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

**6.02x10**molecules (Avogadro’s number)^{23}**1 Torr-liter would then contain about 3.5 x 10**

^{19}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 10**

^{19}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 Î”p is the pressure rise during the time interval Î”t .