A Straight Dope Classic from Cecil's Storehouse of Human Knowledge

# How does a person with a steel plate in his head get through airport metal detectors?

August 7, 1992

Dear Cecil:

Three questions occasioned by a recent plane trip to Denver: (1) How does a person with a steel plate in his head get through the airport security systems and onto his plane? You are allowed to fly if you have a metal plate in your head, aren't you? (2) At what altitude would a can of beer explode if it weren't in a pressurized airplane cabin? (3) Flying over western Iowa and eastern Nebraska, I saw for the first time in my life large circular patches of ground colored differently than the surrounding land--like the familiar rectangular patchwork patterns on farms, only round. What are they?

Cecil replies:

(1) The walk-through metal-detection gates are only the first line of the airport security process. They beep or flash or whatever they do to warn a human security guard that the passenger just passed is carrying some heavy metal. The security guard then checks to satisfy him/herself that the metal detected is not of a dangerous sort; for this purpose, he/she is often equipped with a hand-held scanner, as well as eyes, fingers, and (presumably) a brain, with which he/she should be able to handle unusual and perplexing circumstances such as the one you describe.

(2) Conventional beer cans begin to fail at pressures of about 100 pounds per square inch during the pasteurization process, when the contents are heated to about 140 degrees Fahrenheit. Since atmospheric pressure is approximately 15 PSI at sea level, we deduce that a pressure differential of about 85 PSI is needed to induce rupture. The absolute pressure of the typical beer can's contents at room temperature is about 50 PSI, which means the greatest possible differential (achievable in a perfect vacuum, where atmospheric pressure is zero) is also 50 PSI. In other words, if you keep your beer at room temperature or below, you can get as high as you want without worrying.

(3) The curious round patches you've seen result from an irrigation device known as a self-propelled boom, which is like a very large and elaborate lawn sprinkler. The boom is anchored in the center and, as you can plainly see, spreads water in a circular pattern, covering about 180 acres at a shot. The boom is usually only used in the Corn Belt during a season of poor rainfall.

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