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Leak Test Equipment, Method, Standard

Leak Test Equipment, Method,  Standard

Leak test equipment is a type of nondestructive testing equipment used to measure the escape of liquids, vacuum or gases from sealed components or systems. Some configurations require a separate leak detector or sensor as an input. They are often equipped with various other components such as pumps, calibrators, gauges and cases.

Several methods are available for testing leaks. Bubble testing is used for applications that do not require high sensitivity. A pressurized test object is submerged in a water tank. If bubbles emerge there is a leak. Specialized soap films applied to the surface of the test object are used to aid the process by lowering the surface tension and increasing the wetting ability of water. With the colorimetric method, the surface is treated with a developer and reacts with the activating tracer (the leaking fluid), causing a visible color change to take place at the point of leakage. Developer application method is aerosol spray, paint or treated paper. Colorimetric developers are used to leak test large single and double walled tanks, pressure and vacuum vessels, laminated, lined or double-walled parts, complex piping systems, flexible containers such as aircraft fuel tanks, glass-to-metal seals in hybrid packages and systems that inherently contain or will contain the tracer (water, ammonia, chlorine or oil). For electronic gas detection, the test object is pressurized with a tracer gas and a sniffer probe is used to scan the object for leaking gas.

In the mass flow method, any pressure change measured by a pressure sensor is compensated for by inputting air into the test part. The amount of air that enters is measured by a flow sensor, directly determining the leak rate of the part. Leakage flow is directed across a heating element. The temperature change across a temperature transducer bridge results in an output voltage proportional to mass flow. Another method is the mass spectrometer method. This process involves the pressurizing of the test object with a helium mixture inside an airtight pressure chamber. The air is then evacuated from the chamber, creating a pressure gradient between the internal volume of the part and the vacuum. The helium molecules move out of the part through any leakage. A mass spectrometer then samples the air inside the chamber and measures the partial pressure of helium. The pressure differential or decay method involves the initial inflation of the test object and the establishment of a reference pressure. After a designated amount of time, the pressure is measured again and the initial and final measurements are compared. The change in pressure can be used to calculate the leak rate given the internal volume of the device. Pressure decay is able to detect minute changes in pressure. An ultrasonic leak detector, sometimes called sonic or choked flow or turbulent flow, is another leak detection method. The turbulent flow of a fluid across a pressure boundary creates acoustic waves. These waves can be transmitted through the medium of the fluid itself, through the containment structure, or through the air surrounding the containment structure. An ultrasonic detector is used to pick up these sounds.

Leak testers can have analog voltage, current or a frequency or pulsed output. The output can also be a change in state of a switch or an alarm. Digital outputs can be either parallel or serial or other computer signals. Likewise, displays for these instruments can be analog, digital or video type readouts. Popular features available for some leak testing equipment include timers, triggers, alarms, data logging or memory and self-calibration.

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