  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.
