- Turn on your mass spec.
- Enter leak check mode.
- Start He gas flow. You want to use a small tube or even a syringe needle to leak a gas flow just above ambient (when you can barely hear gas hissing out).
- Leak He gas into possible areas of leaking in your chamber (flanges, bellows, valves, ceramic feedthroughs, water cooling lines, etc...). If your chamber pressure was better before, you should concentrate your leak check to areas that have been added or changed. Watch for the Helium signal to increase. When the signal increases, this corrolates to a leak being at the location you are leaking you are leaking He at. Possibly, if your system is large, there may be a delay. Because of this, leave enough time leaking at a particular location to identify if there is a delayed He signal increase.
- If this fails, but you are sure there is a leak, try using the balloon method: using plastic bags (grocery, trash, etc), section off all areas of your chamber into different bags using tape to complete the seal. Poke your He leak into the bag and balloon the gas into the bag. If there is no mass spec signal increase, move on to the next balloon. If you see a signal increase with a particular balloon, then you have narrowed down your area of search. You know the leak is contained somewhere in there.
More detailed explanation here.
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