What is pressure?

Our planet is surrounded by air, which like any substance has mass. The mass of air, under the influence of gravitational force of 1g, creates a pressure approximately equal to 1 bar at sea level everywhere. The map shows isobars, which connect points with the same pressure. Areas of high pressure are usually characterized by "good" weather. Rain and stormy weather are typical in low-pressure zones. Air pressure is usually indicated in pascals or mmHg: 1 mbar = 100 Pa; 1 mmHg = 1.3 mbar.

Air Column

1 liter of air has a mass approximately equal to 1 gram. An air column 10 meters high with a base area of 1 sq.cm² (1 liter) creates a pressure approximately equal to 1 g/cm² = 1 mbar.
At an altitude of 1000 m above sea level, the pressure of the air column is approximately 900 mbar. This effect has long been used by devices such as altimeters in airplanes.

Water Column

Water is approximately 1000 times heavier than air, accordingly, a water column 10 m high with a base area of 1 cm² will exert a pressure of 1 kg/cm² (approx. 1 bar). Water pressure increases by 1 bar every 10 m of depth. Thus, at a depth of 100 m, the pressure will be 10 bar higher than at the water surface. The absolute pressure will be 11 bar (1 bar atmospheric pressure + 10 bar relative pressure).

Measuring Pressure Using Scales

The first pressure measurements were made using liquid columns. To this day, pressure in meteorology is measured using a mercury column.

Measurement of relative pressure

Measurement of absolute pressure

Pressure = Force over Area

For high pressures, using liquid columns is extremely impractical. They are replaced by weights on top of a piston in a cylinder. The force "K" affects the pressure of the liquid, and accordingly the level. As a result, the weight can rise/fall. If the piston area is 1 cm² and the weight is 100 kg, the weight begins to rise at a pressure greater than 100 kg/cm² (about 98.1 bar) under standard gravitational force 1g. This principle allows very precise pressure measurement.