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Transpector SPS Operating Manual
Figure 4-4 Pressure Ranges for Sampling for Transpector SPS Inlet Orifices and Sniffers
(some orifices shown may not be available)
4.5.2 High Pressure Sampling: Orifice Bypass
When process pressures exceed 10 Torr, the process gas is dense enough that the
gas molecules collide with each other more often than with the walls of the
sampling system. In this transition or viscous flow regime, the time constant for
detecting changes in process gas composition becomes dominated by the time it
takes for the gas species that changes in the process to diffuse through the gas
matrix and arrive at the sampling orifice. This diffusion time is proportional to the
process pressure and the square of the distance from the process to orifice. This
sampling method effectively shortens the diffusion distance from process change
to the sampling orifice by drawing a small quantity (10 sccm) of the process gas
through the sampling valve and bypass valve to the interstage port of the turbo
molecular pump. When the process gas flows in this manner, the diffusion distance
of the gas is reduced to about 2 cm and the response time for changes in the
process gas is significantly reduced.
