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 .

What is outgassing in vacuum science?

The generation of gas resulting from the desorption is known as outgassing. The outgassing constant is defined as the rate at which gas appears to emanate from unit area of surface, and is usually measured in units of Torr.Liter.Sec-1.Cm-2.     

If the temperature of the material is raised (baking), the outgasssing rate rises to a peak value.

Together with the acceleration of desorption, heating may also have the effect of causing activated chemisorption of physically adsorbed gas (in particular water vapour), which can then be desorbed only by prolonged heating at much high temperatures.

Chemi-adsorbed water vapor continues to be evolved at temperatures in excess of 300 degC. It should therefore appear that a degassing programme should begin with pumping at room temperature to remove physically adsorbed water vapor, before baking is commenced.

What is desorption?

When a material is placed in Vacuum, the gas which was previously adsorbed begins to desorb i.e. to leave the material.

The desorption is influenced by

1.       Pressure

2.       Temperature

3.       Shape of material

4.       Kind of its surface

The pressure has a basic influence on the desorption phenomenon since according to its tendency of increasing over or decreasing below the equilibrium , the phenomenon of sorption or that of desorption appears.

The temperature has a clear influence on desorption phenomena. Desorption is endothermic, thus it is accelerated by increase of temperature.

The shape of the material influences desorption either if the gas is adsorbed or absorbed.

If the gas is adsorbed, then only the amount of surface is the influencing factor, but if the gas has to diffuse from the interior of the material to the surface, then the third dimension “thickness” is also influencing the rate of desorption.