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In the laboratory, we regularly conduct leak tests on gas cylinders, pipelines, instruments, etc. The level of airtightness has a significant impact on the safety of our experiments as well as on the accuracy of the results obtained. For example, when measuring PPM-level CO and CO2, if there is a leak in the pipeline, the measurement results will be highly inaccurate ； Another example is a leaking gas cylinder, which not only affects the quality of the gas but also poses a potential explosion risk. The common leak detection methods are generally as follows: 1) Intuitive perception: that is, using listening and smelling. This is also our most direct and commonly used method for making initial judgments. For example: if a hissing sound is heard from the gas cylinder or a strong, pungent odor is detected, the leak location can be determined based on these visual and sensory indications. This method is simple, but it has limitations; it is not suitable for highly toxic gases and is not effective for minor leaks. 2) Leak detection liquid: Leak detection liquid is the most commonly used method. If none is available, one can mix some dishwashing liquid or soapy water on their own; apply this mixture to areas such as gas cylinders, pipelines, and instruments where leaks are suspected. If bubbles form, it indicates a leak. It is important to note that for some precision instruments and electronic components in areas where water spots should not form, we can use cotton balls dipped in the leak detection liquid to apply it, or use sponge pieces dipped in the same liquid to cover those areas. This method is widely used and accurate, but note that soap water must never be used for checking oxygen cylinders for leaks, to prevent the fats in the soap water from coming into contact with oxygen and causing a violent oxidation reaction. 3) Pressure retention method: For testing for leaks in certain pipelines or gas cylinders, we can apply pressure and observe whether the pressure drops over a certain period of time; if the pressure does drop, it indicates the presence of a leak. In this way, we can apply pressure in stages, gradually narrowing down the area to locate the leak. 4) Bubble method: For some complex components or highly toxic gases, we can wrap a flexible rubber tube or a airtight bag around the suspected component, with the other end placed in water or connected to a balloon. If bubbles appear in the water or the balloon expands, it indicates a leak. This method is most suitable for complex components where water cannot be present or for detecting highly toxic gases. 5) Reagent method: This approach is commonly used for detecting certain highly toxic gases. To check a liquid ammonia cylinder, a red litmus paper moistened with water can be brought near the leak site of the cylinder; if the paper changes from red to blue, it indicates a leak.