Drift time response of the MDT chambers to a variation of the gas mixture

Using the cosmic ray stand at Laboratori Nazionali di Frascati, the drift time response of a MDT-BML chamber is studied, in different positions with respect to the incoming gas flow, during an induced variation of the gas mixture. The chamber under test is equipped with two different gas distribution systems, fully parallel and serial-parallel. Large differences are observed between the drift times measured at different locations with respect to the gas flow during the transient. At the Atlas-like flow of one gas volume exchange per day, the differences reduce to a negligible amount as soon as stationary conditions are reached.