NDT - Non-Destructive Testing Leak Testing Techniques Questions and Answers — Questions and Answers
Question 1: A technician must verify the integrity of a hermetically sealed electronic component for a deep-space satellite, where even a minuscule leak is unacceptable. Which of the following leak testing methods offers the highest sensitivity for this application?
- Bubble emission testing
- Helium mass spectrometer testing (Correct answer)
- Halogen diode detector testing
- Absolute pressure leak testing
Correct answer: Helium mass spectrometer testing
Helium mass spectrometer testing is the most sensitive practical leak detection method available, capable of detecting extremely small leaks (down to 10-12 mbar·l/s). This level of sensitivity is critical for high-reliability applications like space hardware where leakage cannot be tolerated.
Question 2: When selecting a tracer gas for leak testing, which of the following is a critical property for ensuring high sensitivity and minimizing false readings?
- High molecular weight to prevent rapid dispersion
- Chemical reactivity with the test object's surface
- Low background concentration in the atmosphere (Correct answer)
- High visibility under ultraviolet light
Correct answer: Low background concentration in the atmosphere
A good tracer gas should be rare in the atmosphere to provide a high signal-to-noise ratio, making it easier to detect leaks without interference from ambient air. Helium is often used because its atmospheric concentration is very low (about 5 ppm), which prevents high background signals from masking a true leak.
Question 3: An inspector is performing a leak test on a large vessel that has been pressurized with a helium-air mixture. The inspector moves a probe connected to a mass spectrometer over the exterior surface of the vessel to pinpoint the location of any leaks. What is this specific technique called?
- Tracer probe technique
- Pressure decay method
- Detector probe technique (Correct answer)
- Bubble immersion technique
Correct answer: Detector probe technique
This method is the detector probe technique, commonly known as "sniffing." It involves pressurizing the component with a tracer gas and using a detector probe on the outside to find where the gas is leaking out. The tracer probe technique is the opposite, where a vacuum is pulled on the component and tracer gas is sprayed on the exterior.
Question 4: During a bubble leak test using a soap solution on a pressurized gas line, the technician observes a single, large bubble that forms immediately upon application and then remains without growing or bursting. What is the most likely interpretation of this observation?
- A very large, high-pressure leak is present
- Entrapped air from the initial application of the solution (Correct answer)
- The test solution has too high a viscosity
- A very small leak or "weep" is present
Correct answer: Entrapped air from the initial application of the solution
A true leak will produce a continuous stream of bubbles or a bubble that steadily grows. A single, static bubble that forms immediately is often just air trapped under the solution as it was applied, not an indication of a leak from the pressurized system.
Question 5: Which of the following is a standard unit used to express a leak rate, representing a throughput of gas?
- Pascals per second (Pa/s)
- Atmosphere cubic centimeters per second (atm·cm³/s) (Correct answer)
- Liters per minute (L/min)
- Pounds per square inch per hour (psi/hr)
Correct answer: Atmosphere cubic centimeters per second (atm·cm³/s)
A leak rate is defined as the throughput of gas through a leak, which accounts for volume, pressure, and time. Standard units like atm·cm³/s (also written as atm·cc/s) or the SI unit Pa·m³/s incorporate all three components. The other options are units of pressure change, simple volumetric flow, or pressure change over a long period, not standard leak rate units.
Question 6: Which of the following leak testing methods relies on monitoring the change in pressure inside a sealed component over a specified period to determine if a leak is present?
- Absolute pressure testing (Correct answer)
- Halogen diode testing
- Mass spectrometer testing
- Bubble emission testing
Correct answer: Absolute pressure testing
Absolute pressure testing, a type of pressure change measurement, directly measures the internal pressure of a sealed object over time. A drop in pressure (for a pressurized object) or a rise in pressure (for an evacuated object) indicates a leak. The other methods rely on detecting a specific tracer gas (halogen, helium) or the formation of bubbles.
NDT - Non-Destructive Testing Leak Testing Techniques Questions and Answers
A technician must verify the integrity of a hermetically sealed electronic component for a deep-space satellite, where even a minuscule leak is unacceptable.
Which of the following leak testing methods offers the highest sensitivity for this application?