To achieve a large measurement range from small up to larger leakage rates, a switchover from normal operation to coarse operation occurs. In coarse operation, the sucked-in gas flow is separated over different chokes (D4, D2), wherein the choke that leads to the test gas sensor (16) has a low flow rate. This manner of operation prevents too large a quantity of test gas from reaching the sensor surface (17) and contaminating the sensor. In another alternative, in coarse operation only a part of the sensor surface (17) is coated by the test gas. The other part is flushed.