The reconstruction of the positron trajectory in the MEG-II experiment searching for the $mu^+ to e^+ gamma$ decay uses a cylindrical drift chamber operated with a helium-isobutane gas mixture. A stable performance of the detector in terms of its electron drift properties, avalanche multiplication, and with a gas mixture of controlled composition and purity has to be provided and continuously monitored. In this paper we describe the strategies adopted to meet the requirements imposed by the target sensitivity of MEG-II, including the construction and commissioning of a small chamber for an online monitoring of the gas quality.
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