ASNT NDT III LT cheat sheet

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

Molecular flow conductance for a long tube: C = (π d^3 / (12 L)) √(2π R T / M), where R is gas constant, T temperature, M molar mass.

Bubble immersion test: leak rate (std cm³/s) ≈ (bubble volume)/(time to form bubble) × (bubble count), corrected to standard conditions.

Pressure decay test: leak rate Q = (V ΔP) / (Δt Patm), assuming ideal gas and isothermal conditions; V is test volume.

Helium mass spectrometer leak detector: minimum detectable leak typically 1×10^-10 std cm³/s; calibrated with a certified leak standard.

Vacuum test (hood test): tracer gas applied to outside of evacuated component; leak rate = (signal rise × calibrated sensitivity) / response time.

Ultrasonic leak detection: detects turbulent flow noise; effective for pressure differentials > 5 psi and leak rates > 1×10^-3 std cm³/s.

Halogen diode detector: sensitive to halogen gases; minimum detectable leak ~1×10^-6 std cm³/s; sensor can be poisoned by exposure to high concentrations.

Qualification of LT procedures requires demonstration of repeatability and reproducibility; typically three runs by two operators on a known leak standard.

Calibration of leak detection equipment must be traceable to a national standard (e.g., NIST) and performed at intervals specified in the written procedure.

A leak standard must be certified and have a known leak rate; its uncertainty should be ≤ 1/3 of the tolerance being measured.

Procedure qualification record (PQR) must include test method, tracer gas, pressure, temperature, equipment, and acceptance criteria.

Environmental factors (temperature, background tracer gas, humidity) must be controlled or compensated during leak testing.

Acceptance criteria for leak tests are typically expressed as maximum allowable leak rate in std cm³/s or mbar·L/s.

Personnel performing leak tests must be qualified to the applicable standard (e.g., SNT-TC-1A) with vision acuity and training records.

Ideal gas law: PV = nRT; used to convert leak rates to standard conditions (T=273 K, P=1 atm).

Viscosity of gases increases with temperature; for gases μ ∝ T^0.5 approximately.

Speed of sound in gas: c = √(γ R T / M), where γ is specific heat ratio.

Vacuum units: 1 atm = 760 torr = 101325 Pa = 1.01325 bar; 1 std cm³/s = 0.001 Pa·m³/s.

Leak rate conversion: 1 std cm³/s = 0.001 mbar·L/s = 0.1 Pa·L/s.

Pressure measurement: Bourdon tube for gauge pressure, Pirani gauge for rough vacuum, ionization gauge for high vacuum.

Flow regimes: viscous flow (continuum) when Knudsen number Kn < 0.01; molecular flow when Kn > 1; transitional in between.

NDT methods include VT, PT, MT, UT, RT, ET, LT; selection depends on material, defect type, and accessibility.

SNT-TC-1A: employer-written practice; certification levels I, II, III; training hours and experience requirements are minimums.

CP-189: establishes minimum requirements for qualification and certification of NDT personnel; more prescriptive than SNT-TC-1A.

Level III responsibilities include procedure approval, interpretation of codes, and training/certification of Level I and II personnel.

Certification requires documented training, experience, and passing a general, specific, and practical examination.

Recertification: every 5 years for SNT-TC-1A; annual vision test (Jaeger 1 or equivalent) required.

Level III examination includes a basic exam, method exam, and a practical exam; passing score typically 80%.

Employer must maintain certification records including examination results, experience, and vision tests.