Leak rate

The leakage rate (also: leakage rate) is a measure of the volume or mass units emerging from a body.

In vacuum technology , the leak rate is defined as follows:

The leak rate is the quotient of the pV value of a gas that flows through a pipe cross-section during a period and the period of time. The pV value is the product of pressure and volume of a certain amount of a gas at the prevailing temperature . For an ideal gas , the pV value is a measure of the amount of substance or the mass of the gas at a given temperature . The leak rate depends on the type of gas, pressure difference and temperature.

Very small leaks are often detected with the help of helium leak detectors. The following conditions usually prevail: gas type helium , pressure difference 1013 hPa, temperature 20 ° C. These conditions are also called "helium standard conditions".

Formula symbol and unit of measure

Q or Q _L is usually used as the symbol for the leak rate . The following units of measurement are typically used for the leak rate:

{\ displaystyle 1 \, \ mathrm {\ frac {Pa \ cdot m ^ {3}} {s}} = 10 \, \ mathrm {\ frac {mbar \ cdot l} {s}} \ approx 7 {,} 5 \, \ mathrm {\ frac {Torr \ cdot l} {s}}}

A leak rate of 1 Pa · m ³ / s is given if the pressure in a closed evacuated container with a volume of one cubic meter increases by one pascal in one second .

Relationship between leak rate and hole size

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

Consider a large container with a circular hole 1 mm in diameter . Atmospheric pressure prevails outside the container and vacuum inside. Then all gas molecules that are in a cylinder with a diameter of 1 mm and a height of 330 m "above" the hole would "fall" into the hole in one second at the speed of sound (330 m / s). This corresponds to the following leak rate (= pV value per second):

{\ displaystyle Q = 1013 \, \ mathrm {hPa} \ cdot 330 \, {\ frac {\ mathrm {m}} {\ mathrm {s}}} \ cdot {\ frac {(0 {,} 001 \, \ mathrm {m}) ^ {2} \ cdot \ pi} {4}} \ approx 26 \, \ mathrm {\ frac {Pa \ cdot m ^ {3}} {s}}}

From this estimate and the size of viruses and bacteria , the common terms "bacteria-proof" and "virus-proof" can be assigned corresponding limit leak rates:

Bacteria-tight: bacteria diameter approx. 0.5 µm → Q <10 ⁻⁵ Pa · m ³ / s
Virus density: diameter of small viruses approx. 10 nm → Q <10 ⁻⁹ Pa · m ³ / s

Modern helium leak detectors can detect leak rates up to 5 · 10 ⁻¹³ Pa · m ³ / s. According to the above estimate, this would correspond to a hole diameter on the order of an atomic radius .

Assessment of the tightness of high vacuum systems

When starting up a vacuum apparatus, which consists of at least one vacuum chamber, the so-called recipient, a vacuum pumping station and a pressure measuring device, the question arises as to whether the recipient can be regarded as tight in terms of vacuum technology. The procedure is then as follows:

(i) The recipient is evacuated to the achievable final pressure.

(ii) The valve between the recipient and the vacuum pumping station is closed.

(iii) The pressure increase in the recipient is recorded in small time steps (typically 10s) over a longer period of time.

(iv) The pressure readings are plotted against time.

(v) The curve obtained in this way is used to determine the rise in pressure per time (e.g. in Pa / s or mbar / s) as the gradient.

(vi) The leak rate is obtained by multiplying the pressure increase per time by the determined volume of the recipient.

When evaluating the measured value obtained in this way for the leak rate, it must be taken into account which type of seal was used when the vacuum apparatus was assembled. Today, in the physical-technical area, mainly stainless steel recipients are used, which are either sealed with round cord rings made of rubber or Viton (high vacuum systems) or are sealed with copper seals (ultra-high vacuum systems). Ultra-high vacuum systems are almost completely tight, so the leak rate measurements usually relate to the evaluation of high vacuum systems. The following qualitative values apply:

System very tight: Q <10 ⁻⁶ mbar l / s
System sufficiently tight: Q <10 ⁻⁵ mbar l / s
System leaking: Q> 10 ⁻⁴ mbar l / s

Only this information makes it possible to judge after a vacuum apparatus has been set up whether the tightness of the apparatus corresponds to what is technically possible. If the volume of the apparatus cannot be precisely determined, it should always be assumed to be a little too large in order to obtain an estimate for the worst case.

Web links

Leak rates in fittings , in the English language Wikipedia

Individual evidence

↑ Data sheet of the UL1000Fab helium leak detector from INFICON (PDF, English; 888 kB)
↑ Oerlikon Leybold Vacuum: Basics of Vacuum Technology , published in 2007, p. 144 Available free of charge as a PDF file in German or English under [1] . A very good, informative manual on vacuum technology that has been maintained by Leybold (now: Oerlikon-Leybold) for decades.

[1] Data sheet of the UL1000Fab helium leak detector from INFICON (PDF, English; 888 kB)

[Grundlagen_der_Vakuumtechnik-2] Oerlikon Leybold Vacuum: Basics of Vacuum Technology , published in 2007, p. 144 Available free of charge as a PDF file in German or English under [1] . A very good, informative manual on vacuum technology that has been maintained by Leybold (now: Oerlikon-Leybold) for decades.