SVC Education Program
Detailed SVC Course Syllabus
We go to the expense of purchasing and operating a Helium Mass Spectrometer Leak Detector (HMSLD), because everything leaks. Some leaks are small and of no consequence. Others are large and harmful to system operation.
There are certainly other types of leak detection, besides HMSLD. These include hydrostatic, ionization, radioactive isotope decay and ultrasonic testing, as well as bubble test. But for finding small leaks and quickly determining size of leak, in a repeatable fashion, “helium leak checking” is preferred.
The history of HMSLD, starting in 1939, shows the changes in mass spectrometer construction, as well as general pump improvement.
There is more than one type of helium leak detector, produced by most manufacturers. There are portable and console types (with built in pumps, valves, gauges, etc), and even dedicated mass spec module units, that are built into vacuum systems.
More than one tracer gas (Helium is the most prominent) can be used, with most machines having the capability to be adjusted, over a short mass range. The properties of the chosen tracer gas are discussed, along with advantages and disadvantages, of the available types
The formula for leak detection (Throughput = Pressure times Pumping Speed divided by Time) is discussed, along with the many different, and sometimes confusing, words for this throughput formula.
The group then reviews an operating, portable, hybrid style helium leak detector. The normal start-up of the machine, test cycle sequence and shutdown is discussed. A complete review of the mass spectrometer used in the unit is conducted. Then the attendees disassemble an average mass spectrometer assembly. Gas flow through the plumbing, along with the tracer gas, is illustrated. Discussion follows on the various control valves that allow gross and fine leak detection levels.
A review of average acceptable leak rates, in different industries is covered, with comments on why the values vary.
The three (3) general “sources of vacuum leaks”, mentioned in the literature, are discussed. The ability of the average helium leak detector to find these leaks is reviewed.
Key words in leak detection and their meanings, are discussed, such as; Calibration or Adjustment, Response Time, Clean-up Time, Background, Zero or Null, Purge, Ballasting. Illustration and process examples of each, using the operating leak detector, are discussed.
The group reviews the various techniques of HMSLD leak finding, and their varying names. This includes the three (3) general types of Vacuum Inside techniques, as well as Sniffing. Examples of size of leak found, ease of use, and why one should use one over the other, are discussed.
Various diagrams of vacuum systems are shown, with questioning of where one should couple the leak detector to the system. Advantages/disadvantages of locating the HMSLD at the foreline, above the high vacuum pump, or at the chamber, are reviewed.
A list of general “Do and Don’t” statements, for helium leak testing, is reviewed. Reasons are given for the general statements. Questions are raised on attendee use.
Attendees then do actual leak detection work, with the operating helium leak detector, on sample parts. Some parts are within stated leak rate acceptance values, others beyond. The attendee is asked to define the point of leakage and rate. Some items have multiple small leak areas.
A discussion then follows, on the required maintenance on the mechanical pump, filters and mass spectrometer filament, to maintain proper leak detector operation. Variations are explained between different types of mechanical and high vacuum pumps.
A recap is then conducted on six (6) common problems, found in helium leak detection, to wrap up the program.
Instructor: David B. Webb, Vacua Techniques Company