  The gas-flow reduction factor of the second forward Differential Pumping
Section (DPS2-F) for the KATRIN experiment was determined using a dedicated
vacuum-measurement setup and by detailed molecular-flow simulation of the
DPS2-F beam tube and of the measurement apparatus. In the measurement,
non-radioactive test gases deuterium, helium, neon, argon and krypton were
used, the input gas flow was provided by a commercial mass-flow controller, and
the output flow was measured using a residual gas analyzer, in order to
distinguish it from the outgassing background. The measured reduction factor
with the empty beam tube at room temperature for gases with mass 4 is 1.8(4)E4,
which is in excellent agreement with the simulated value of 1.6E4. The
simulated reduction factor for tritium, based on the interpolated value for the
capture factor at the turbo-molecular pump inlet flange is 2.5E4. The
difference with respect to the design value of 1E5 is due to the modifications
in the beam tube geometry since the initial design, and can be partly recovered
by reduction of the effective beam tube diameter.
