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OM-610-1002-1 Operator’s Manual Rev 37
PDS-2010 SECTION 5 OPERATION page
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is determined by the mean free path of the enclosed gas molecules and the dimensions of the container or pipe
diameter.
5.2.5.1 Mean Free Path
The mean free path is basically the average distance that a gas molecule will travel before colliding with
another gas molecule. At atmospheric pressure, the mean free path is about 2 x 10
-6
inch. The mean free path
is inversely proportional to pressure. As the pressure is decreased, the average distance a gas molecule will
travel before colliding with another gas molecule will increase.
5.2.5.2 Flow Regimes
In typical pump down from atmosphere, the vacuum system may pass through all the different flow regimes
(from turbulent to viscous) to transition and ultimately to molecular flow conditions.
Turbulent flow is usually short lived and associated with the very beginning of a vacuum system being
pumped down. As the vacuum valve is opened, gas begins to flow from the system in a disorderly fashion.
Swirling whirlpools of gas can appear briefly. As things settle down, the turbulence disappears and the gas
leaves the system in an orderly fashion.
Viscous flow (laminar flow) is the flow of a gas through an enclosure under conditions such that the mean free
path of the gas molecule is very small in comparison to the smallest dimensions of the enclosure. The
character of the flow is determined by gas molecule-to-molecule collisions. The gas leaves the system as any
viscous fluid would with the molecules striking each other and dragging other molecules along with them.
Transition flow is the flow of gas through an enclosure under conditions such that the mean free path is of the
order as the dimensions of the enclosure. In this flow regime, the flow characteristics are determined by
collisions of the gas molecules with surfaces as well as with other gas molecules. This is the "gray" area
separating viscous flow and molecular flow.
Molecular flow is the flow of gas through an enclosure under conditions such that the mean free path is much
greater than the dimensions of the enclosure. At these low pressures (high vacuum), the flow characteristics
are determined by collisions of the gas molecules with the enclosure surfaces, and the flow effects from gas
molecule to gas molecule collisions are insignificant. Molecular flow is not really a flow at all; but rather a
random bouncing back and forth of the molecules from wall to wall, with the progress of molecules through
the vacuum system a matter of statistical thermodynamics. The Parylene deposition process occurs between
transitional and molecular flow regimes.
5.2.5.3 Throughput
Throughput in the context of vacuum systems refers to the amount (usually expressed in volume) of gas that
passes a certain point in a given time. The throughput of a vacuum pump at a particular operating pressure is
referred to as pumping speed.
Another related topic to throughput is conductance. Similar to the term in electricity, conductance refers to the
throughput of a gas in relation to the pressure drop across a particular segment of a vacuum system. The
higher the conductance, the easier it is for a gas to flow.
5.2.6 Outgassing
If all the gases to be removed from a vacuum chamber were located exclusively in the free volume bounded
by the vessel walls, system pump down would occur relatively quickly. However, in practice this will never