One of the biggest issues that you will run into working with cryogenic fluid systems is finding and fixing leaks, especially those leaks that open up at low temperature. A helium leak detector and a bottle of soapy water does wonders for finding leaks at room temperature, but in order to get components down to working temperature in the cryostat you have to enclose the entire experiment in a vacuum chamber and cool everything down via cryocooler – not a very easy environment to isolate the location of a leak! An example of this is that in some cases (usually poor joints between multiple different materials), brazed joints will have no leakage at room temperature, but new leaks will open up after getting really cold. We have had this happen anywhere from 80 to 200 Kelvin, and as these temperatures are far below your typical room conditions, they’re very difficult to isolate. To try and speed up the process of checking if leaks would open, our recommended practice for all cold joints and fittings is therefore to test in liquid nitrogen (LN2) before installation on the system.
General Instructions for LN2 Leak Checking, based off ASTM E499/E499M – 11 Test Method A:
- Cap the test specimen and connect to a helium bottle. Pressurize the test specimen with helium to the working pressure of the specimen.
- Dip the test specimen into LN2 to cool it down to the LN2 temperature of approximately 77 K.
- Sniff all fittings, welds, and solder joints with the mass spectrometer by passing the sniffer probe over likely leak points. Start at the bottom of the specimen 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. Identify 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. When testing with a short line from the helium bottle, be aware that the regulator connection into the bottle often leaks at a higher rate than is acceptable for our cryo experiments, and can cause false alarms (This can usually be remedied with longer lines to move the specimen farther from the bottle and by keeping the specimen low, well below the bottle).
- If any leaks are identified, take corrective action and perform this procedure again until leaks are no longer detected.
This is recommended for cryo-rated valves, small pressurized vessels, or any other cold equipment that you don’t mind completely submerging in LN2. It will definitely save yourself a lot of time by doing this every time!