This photo shows a 42 inch bellows made by PT&P being tested by Leak Detection Specialist Inc. After sealing the bellows a full vacuum is pulled on the inside, then helium is sprayed over the surface of all welds. A pump pulls any gas which leaks to the interior of the bellows to a special mass spectrometer designed to detect the helium. If helium is detected, the point of the surface where it was sprayed is marked for repair. When no leaks are found, the flow rate of helium to the mass spectrometer is measured against a standard for aluminum, in this case 0.000001 cc/sec. Helium molecules are so small, they will penetrate aluminum at a slow rate when the pressure difference of a full vacuum is applied.
A mass spectrometer functions by accelerating a narrow beam of ions of a gas sample through a magnetic field. When they reach the magnetic field, the amount they are deflected depends on their charge-to-mass ratio. In this application the detector is set to record helium ions. In order to measure the flow rate of the helium when no leaks are detected, the entire system is sealed in a plastic “bag”. LDS can measure helium flow rates as small as 0.00000001 cc/sec.