At this type of test the suspected areas of the test object are carefully explored by means of a detector probe, a “sniffer”, which is connected to the leak detector. The object to be tested are under helium search gas overpressure. The sensitivity of the method of the method and the accuracy of localising any leaky points depend on the nature of the search gas, the design of the sniffer and the time constant of the actual leak testing device. The sniffer test can for example be used for examination of vessels, heat exchangers, seals, flange connections, weld seams etc..