The leak rate (also leakage rate) is a measure of the amount of substance (mass),
flowing due to a leak.
In vacuum technology the leak rate is defined as follows:

The leak rate is the ratio of the pV value of a gas flowing through the cross section of a pipe during a period, to the period. As such the pV value is the product of the pressure and volume of a certain amount of a gas at the respective prevalent temperature. For an ideal gas at a given temperature the pV is a measure of the amount of substance or the mass of the gas.

The leak rate depends on the type of gas, difference in pressure and temperature.
Very small holes are often detected with the help of helium leak detectors. Here the following conditions mostly apply: Type of gas helium, difference in pressure 1013 hPa, temperature 20 °C. The conditions are also known as the 'helium standard conditions'.

Q or QL are mostly used as the symbols for leak rate. The following measurement units are typically used for the leak rate:

A leak rate of 1 Pa*m3/s is given if the pressure rises by one Pascal in one second in a sealed evacuated bag with a volume of one cubic metre.

The following rough estimation gives an idea of the relationship between the geometric hole size and the associated leak rate:

We will assume a large container with a circular hole of a diameter of 1mm. Outside the container is atmospheric pressure, inside is a vacuum. Then all gas molecules found in a cylinder of a diameter of 1 mm and height of 330 m and 'over' the hole would 'fall' into the hole at the speed of sound (330 m/s) in one second. This corresponds to the following leak rate (= pV value per second):

From this estimation and the sizes of viruses and bacteria, corresponding threshold leak rates can be assigned to the common terms 'bacteria-proof' and 'virus-proof'.

Bacteria-proof: Diameter of bacteria approx. 0,5 µm -> Q < 10-5 Pa*m3/s

Virus-proof: Diameter of small viruses approx. 10 µm -> Q < 10-9 Pa*m3/s

Modern helium leak detectors are capable of detecting leak rates of up to 5*10-13 Pa*m3/s. According to the above estimation, this would correspond to a hole diameter of the size of the radius of an atom.

In the technical sense, sealed system only means 'free from leaks corresponding to the area used." When looked at more closely, the requirements on technical systems vary greatly.

Leaks are split into the following groups depending on the type of the characteristic:

• Hole leak (clearly identifiable)
• Turbulent leak (whistling gas leak)
• Laminar leak (leaks in detachable or non-detachable connections such as flanges, welding seams etc.)
• Molecular leak (also pore leak; leaking through the finest pores or breaches in the polycrystalline structure of a material)
• virtual leaks (apparent leaks caused by liquids evaporating or outgassing)
• Cold/warm leaks (reversible leaking that only occurs with extreme temperature loads)
• Valve leak or flap leak (leaking with preferential direction of flow)
• Lambda leak (leaking when helium liquefies)
• Small leak in the traditional sense is the material-specific permeation of a material.