Pressure in general

Pressure is defined as force per unit area.
Pressure p [Pa] is thus the ratio of force F [N]
to area A [m²]: p=F/A

Other units:
1 atm = 1013.25 hPa
(atmospheric pressure, pressure at sea level)
1 mm mercury at 0°C = 1 torr = 133.322 hPa
1 mm water at 4°C = 0.0980665 hPa (1 m water column is equivalent to approx. 0.1 bar)
1 psi = 68.9475 hPa (pound-force per square inch)

Relative and absolute pressure

Absolute pressure:
Absolute pressure is gradually increasing starting at vacuum. Therefore, in a vacuum the absolute pressure is 0.

Relative pressure:
Relative pressure is the pressure difference to ambient pressure, i.e. with over-pressure, we have a positive relative pressure.

Temperature

Temperature is an important quantity when measuring the flow of gases. This is due to the fact that gases are very sensitive to changes in temperature, e.g. these changes have an effect on pressure and density.

Units:
0 Kelvin = -273.15°C = -459.67°F (absolute zero)
0°C = 32°F = 273.15K
100°C = 212°F = 373.15K

Humidity

Air is able to store a certain amount of water in the form of vapour. This storage capacity increases as air temperature increases, thereby also increasing the vapour pressure. Air humidity content is indicated by two measures.

relative humidity rH [%] (0 % rH = dry air)
absolute humidity a [g/m³]

Atmospheric air

The constituents of clean, dry air are (in order of volume):
78.13 % nitrogen
20.94 % oxygen
0.93 % argon
0.03 % carbon dioxide
0.01 % hydrogen
smaller amounts of inert gases (helium, xenon ...)

Density

Density is defined as the ratio of mass to volume.
Density [kg/m³] therefore is the quotient of mass m [kg]
and volume V [m³]: density = m/V

The density of air depends on pressure, temperature and humidity.
Density of air: 1.189 kg/m³ at 1000 hPa, 20 °C and 0 % rF

Actual volume flow

The actual volume flow is the unit of the volume of a medium flowing through a certain diameter (for instance through a pipe) per unit of time.

Units::
1 m³/s = 60 000 l/min
1 cfm = 28.317 l/min (foot³/minute)

Mass flow

Mass flow is the unit of the mass of a medium flowing through a certain diameter (for instance through a pipe) per unit of time.

Unit::
1 m³/s = 60 000 l/min

Standardized volume flow

The standardized volume flow is a fictional quantity and is derived from the relation of mass flow to a defined standard density. It is the volume flow a given mass flow would generate under standard conditions.

Viscosity

Viscosity is the "thickness" of a medium. High viscosity stands for siziness, a low viscosity for fluidity. A distinction is made between dynamic and kinematic viscosity. Viscosity of gases depends on temperature and pressure.

Reynolds number

The Reynolds number defines the ratio of inertial forces to viscous forces affecting the flow particles.
The dimensionless Reynolds number is defined as the quotient of the product of velocity times a characteristic length to the kinematic viscosity.