We live at the bottom of a sea of air, surrounded by a constant pressure equal to 101 kPa (14.7 psi) over our entire bodies. For the most part, we don’t notice the pressure air exerts on us. But if you’ve flown on a commercial airplane, you may have noticed some of the effects of changing that air pressure. Flexible sealed containers, like bags of chips or bottles of water, change their shape dramatically over the course of a flight because the air pressure inside them can be greater than the air cabin pressure at altitude. In the video above, Nick Moore measured his in-flight cabin pressure as 84 kPa (12psi), which is equivalent to about 1500 m (5000 ft) above sea level. Why do airlines keep the cabin pressure lower in flight? The biggest reason is because the airplane, like the in-flight snack, is a pressure vessel. At cruising altitudes the outside air pressure is about 24 kPa (3.5 psi). To keep the interior of the cabin habitable, the fuselage of the airplane has to hold a higher pressure. The larger the difference between the interior and exterior pressures, the greater the stress the airplane must withstand. Keeping the air pressure in flight a little lower makes the engineering a little easier and does the occupants no harm. (Video credit: N. Moore)
Tag: pressure vessel
Popcorn Popping
The familiar popping behavior of popcorn is the combination of several events. When heated, unpopped kernels act like pressure vessels, managing to contain their boiling water content until a critical temperature of 180 degrees Celsius. At this temperature, nearly all kernels fracture. Popcorn’s jump doesn’t come from the fracture, though. Most of its acrobatics occur when a leg of starch branches out of the popping kernel. The starch acts somewhat like a muscle – after being compressed against the ground, it springs back, propelling the corn upward. Finally, by synchronizing high-speed video and audio recordings of popping corn, researchers determined that the pop in popcorn is not caused by fracture or rebound but instead is the result of the release of water vapor. (Image credit: TAMU NAL, source; research credit: E. Virot and A. Ponomarenko; submitted by Chad W.)