Search This Blog

Sunday, September 20, 2015

WHAT IS LEAK RATE IN VACUUM SCIENCE?



Ideally it is impossible to build a vacuum system with out any leak. Without active pumping the pressure in a real system will rise with time. 

Pressure rise is produced by outgassing and by gas molecules penetrating through leaks from the outside into the vacuum system.


 The leak rate must be small enough not to prevent to reach the required pressure level.

A real leak will yield a linear pressure rate-of rise curve. The slope of the curve is a function of the leak rate and the volume of the system: 

 
Leak is related to how many molecules per unit time are being admitted into volume.
According to the gas laws, if volume, temperature and pressure are specified, it is possible to determine how many molecules are there inside the volume.

Torr-liter defines the molecules contained in a one liter volume at a pressure of 1 Torr at 0 °C.

 22.4 liters of gas at 760 Torr and 0 °C equals 6.02x1023 molecules (Avogadro’s number)

 1 Torr-liter would then contain about 3.5 x 1019 molecules.

std cc represents the number of molecules contained in a 1 cc volume at a pressure of 760 Torr and 0°C

1 std. cc contains about 2.7 x 1019 molecules 

 The leak rate is defined as the pV-throughput of a gas through a leak.
 It is a function of the type of gas, pressure difference and temperature.

 In a system of volume V the leak



Here Δp is the pressure rise during the time interval Δt .