In pressure testing, the object to be tested contains helium gas, the pressure of which is greater than one atmosphere. A “sniffer” probe is connected to the leak detector through a flexible hose. When the sniffer probe approaches a leak, the escaping helium enters the vacuum system by way of the sniffer probe and is detected by the mass spectrometer tube. This method of leak detection is useful for testing large tanks and other objects which are not easily evacuated or which cannot withstand the stress of internal vacuum. However, this method is less sensitive than vacuum testing because the sniffer will take in some surrounding air, which dilutes the helium concentration taken into the leak detector.