![]() ![]() How do I determine the source of excessive outgassing? The flowchart in Figure 2 outlines the initial steps you should follow – starting with the door seal – when troubleshooting why your vacuum furnace will not pump down. As such, it tends to be the cause of pump-down issues about 80% of the time. The door seal on a vacuum furnace sees some of the highest wear-and-tear through normal operation. If the pumping system is operating correctly, then one of these failure modes is likely the cause: a damaged door seal, vacuum furnace leaks and/or hot-zone contamination. If your vacuum furnace won’t pump down, the first thing you should do is verify the functionality and blank-off of the mechanical pumping system. Once you have identified all of the vacuum furnace and/or inert-gas leaks, you must go back and address each of the individual leaks. You should then leak check the system using a helium mass spectrometer. Next, you can use the furnace’s pumping system to pump the entire backfill system and pull a vacuum on the complete system. It will also draw a vacuum on the gas reservoir tank and the evaporator stand all the way back to the closed outside valve. Once you open the backfill valve and depressurize the backfill reservoir tank back to an atmospheric condition, the furnace will start drawing a vacuum on the lines. This process should be performed by a qualified expert and according to safety guidelines. Manually open the backfill valve on the furnace by electrically energizing it open.Depressurize the backfill reservoir tank.Pump down the furnace using the furnace’s normal pumping system. ![]() This isolates the outside liquid system from the evaporator stand, the backfill reservoir tank and the furnace and all of its ancillary partial-pressure and backfill lines. To perform vacuum leak testing, you would: It involves isolating the liquid-nitrogen or liquid-argon storage system from the furnace, ensuring the entire system is in vacuum and then using a helium mass spectrometer to leak check the system. The more precise method is vacuum leak testing. If it starts to bubble in any tested places, you know you have found a leak. It involves spraying a leak-detector solution on known suspect areas. Thin-film leak testing is the first method, and it is often the easiest and quickest way to check for inert-gas leaks on the pressurized side. They are thin-film (soap-bubble) leak testing and vacuum leak testing. ![]() There are also two common methods for locating inert-gas leaks. This leak is then reported by the helium mass spectrometer. Once you charge the test piece with the trace gas, the probe sniffs around the part for any gas that may be escaping through a leak. Test-piece examples include the heat exchanger, water-cooled flange, water-cooled fans and shafts, power feed-through, water jacket, etc. To use this technique, you should charge – or pressurize – a test piece with a trace gas, such as helium. This is for applications where it’s not feasible to evacuate what you’re testing for leaks. Then there is the sniffer probe technique. It involves pumping down the furnace, applying helium to the exterior of the furnace (e.g., with a helium mass spectrometer connected) and looking for a response. The spray probe technique seen in Figure 1 is the traditional form of leak checking. There are two primary techniques used to leak check a vacuum furnace: the spray probe and the sniffer probe methods. What should I do about poor ultimate vacuum?įaced with these questions – and the frequency at which they are asked – we decided to compile a short guide that covers our experts’ answers to these common challenges. ![]()
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