ASNT NDT III LT flashcards

32 cards. Read the term and the definition here. The deck at the top of this page is optional.

Leak rate formula

For viscous flow through a capillary, Q = π d^4 ΔP / (128 μ L), where d is diameter, L is length, μ is viscosity, and ΔP is pressure drop.

Bubble immersion test

A leak testing method where the component is pressurized and immersed in liquid; leaks are indicated by bubbles. Leak rate is estimated from bubble size and formation rate.

Helium mass spectrometer

A sensitive leak detector that measures helium partial pressure; minimum detectable leak typically 1×10^-10 std cm³/s. Requires calibration with a certified leak standard.

Pressure decay test

A leak test where the component is pressurized and the pressure drop over time is measured. Leak rate Q = (V ΔP) / (Δt Patm), assuming isothermal ideal gas.

Vacuum hood test

A tracer gas leak test where a hood encloses the component and vacuum is applied; leaking tracer gas is detected. Leak rate is derived from signal rise and calibration.

Ultrasonic leak detection

Detects high-frequency sound produced by turbulent gas flow through a leak. Effective for pressure differentials >5 psi and leak rates >1×10^-3 std cm³/s.

Halogen diode detector

A leak detector sensitive to halogen gases; minimum detectable leak ~1×10^-6 std cm³/s. Sensor can be poisoned by high concentrations of halogen.

Tracer gas selection

Tracer gas should have low background concentration, high detection sensitivity, and compatibility with the test component. Helium is common due to low atmospheric concentration.

Leak standard calibration

A certified leak standard with known leak rate is used to calibrate leak detectors. Its uncertainty should be ≤1/3 of the tolerance being measured.

Procedure qualification record

Documentation that a leak testing procedure has been qualified, including test method, tracer gas, pressure, temperature, equipment, and acceptance criteria.

Repeatability and reproducibility

Qualification of LT procedures requires demonstration of repeatability (same operator) and reproducibility (different operators) typically via three runs by two operators.

Environmental factors in LT

Temperature, background tracer gas, and humidity can affect leak test results; they must be controlled or compensated during testing.

Acceptance criteria

The maximum allowable leak rate specified for a component, typically expressed in std cm³/s or mbar·L/s, based on design requirements.

Personnel qualification for LT

Personnel performing leak tests must be qualified to a recognized standard (e.g., SNT-TC-1A) with documented training, experience, and vision acuity.

Ideal gas law

PV = nRT, used to convert leak rates to standard conditions (T=273 K, P=1 atm). Essential for comparing leak rates measured at different conditions.

Viscosity temperature dependence

For gases, viscosity increases with temperature approximately as μ ∝ T^0.5; this affects leak rate calculations for viscous flow.

Speed of sound in gas

c = √(γ R T / M), where γ is specific heat ratio, R is gas constant, T is temperature, and M is molar mass. Used in acoustic leak detection.

Vacuum unit conversions

1 atm = 760 torr = 101325 Pa = 1.01325 bar. 1 std cm³/s = 0.001 Pa·m³/s = 0.001 mbar·L/s.

Knudsen number

Kn = λ / L, where λ is mean free path and L is characteristic length. Kn < 0.01 indicates viscous flow; Kn > 1 indicates molecular flow.

Pirani gauge

A thermal conductivity vacuum gauge used for rough vacuum measurement (typically 10^-3 to 10 torr). Not suitable for high vacuum.

Ionization gauge

A vacuum gauge that ionizes gas molecules and measures ion current; used for high vacuum (10^-3 to 10^-10 torr).

NDT method selection

Choice of NDT method (VT, PT, MT, UT, RT, ET, LT) depends on material, defect type, and accessibility. Leak testing is for detecting leaks, not internal defects.

SNT-TC-1A

A recommended practice for NDT personnel qualification and certification. Employers write their own practice; certification levels I, II, III with minimum training and experience hours.

CP-189

A standard that establishes minimum requirements for qualification and certification of NDT personnel. More prescriptive than SNT-TC-1A.

Level III responsibilities

Level III personnel approve procedures, interpret codes, and train/certify Level I and II personnel. They also must pass a basic, method, and practical exam.

Certification examination

Requires passing general, specific, and practical examinations. Passing score typically 80%. Records must be maintained by the employer.

Recertification requirements

SNT-TC-1A requires recertification every 5 years, including annual vision test (Jaeger 1 or equivalent) and documented continuing experience.

Vision acuity test

Annual vision test required for NDT personnel; must demonstrate near vision acuity of Jaeger 1 or equivalent, and color perception as applicable.

Employer certification records

Employer must maintain records of training, experience, examination results, and vision tests for each certified individual.

Leak rate units conversion

1 std cm³/s = 0.001 mbar·L/s = 0.1 Pa·L/s = 0.001 Pa·m³/s. Standard conditions are 0°C and 1 atm.

Molecular flow conductance

For a long tube, C = (π d^3 / (12 L)) √(2π R T / M), where d is diameter, L is length, R is gas constant, T is temperature, M is molar mass.

Background tracer gas

Ambient concentration of tracer gas can cause false signals; must be minimized by ventilation or using a tracer with low atmospheric concentration.