When a soccer ball is deflating, gas escapes till the inside pressure equals the atmospheric pressure. At this point, the pressure gauge reads zero. Most gauges are calibrated to measure zero at atmospheric pressure, and the pressure measured is called the gauge pressure. The absolute pressure is the total pressure, including gauge pressure and atmospheric pressure. There are two important classes of pressure gauges: the manometer, which measures gauge pressure, and the barometer, which measures atmospheric pressure. A manometer is a U-shaped glass tube containing water or mercury. One end of the tube is exposed to atmospheric pressure and the other is connected to a container of unknown pressure. The pressure at the bottom of both ends is compared, and the gauge pressure is estimated. A mercury barometer consists of a single long tube, closed at one end. The tube is filled with mercury and inverted over a mercury dish. The mercury level in the tube rises and falls, depending on the pressure outside the tube. From the mercury level, the atmospheric pressure is estimated.