  The KArlsruhe TRItium Neutrino experiment (KATRIN) aims to measure the
effective electron anti-neutrino mass with an unprecedented sensitivity of
$0.2\,\mathrm{eV}/\mathrm{c}^2$, using $\beta$-electrons from tritium decay.
The electrons are guided magnetically by a system of superconducting magnets
through a vacuum beamline from the windowless gaseous tritium source through
differential and cryogenic pumping sections to a high resolution spectrometer
and a segmented silicon pin detector. At the same time tritium gas has to be
prevented from entering the spectrometer. Therefore, the pumping sections have
to reduce the tritium flow by more than 14 orders of magnitude. This paper
describes the measurement of the reduction factor of the differential pumping
section performed with high purity tritium gas during the first measurement
campaigns of the KATRIN experiment. The reduction factor results are compared
with previously performed simulations, as well as the stringent requirements of
the KATRIN experiment.
