I want somebody to explain this to me. We were in Georgetown, Colorado, at an elevation of–well, the exact number escapes me, but it was over 9,000 feet above sea level. We stopped at a Mini-Mart for snacks and there was a display of Pringles cans with the top paper vacuum seals all bulging out tremendously, and a stack of lids next to it, and a sign that said, “These Pringles are not bulging because there’s something wrong with the chips. The high altitude makes the lids pop off. Please take a lid with you.”
So, why is that? If it’s a vacuum inside the can, it shouldn’t matter how hard the outside pressure is pushing on it, whether it’s 600 or 9,000 feet above sea level. If it isn’t a real vacuum inside the can, the air molecules inside it ought to be the same number of molecules as they were at the factory lower down, and they ought to be pushing on the lid with the same amount of force that they were using at the factory. It shouldn’t matter how hard the outside pressure is pushing on the lid.
Obviously I’m missing something here.
We didn’t open the can, and by the time we got to the middle of Missouri, the seal had gone down and we could get the lid on again (and just in time, too, because the back seat was going CRAZY. “Why CAN’T we open those?”)
The air molecules inside ARE pushing with the same amount of pressure- the difference is that the outside pressure is less, so the lid bulges out. The lid isn’t strong enough to withstand the net force without bulging. It’s like a ballon. If the can were a strong, rigid cylinder, it wouldn’t do that.
It obviously isn’t a good vaccuum seal. And if the cans were sealed at a lower altitude, then the inner air would be that of the lower factory pressure. When the cans were at the higher altitude, there was lower pressure on the outside pushing in, and greater pressure inside the cans pushing out. That’s why the molecules inside the can tried to spread out and bulged the seal. It was the greater inner pressure trying to equal the outside pressure. You did go to junior high, right?
Actually it must be a decent seal, otherwise leakage would equalize the pressure inside and outside, and no bulging would happen at all (especially since the trip from low-altitude factory to high-altitude store was probably by truck and hence slow enough for a small, slow leak to equalize pressures along the way).
Also, air pressure is higher at lower altitudes, so the “factory pressure” would be higher, not lower, assuming the can isn’t vacuum packed (which it appears not to be given what happens at high altitude).
Arjuna34, by “vacuum seal”, I was referring to vacuum itself, not the seal’s likeliness to leak. And as for the pressure switch, give me a break…it’s not like i re-read what I type.
Being in the middle of Missouri would make anything go down pretty fast.
The can bulge is simple. It’s because the cabin is pressurized to 8,000 feet. This makes cans bulge and passengers lethargic so the flight attendants don’t have too much trouble with them. Think of it this way: If the cabin wasn’t pressurized, the Pringles cans would start imploding about 10,000 feet. Then the passengers would start popping like huge ticks about 28,000 feet. This is why all mountain climbers wear oxygen masks and gird their loins.
