Helium is used as a tracer gas to detect leaks for several reasons. These include the fact that it constitutes only ~ 5 ppm in air so that background levels are very low. Helium has also relatively low mass so that it is ‘mobile’ and is completely inert/non-reactive. Helium is also non-flammable and generally widely available and low cost.
This association with helium is one of the reasons why one of the most accurate and rapid leak detection methods employs helium as the tracer gas, and a mass spectrometer for the analyzing/measuring. Furthermore, helium is chosen as a tracer gas because it is light, very quick and absolutely harmless.
Helium detection works in the following way: the unit being checked is either pressurized from within or else pressured from without with helium. The gas from any potential leaks is collected and pumped into the mass spectrometer for analyzing, and any value above the background trace of helium is evidence of a leak. The spectrometer itself works in the following way: any helium molecules sucked into the spectrometer will be ionized, and these helium ions will then “fly” into the ion trap where the ion current is analyzed and recorded. Based on the ionization current the leak rate is then calculated.
The reference (or background) reading for helium is an important part of the process. This reference reading provides the “background noise” for helium, which can be thought of as the ambient level of helium. The majority of this background helium is contained in between 100 and 150 micro-layers of gas molecules and is permanent gas (contained in the air) that is in the leak detector, pumps, test part etc. The removal of this surface helium is called “degassing” and starts when all the gas has been pumped out, once the molecules have been “desorbing” from the inside surface of the metal. This desorption starts at a pressure of about 10-1 mbar. Such degassing by lowering the pressure or by heating the chamber surface is not unusual, but even then does not totally eliminate all the gas at the surfaces. In addition to surface helium, “standby” helium is also contained in O-rings which act like sponges, whilst also providing a good indication of how clean the unit is. Modern helium leak detectors are able to constantly measure and calculate this internal (background) level and automatically subtract this from the leak rate measurement.
(1)Negative pressure method
Vacuum the sealed chamber inside the inspected product.
Apply helium gas on the outer surface of the product under inspection
Leak detector detects changed in helium content inside the workpiece.
The precise location of the leak can be realized.
(2)Positive pressure method
The sealed chamber is filled with helium under certain pressure.
When a leak occurs, helium leaks through the leak through the surrounding atmosphere.
Detection of helium concentration increments in surrounding atmosphere by suction method.
The precise location of the leak can be realized.
(3)Back pressure method
Put the test piece into a pressure helium tank and vacuum the tank. After pressure below 100Pa, pure helium specified by GB 4844-1984 for a charge time greater than 20s.
When the pressure reaches the required helium filling pressure, hold it up to the required time.
When the inspected is removed, the pressure time is greater than20s.
The inspection will be blown above 5min with nitrogen or dry air.
(4)Leak detection method
Place the inspected product in a pressurized helium chamber for a certain period of time.
Remove the residual helium from the surface.
Put the inspected product into the leak detector connected with the leak detector.
Take the maximum leakage rate for a certain time.
(1)Helium spray testing process
◆After assembling the workpiece, use the corresponding tooling to connect the workpiece to the leak detector.
◆After confirming that the connection is OK, press the “Start” button of the leak detector, and the leak detector enters the vacuum leak detection state.
◆Observe that the background value displayed on the leak detector screen reaches 4.0x 10-10Pa·m3/s.
◆Use 99.99% or 99.999% helium to spray helium detection on the suspected leak points of the workpiece.
◆Spray helium for 1s for each suspected leak point, and observe the background value of the leak detector for 3-5s. Observe whether the background value of the leak detector increases or whether the leak detector has an alarm output.
◆Follow step 5 to detect other suspected leak points in sequence.
◆The order of helium spray detection is from top to bottom, and confirm the large leak point first.
◆After the detection, press the “Stop” button of the leak detector first, and wait for the standby screen to appear on the leak detector display before disassembling the workpiece.
(2)Sniffer gun testing process
◆After assembling the workpiece, use the corresponding tooling to fill the workpiece with a certain pressure of 99.99% or 99.999% helium.
◆After confirming OK, connect the suction gun to the leak detection port of the leak detector. Press the “Start” button of the leak detector, and the leak detector enters the vacuum leak detection state.
◆Observe that the background value displayed on the leak detector screen reaches about 5.0x 10-8Pa·m3/s. Relatively stable state.
◆Use helium to detect the suspected leak points of the workpiece. Use the suction gun from bottom to top, 1-3mm away from the surface of the inspected part, and the moving speed is not more than 10mm/s to detect the helium concentration increment in the atmosphere around the inspected product.
◆Follow step 4 to detect other suspected leaks in sequence.
◆First confirm the large leak point, and then detect the small leak point after handling it. .
◆After the test, press the “Stop” button of the leak detector first, and then remove the suction gun after the standby screen appears on the leak detector display.
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