India, being a home for so many hill stations, makes life much more enjoyable. For most of us, summer vacation means a vacation in the lap of Himalayas. Many people must have experienced packets of chips exploding in high altitude regions of Himalayas. Have you ever wondered that how the change in altitude effects the tyre air pressure? If not, now is the time to learn about the relatively small change in tyre air pressure that will be easy to adjust for altitude change.
What is Atmospheric pressure ? It is the force exerted by the weight of the air molecules on the object. Although air molecules are invisible to us, they do have mass and take up space. At a sea level, air pressure is 14.7 pounds per square inch (psi) but as the altitude rises, air pressure keeps decreasing. For instance, at 10,000 ft only 10.1 psi of atmospheric pressure acts on an object.
Most people confuse atmospheric pressure with gauge pressure. There is a big difference between gauge pressure and atmospheric pressure. If you remove the tyre valve and let all the air out of the tyre, then the pressure gauge will read 0 psi while the actual pressure is 14.7 psi because the tyre is experiencing atmospheric pressure of 14.7 psi. Therefore, due to opened valve, the pressure inside and outside maintains an equilibrium as the flow of air depends on the air pressure difference.
Tyres when mounted onto the wheels, forms an airseal that acts as a flexible barrier between the pressure inside the tyre and the atmospheric pressure. Air inside the tyre retains the same volume of air irrespective of the altitude change but because of lowered atmospheric pressure at high altitude, air inside the tyre exerts more force on it. If the tyre pressure is set at sea level using a gauge and then the same gauge is used again on the same tyre at high altitude, pressure reading would be higher than what was set at sea level. For example: If 30 psi pressure was set at sea level, then the same tyre will read 4-5 psi higher at an altitude of 10,000 ft. In case, when initial pressures were set at high altitude then the pressure reading will reduce at sea level.
However, ambient temperature is another factor that effects the tyre pressure. Tyre pressure change about 1 psi for 10 degree Fahrenheit change in ambient temperature. Cold ambient temperature reduces the tyre pressure and vice-versa. That is why keeping a pressure gauge and a pump in your car always comes handy.
Table below shows the change in atmospheric pressure due to altitude-
|Sea level||14.7 psi|
|1,000 ft||14.2 psi|
|2000 ft||13.7 psi|
|3000 ft||13.2 psi|
|4000 ft||12.7 psi|
|5000 ft||12.2 psi|
|6000 ft||11.7 psi|
|7000 ft||11.3 psi|
|8000 ft||10.9 psi|
|9,000 ft||10.5 psi|
|10,000 ft||10.1 psi|