Definition and Measurement of Pressure: Atmospheric Pressure, Barometer, and Manometer

Recall that a gas always assumes the shape and volume of the vessel in which it is contained. Microscopically, a gas consists of moving particles that collide with each other and the walls of their container.

Each collision with the wall exerts a small force on the area of contact. The forces resulting from a large number of such collisions add up to its pressure.

Fewer gas particles lead to fewer collisions, a lower force per unit area, and hence, a lower pressure. Pressure, therefore, is directly proportional to the number of particles in a given volume, or the density of the gas.

Air — a mixture of different gaseous atoms and molecules — exerts pressure on everything on the surface of the earth. This is atmospheric pressure, and it is measured using a barometer. A traditional barometer is a mercury-filled glass tube that is inverted into a dish containing mercury.

In response to pressure exerted by the atmosphere, the height of the mercury column rises or falls. Measuring the height provides a measure of the atmospheric pressure.

At sea level, the atmospheric pressure is 1 atm, which raises the column of mercury to a height of 760 millimeters of mercury. At higher altitudes, where the air is less dense, the height of the mercury column falls, indicating a lower atmospheric pressure.

The pressure of a gas trapped in a container is measured using a manometer. A closed-end manometer consists of a mercury-filled U tube. One end is sealed in a vacuum, and the other end is connected to the container that is filled with the gas sample.

The gas particles push down on the mercury in one end of the tube, creating a height difference that corresponds to the pressure exerted by the gas.

In an open-end manometer, one end is left open to the atmosphere. This measures the pressure of the gas sample relative to atmospheric pressure.

When the gas exerts a pressure higher than the atmospheric pressure, the height difference is added to the atmospheric pressure. This yields the pressure of the gas.

Conversely, when the atmosphere exerts a pressure greater than the gas sample, the height difference is subtracted from the atmospheric pressure to get the pressure of the gas.