A $10^{-3}$ drift velocity monitoring chamber.

The MEG II experiment searches for the lepton flavor violating decay $\mu\rightarrow e^{+}\gamma$. The reconstruction of the positron trajectory is performed by a drift chamber, operated with a mixture of He and $i\mbox{-}C_{4}H_{10}$. For a continuous monitoring of the quality of gas, we plan to install a small monitoring chamber, that allows to give a prompt response about drift velocity variations at the $10^{-3}$ level. The monitoring chamber will be supplied with the gas mixture coming from the inlet and the outlet of the detector to determine if any gas contamination originate inside the main chamber or in the gas supply system. The chamber is a small box with cathode walls, that determine a highly uniform electric field inside two adjacent drift cells. Along the axis separating the two drift cells, four staggered sense wires alternated with five guard wires collect the drifting electrons. The trigger is provided by two $^{90}Sr$ weak calibration radioactive sources placed on top of a two-thin-scintillator-tiles telescope. We will present a detailed description of the chamber layout and its simulations and the preliminary measurements.