Air pressure is measured using an instrument called a barometer. The most common type of barometer is a mercury barometer, which was developed in the 1600s and consists of a small basin of mercury with an upside-down glass tube placed in the mercury. As air pressure increases, the pressure forces more mercury into the tube, and when pressure decreases, the level of mercury in the tube goes down. The height of mercury in the glass tube can be measured in inches or centimeters. Other types of barometers include aneroid barometers, which use a sealed metal container with a partial vacuum inside that expands or contracts with changes in air pressure, and digital barometers, which use electronic sensors to measure air pressure. Atmospheric pressure is commonly measured in units of millibars (mb) or hectopascals (hPa), with the standard pressure at sea level being 1013.25 mb or hPa. One atmosphere (atm) is also a standard unit for measuring barometric pressure, and it equals 29.9213 inches of mercury (inHg) .