Gas molecules, hitting about a surface of the body, for example, on the wall of the vessel, challenging it. The magnitude of this pressure the more, the greater the average kinetic energy of translational motion of gas molecules and their number per unit volume. Remember, the pressure is measured by, the current per unit surface area:

p=F_D/S

Derive the unit of pressure in SI:

p= 1N/1m²= 1 N/m²=1 (kg*m/s²) /m²=1 kg/(m*²) = 1 PA.

The SI unit of pressure is Pascal taking (PA). Pressure, in which area in 1 m² the force of the pressure in 1H, is called Pascal. In practice, there are still other units of pressure: technical atmosphere, the millimeter of mercury, physical atmosphere. Technical atmosphere (at) is called pressure, generated force in 1 kgf to the square in 1 cm²:

1 at = 1 kgf/cm² = 9,81*10⁴N/m2=9,81*10⁴ PA.

The millimeter of mercury (mm Hg. article) is called pressure, produced by the column of mercury height in 1 mm on a horizontal surface. Since the pressure inside the liquid at a depth h, due to its weight, is calculated by the formula p=pgh is easy to establish the following relation:

1 mm Hg.PT.=(13,6*10³kg/m³) *9,81m/s²*10^-3m=133 PA

Physical atmosphere (ATM) is called pressure, produced by the column of mercury height 760 mm on a horizontal surface:

1 ATM=1,033 at=1,013*10⁵ PA

This value is taken as normal atmospheric pressure. A device for measuring pressure is called a manometer. Large gas pressure measured metal manometrum (rice. 3.6). Main it detail is curved metal tube And. Its open end soldered to the tube In, and closed is connected with an arrow. If you open the valve To, the gas rushes into the tube and straightened it. The reason is, the area of the convex surface of the tube And more concave, and the pressure of the gas equally in all directions; therefore, the pressure inside the tube on the convex surface more, than on the concave, hence the extension tube And. End moves the arrow on the gauges.

For measuring very small pressures (vacuum) apply Wukryty liquid gauge (rice. 3.7). If you connect it to open end of the vessel, in which low gas pressure, the level of a liquid in a closed knee drop. The pressure in the vessel can will find on the difference levels in closed and open knees pressure gauge.