To test for a leak in a vacuum furnace, there are several methods that can be used:
1. Sound Detection: One method is to listen for any sound emanating from the leak. A hissing or whistling sound may indicate the presence of a leak.
2. Vacuum Decay: Another method is to use a solvent to penetrate the leak and observe any vacuum decay on the vacuum gauges. If there is a leak, the vacuum level will drop.
3. Helium Leak Detector: The use of a helium leak detector, such as a mass spectrometer, is a highly sensitive method for detecting leaks. Helium gas is introduced into the furnace, and if there is a leak, the helium will escape and can be detected by the leak detector.
It is important to note that if a leak is detected in a vacuum furnace, it should not be used for production until the leak is repaired. One common practice is to backfill the furnace with nitrogen without opening the chamber to atmosphere. All thermocouple fittings and other vacuum feed-thrus should be tightened. The furnace can then be re-tested for leak rate. If the second leak rate test fails, it indicates that more extensive maintenance, possibly including helium leak checking, is required.
To prevent leaks in vacuum furnaces, a comprehensive preventive maintenance program is essential. This includes proper care of pumps, O-rings, and flange sealing surfaces, as well as regular inspection of vacuum feed-thrus. Continuous monitoring of vacuum levels during processing can also help identify potential problems before they become major repairs.
To perform a leak rate test, the furnace should be pumped down to a high vacuum level, such as 3 x 10-5 torr, on the ion gauge. The vacuum-level reading on the vacuum TC gauge should be recorded. After isolating the vacuum chamber from the pumps, the vacuum level on the vacuum TC gauge should be recorded again after 15 minutes. The difference between the initial vacuum reading and the 15-minute reading is multiplied by 4 to calculate the leak rate per hour of the vacuum chamber. Acceptance criteria for leak rate can vary depending on governing specifications, but a leak rate of 20 microns/hour is generally acceptable for most applications. However, a well-sealed furnace can achieve a leak rate of less than 5 microns/hour, with some even achieving a level of less than 1 micron/hour.
In summary, testing for leaks in a vacuum furnace can be done through sound detection, vacuum decay, or using a helium leak detector. Regular preventive maintenance and monitoring of vacuum levels are important to minimize downtime due to leaks.
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