The common definition of vacuum refers to the state in which the pressure in a space is lower than atmospheric pressure. These pressures are generated by gases or vapors uniformly distributed in that space.
The standard definition of vacuum is “the pressure and particle density of a gas in a vessel are lower than those of the surrounding ambient atmosphere, or the pressure is lower than 300mbar. That is: lower than the standard atmospheric pressure at the Earth's surface.
The significance of the 300 mbar specified in the standard definition becomes obvious when the barometric formula is considered. Because the weight of the air column over a given area decreases with increasing height, atmospheric pressure decreases with increasing altitude.
Formula 1-1: Barometric formula
| $p_h$ | Air pressure at altitude $h$ |
| $p_0$ | Sea-level atmospheric pressure = 1,013.25 mbar or 101,325 Pa |
| $g$ | Acceleration due to gravity = 9.81 m s-2 |
| $\rho_0$ | Air density at sea level at 0 °C = 1.293 kg m-3 |
For simplification, assuming that air density, acceleration due to gravity, and atmospheric pressure at sea level are constant, we obtain:
Formula 1-2: Numerical barometric formula
From the formula, the altitude corresponding to a halving of gas density, i.e. ph = p0 /2, is h1/2 = 5,548 m. In other words: for every 5,548 km increase in altitude, the pressure halves.
If the altitude in the formula is replaced by the height of Mount Everest, the resulting pressure is 335 mbar, or 33,500 Pa or 335 hPa expressed in formal SI (International System of Units) units. This explains why the standard definition specifies 300 mbar as the lowest pressure at the Earth's surface.
In this manual, we use the SI unit Pa with the prefix (hecto) as the pressure unit (hectopascal), thereby equating the standard-compliant SI unit with the mbar values commonly used in Europe.
At the cruising altitude of passenger aircraft, about 10,000 m above the Earth's surface, the pressure has already dropped to 290 hPa. Weather balloons rise to altitudes of up to 30 km, where the pressure is 24 hPa. Weather satellites fly in sun-synchronous polar orbits at an altitude of about 800 km. Here the pressure has dropped to about 10-6 hPa. The farther from Earth, the lower the pressure becomes. The lowest pressure known to date exists in interstellar space.
In technical applications, pressure is not always expressed as an absolute value, but as a value relative to atmospheric pressure. Pressure ranges below atmospheric pressure can be expressed as negative numbers or percentages. Examples include liquid-level pressure gauges, pressure gauges, and pressure regulators used in gas cylinders, vacuum suction cup lifting devices, and vacuum conveying systems.
