Leak Detection using the Adixen Graph D+ Variable Mass Spectrometer
The Adixen Graph D+ variable mass spectrometer (mass spec) is a versatile instrument. We typically use it to perform leak detection of our cryostats and associated equipment that will be flowing hydrogen at cryogenic temperatures, often through a high vacuum environment. During leak detection, we either flow helium gas through the apparatus if it is to be pressurized and detect leaks externally with a ‘sniffer’ probe, or we pull a vacuum on the experimental apparatus with the mass spec and flow helium around fittings. Either way we can detect leaks down to 1.1E-7 mbar liter/sec.
The mass spec is also capable of detecting hydrogen. We use this feature when performing hydrogen permeability tests of polymers and 3D printed materials. Why not just use hydrogen for leak detection if we are going to flow hydrogen during the experiment? The atomic radius of helium is 31 picometers, while the molecular or Van der Waals radius of a hydrogen molecule is ~120 picometers. Thus, the smaller size of the helium atom assures us that the hydrogen leak rate will be less than our typical acceptable helium leak rate of less than 10-6 mbar liter/sec.
Guidelines to use the mass spectrometer with helium for leak checking, based off ASTM E499/E499M – 11 Test Method A:
- Pressurize the system with helium to 40 psia.
- Purge the system by opening the experiment’s gas purge valve. Close the purge valve and repeat at least two more times. Leave helium in the system after purging.
- Pressurize with helium to the expected system use pressure, if higher than 40 psia.
- Sniff all fittings, welds, and solder joints with mass spectrometer by passing the sniffer probe over likely leak points. Start at the bottom of the assembly and work your way up, holding the probe on or not more than 1mm from the surface. Do not move the probe faster than 20mm/s.
- Continue sniffing in an orderly procedure from bottom to top. Mark any leaks so they can be remedied. Be aware that helium will rise, so a leak above a previously found leak may not actually exist. It is also important to be aware of the airflow in the room, as helium can be blown around the experiment and produce small “leaks” that don’t actually exist.
- If any leaks are identified, take corrective action and restart this procedure.
- Leave positive pressure helium in system.
- Replace cap on inlet line after removing helium gas line. Store the helium bottle back in the bottle closet.