High precision tracking using gaseous detectors mainly has two basic requirements: the first being a stable gas composition, the latter being a well-known space-drift time relation. But due to several reasons such as e.g. contamination by water or air a change of the gas composition might occur and thus influence the real space-drift time relation. To continuously monitor the composition of the gas therefore is the main design goal of the assembly introduced in this article. It is reached by a measurement of the drift velocity of electrons inside the gas as function of different electric field strengths. Finally, comparisons with Monte Carlo-simulations are drawn to extract the cause of the drift velocity change.
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