tire, and the second is an indirect system that measures tire rotational speed to check the tire pressure. But, the mechanic had the answer, but not the explanation as to why the pressure was lower at low temperatures.
Fortunately, we have Jacques Charles to thank, as he is responsible for discovering the relationship between temperature and pressure in an ideal gas. He did not publish his findings in the 1780’s, but later, in the early 1800’s, Gay-Lussac did attribute the discovery to Charles. In today’s language we know that if the amount of air in a tire remains constant, the pressure will be directly proportional to the temperature. So, if a tire has 35 PSI, (Pounds per Square Inch) at 68 F (20 C), and the temperature drops to 32 F, (0 C), the tire pressure will go down. As a result, depending on the tolerance of the sensor, the warning light and low tire symbol may come on. So, to state the law according to Charles:
The volume of a given mass of an ideal gas is directly proportional to its temperature (in Kelvin), if pressure and the amount of gas remain constant; that is, the volume of the gas increases or decreases by the same factor as its temperature. This can be seen by the following formula:
V/T = k, where V is the volume of the gas, T is the temperature of the gas, and k, which is a constant. So, if the temperature drops really low in relation to the tire temperature, expect a warning. The best rule to follow is to test tire pressure when the tires are cold, and use a reliable pressure gauge. Fortunately, neither Charles nor Gay-Lussac had to worry about their tires!
Fortunately, we have Jacques Charles to thank, as he is responsible for discovering the relationship between temperature and pressure in an ideal gas. He did not publish his findings in the 1780’s, but later, in the early 1800’s, Gay-Lussac did attribute the discovery to Charles. In today’s language we know that if the amount of air in a tire remains constant, the pressure will be directly proportional to the temperature. So, if a tire has 35 PSI, (Pounds per Square Inch) at 68 F (20 C), and the temperature drops to 32 F, (0 C), the tire pressure will go down. As a result, depending on the tolerance of the sensor, the warning light and low tire symbol may come on. So, to state the law according to Charles:
The volume of a given mass of an ideal gas is directly proportional to its temperature (in Kelvin), if pressure and the amount of gas remain constant; that is, the volume of the gas increases or decreases by the same factor as its temperature. This can be seen by the following formula:
V/T = k, where V is the volume of the gas, T is the temperature of the gas, and k, which is a constant. So, if the temperature drops really low in relation to the tire temperature, expect a warning. The best rule to follow is to test tire pressure when the tires are cold, and use a reliable pressure gauge. Fortunately, neither Charles nor Gay-Lussac had to worry about their tires!
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