NDT Leak Testing Techniques 5 — Questions and Answers
Question 1: Which NDT leak testing method relies on detecting the ultrasonic frequencies generated by turbulent gas flow through a leak?
- Radiographic testing
- Ultrasonic leak detection (Correct answer)
- Eddy current testing
- Liquid penetrant testing
Correct answer: Ultrasonic leak detection
Gas escaping through a leak creates turbulence that generates ultrasonic frequencies (typically 35–45 kHz), which are detected by ultrasonic leak detectors.
Question 2: In mass spectrometer leak testing, what is the 'background' signal and why must it be managed?
- It is the signal from the reference leak used for calibration purposes
- It is the residual helium already present in the detector environment that can mask small leak signals (Correct answer)
- It is electrical noise in the detector electronics only
- It refers to vibrations from nearby equipment affecting results
Correct answer: It is the residual helium already present in the detector environment that can mask small leak signals
Background helium from prior testing or ambient sources raises the baseline signal, potentially masking small leaks and reducing sensitivity.
Question 3: What is the correct action when a leak is found in a pressure system during a hydrostatic test?
- Weld the leak while the system remains pressurized to save time
- Depressurize the system completely before any repair is attempted (Correct answer)
- Reduce pressure to half and continue the test while marking the leak location
- Apply sealant immediately under pressure to stop the leak
Correct answer: Depressurize the system completely before any repair is attempted
Safety requires full depressurization before any repair work, as working on pressurized systems poses severe injury risk.
Question 4: Which leak testing technique would be most appropriate for verifying the integrity of a spacecraft fuel tank before launch?
- Soap bubble test at ambient pressure
- Helium mass spectrometer leak testing to detect micro-leaks (Correct answer)
- Hydrostatic testing with water fill
- Visual inspection only
Correct answer: Helium mass spectrometer leak testing to detect micro-leaks
The extreme sensitivity of helium mass spectrometer testing makes it the preferred method for critical aerospace applications where micro-leaks could be catastrophic.
Question 5: What does a 'fine leak' test specification of 1×10⁻⁸ atm·cc/s indicate?
- The test pressure must be 10 atmospheres for 8 seconds
- The maximum allowable leak rate is one hundred-millionth of an atmosphere-cc per second (Correct answer)
- The test uses 8 cycles of pressurization at 10 atm
- A leak of 8 micrometers diameter is acceptable
Correct answer: The maximum allowable leak rate is one hundred-millionth of an atmosphere-cc per second
1×10⁻⁸ atm·cc/s means the component must not leak more than one hundred-millionth of an atm·cc of gas per second — a very tight fine leak specification.
Question 6: What is the main limitation of the soap bubble / liquid film leak test method?
- It cannot be used on metal components
- It has low sensitivity and may miss very small leaks (Correct answer)
- It requires specialized equipment costing over $100,000
- It can only detect leaks in horizontal surfaces
Correct answer: It has low sensitivity and may miss very small leaks
Soap bubble tests are a gross leak method with limited sensitivity; very small leaks may not generate visible bubbles within the observation time.
Question 7: In NDT leak testing documentation, why must the test pressure, test medium, hold time, and acceptance criteria all be recorded?
- Documentation is only required when leaks are found
- Complete records ensure traceability, repeatability, and compliance with quality and safety standards (Correct answer)
- Only the acceptance criteria need to be documented for audits
- Documentation is voluntary unless specified by the customer
Correct answer: Complete records ensure traceability, repeatability, and compliance with quality and safety standards
Full test documentation provides traceability for quality assurance, supports repeat testing, and demonstrates compliance with applicable codes and standards.
NDT Leak Testing Techniques 5
Which NDT leak testing method relies on detecting the ultrasonic frequencies generated by turbulent gas flow through a leak?