A gas is a state of matter that has neither a definite volume nor a definite shape, consisting of particles that are far apart and move randomly to fill their container. The particles of a gas are not held together by intermolecular forces, unlike molecules in a liquid or a solid. The atoms or molecules that make up a gas have a great deal of kinetic energy and move around their container, frequently bouncing off the walls of the container. These repeated collisions between the molecules of a gas and the container give rise to gas pressure. Pressure is force applied perpendicular to a unit area of surface. Thus, gas pressure is the force gas exerts on its container.
Gas pressure is measured using various units. The pressure of the atmosphere as it relates to weather is often measured in millimeters of mercury (mm Hg). An atmosphere (atm) is the unit of pressure (force per unit area) equal to 760 mm Hg. The average atmospheric pressure at sea level is defined as 1 atmosphere. Atmospheres are a useful measurement when considering ambient air pressure. These measurements are taken using a barometer, an open tube filled with liquid such as mercury and sealed at one end under a vacuum, used to measure atmospheric pressure (of gas, in mm Hg). Atmospheric pressure pushes down on the surface of the mercury in the reservoir, pushing it up into the open end of the tube. Some barometers instead use a corrugated chamber attached to a lever that moves a needle. The chamber increases and decreases in volume in response to atmospheric pressure changes. A barometer may also use the unit bar, a unit of pressure equal to 100,000 Pa, instead of mm Hg.Another common measurement of gas pressure is pounds per square inch (psi), which, as the name implies, describes the pounds of force applied to a square inch (area) of a container. This measurement is commonly used in tire inflation. The torr is an absolute unit of pressure, defined as 1/760 atm. The pascal (Pa) is the SI unit of gas pressure, although it is quite small for large areas, so the kilopascal (kPa) is often used instead (). Because of the small size of the pascal, many researchers prefer atmospheres for ease of use, even when other measurements are made using SI units, fundamental units of measurements in the International System of Units. Various gauges can be used to measure gas pressure. A manometer is an instrument that measures gas pressure (in mm Hg) via a tube of liquid that is open at both ends in analog devices, or electric channels, in digital devices. In an analog manometer, liquid is present in a tube under vacuum at one end. The other end is connected to the gas being measured. The pressure of the gas pushes the liquid up the tube. In a digital manometer, a transducer works a sensor to detect changes in pressure across two inputs.