Results from the 1998 test beam of the Calypso chamber filled with an Ar-CO2 gas mixture.

In the summer 1998 we have exposed to the Cern H8 test beam the BIL prototype Calypso lled with an Ar (93%) CO2 (7%) gas mixture, which is the current baseline for the Atlas MDT chambers. A possible problem of this gas mixture is the important variation of the drift velocity along the drift path. The aim of the test was to assess the possibility of operating the chamber and measuring the r-t relation in such conditions. Moreover we checked the performance of the chamber with a large-area (50 50 cm 2 ) beam using a hodoscope trigger and two precision