Helium mass spectrometer leak detector—differences between positive pressure helium testing and negative pressure helium testing

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The common helium mass spectrometry leak detection methods are the positive pressure method and the negative pressure method, each with its own advantages; the choice can be made based on the actual circumstances. The advantage of positive-pressure helium testing is its simplicity of operation and fast testing speed, while the disadvantage is its susceptibility to interference ； The advantage of negative-pressure helium testing is that it is less affected by environmental factors, has higher sensitivity, and good stability; the disadvantage is that it is more complex to operate. 1. Negative pressure helium leak detection (helium spraying method): In negative pressure leak detection, the area to be inspected is connected to the detection port of the leak detector, and helium gas is continuously sprayed onto the suspected leak site using a spray gun. Helium enters the leak detector through the leakage holes and is detected. Applicable industries: coating industry, vacuum furnaces, vacuum accessories, vacuum valves, pressure vessels, vacuum systems, etc. Depending on the product, different sizes of fixtures or auxiliary tools need to be selected. For example, leak detection of tubes and shells. Once the leak detector is operating properly, the leak rate can be calibrated using a standard leak hole, allowing for leak detection through spraying on the pipe casing. First, fix the fixture at the leak detection port, place the tube shell to be tested on the rubber plate above the fixture, evacuate it to a predetermined vacuum level using the auxiliary pumping system, and then connect it automatically to the instrument’s measurement system. Then, it usually takes 1 to 3 seconds to continuously inject helium into the shell using a spray gun. When the housing leaks, helium will enter the mass spectrometry section of the instrument through the leak holes, and the magnitude of the leak is displayed directly on the leak rate meter. This method can not only determine the location of the leak, but also measure its size. The entire testing cycle takes only one minute. 2. Positive pressure helium leak detection (inspiration method). Positive pressure leak detection is the opposite of negative pressure leak detection. The suction gun is connected to the testing port of the leak detector, which is filled with helium at a specified pressure. The helium leaking from the leakage hole is drawn into a leak detector for detection. Performing negative pressure leak testing on large containers or internal exhaust pipes is very uneconomical, and the leak detection speed is slow. Positive pressure leak testing is generally used. Applicable industries: special valves, high-pressure pipelines, refrigeration systems, steam turbine units, etc. When using positive pressure leak detection, it is important to first calibrate the sensitivity of the instrument’s suction nozzle, and then fill the container with helium at a pressure higher than atmospheric pressure ； Secondly, some containers have a thin-plate structure. It is recommended to create a fixture outside the container to prevent deformation under high pressure and equipment damage. Third, when the suction gun moves along the weld, its speed should not be too fast; it should be kept at about 1 mm from the surface to ensure the sensitivity of the suction gun. Make the probe in the shape of a horn. The advantage of the positive pressure method is that it does not require a supplementary vacuum system, allows for precise positioning, and enables testing at any working pressure. The disadvantages of the positive pressure method are its low detection sensitivity, high uncertainty in the detection results, and significant dependence on the measurement environment conditions; for example, when the air contains 5 ppm of helium, the leak detection sensitivity of the positive pressure method is lower compared to that of the negative pressure method.