Leak Testing

Leak Testing (manufacturing-quality/ndt/leak-testing)

Use when the task is leak testing an aerospace part or system: computing the leak rate from a pressure decay or vacuum decay measurement, sizing the test time a gauge resolution needs, converting a measured helium leak to the equivalent air leak and back, translating an immersion bubble observation into a leak rate, recommending the leak test method from the required sensitivity and access, and dispositioning the part against the maximum allowable leak rate. This leaf implements the leak-rate measurement math and the method screening in pure Python, stdlib only. It pairs with manufacturing-quality/ndt/ndt-method-selection for the broader NDT method screening context (leak testing sits beyond the five volumetric and surface methods that leaf screens), with manufacturing-quality/as9100/calibration-control for gauge resolution, and with manufacturing-quality/as9100/risk-management for the process risk context.

Domain quick reference

• Leak rate unit: standard cubic centimeters per second (scc/s), the gas volume at standard temperature 293.15 K that passes the leak per second.
• Pressure and vacuum decay: q = V_cc * dP_atm / t * (STD_TEMP_K / temp_K), with V_cc = volume_L * 1000 and dP_atm = dP_bar * BAR_TO_ATM, BAR_TO_ATM = 0.986923. Pressure decay watches the pressure fall in a sealed part; vacuum decay watches the pressure rise in an evacuated chamber. Same math.
• Gauge resolution time: invert the decay equation, t = V_cc * dP_atm / q_target, the test time that makes a target leak produce a pressure change above the gauge resolution.
• Helium to air conversion: q_air = q_he * sqrt(M_HE / M_AIR), with M_HE = 4.003 and M_AIR = 28.97 g/mol, so the factor is sqrt(4.003 / 28.97) = 0.3717. This is the documented typical molecular flow relation; a viscous flow leak sits closer to the viscosity ratio, which the approved procedure resolves.
• Bubble (immersion): q = (4/3) * pi * (d/2)^3 * bubbles_per_s with d in cm, the per bubble volume times the bubble count.
• Method screening thresholds (typical values, module constants): helium mass spectrometer hood when the required sensitivity is at or below 1e-6 scc/s (MS_THRESHOLD); helium sniffer when localization is needed and the sensitivity is at or below 1e-5 scc/s (SNIFFER_THRESHOLD); pressure or vacuum decay when only one side is accessible and the part cannot be immersion tested; bubble when localization is needed and the sensitivity is at or below 1e-2 scc/s (BUBBLE_THRESHOLD); otherwise pressure decay. A helium mass spectrometer resolves leaks down to about 1e-9 scc/s He (HELIUM_MS_MIN_DETECT_SCCS).
• Disposition: accept when measured <= max allowable; reject when measured exceeds the allowable by a ratio above 1.25 (REVIEW_RATIO); review in the band between. margin_db = 10 * log10(max_allowable / measured), positive on accept, negative on reject.
• AS9100 frames the special process control context; the relations above are standard engineering methodology, summary-only.

Workflow

1. Record the test: part internal volume in liters, pressure drop in bar and the test time, then compute the leak rate with pressure_decay_rate (or vacuum_decay_rate for an evacuated chamber, chamber_volume_L in liters). Both return scc/s at standard temperature.
2. Check the measurement is meaningful: gauge_resolution_time(volume_L, gauge_res_bar, target_sccs) gives the seconds needed for the target leak to show above the gauge resolution. If the planned test is shorter, extend it.
3. Convert units when the reading is helium based: helium_to_air(q_he_sccs) for the air-equivalent leak of a helium mass spectrometer reading, air_to_helium(q_air_sccs) to go the other way.
4. For an immersion bubble observation, convert the bubble stream to a leak rate with bubble_leak_rate(bubble_diameter_mm, bubbles_per_s).
5. Screen the method with method_recommendation(required_sensitivity_sccs, access_both_sides, need_localization, part_pressure_capable), which returns the method and the rationale from the deterministic threshold chain.
6. Disposition the part with disposition(measured_sccs, max_allowable_sccs, method) and read the verdict plus margin in dB, or use helium_ms_verdict(detected_sccs_he, limit_sccs_air) for a helium reading against an air limit.
7. For a one-call decay summary, summarize(volume_L, dP_bar, time_s, max_allowable_sccs, method) returns the leak rate, method, verdict and margin in one dict.
8. Confirm the deterministic checks with the contract test scripts/test_leak_testing.py.

Worked example

A 50 L fuel tank is pressure decay tested: 0.02 bar drop in 600 s.

• Leak rate: q = 50000 cc * 0.019738 atm / 600 = 1.645 scc/s (pressure_decay_rate(50, 0.02, 600), module value 1.644872). Against a 2 scc/s maximum allowable the disposition is accept with margin 10 * log10(2 / 1.645) = 0.849 dB.
• Gauge check: with a 0.001 bar gauge resolution and a 0.05 scc/s target leak, gauge_resolution_time(50, 0.001, 0.05) = 986.9 s (module value 986.923), so a 600 s test cannot catch a 0.05 scc/s leak.
• Helium conversion: a helium mass spectrometer reading of 1.0 scc/s He is 0.3717 scc/s air (helium_to_air), so a detected 1e-8 scc/s He leak is 3.7e-9 scc/s air equivalent, accept against a 1e-8 scc/s air limit (helium_ms_verdict).
• Bubble: a 3 mm bubble at 1 per second is (4/3) * pi * (0.15 cm)^3 = 0.01414 scc/s (bubble_leak_rate(3.0, 1.0), module value 0.014137).
• Method screening: a sealed valve needing 1e-8 scc/s sensitivity routes to the helium mass spectrometer hood; a part with 1e-4 scc/s sensitivity that needs localization and has both sides accessible routes to bubble immersion; a one-sided part that holds pressure routes to pressure decay.
• Disposition bands: 1.0 vs 2.0 scc/s accept with 3.01 dB margin; 3.0 vs 2.0 reject (ratio 1.5 above 1.25); 2.4 vs 2.0 review (ratio 1.2 in the band).

Verification

• Confirm pressure_decay_rate(50, 0.02, 600) returns 1.644872 scc/s, within rounding of the 1.645 scc/s anchor, and equals the direct formula V_cc * dP_atm / t at standard temperature.
• Confirm gauge_resolution_time(50, 0.001, 0.05) returns 986.923 s, within rounding of the 986.9 s anchor, and that doubling the target halves the time.
• Confirm helium_to_air(1.0) equals sqrt(4.003 / 28.97) = 0.371722 and that air_to_helium(helium_to_air(x)) recovers x within 1e-12.
• Confirm bubble_leak_rate(3.0, 1.0) equals (4/3) * pi * 0.15^3 = 0.014137 scc/s.
• Confirm method_recommendation picks the helium mass spectrometer hood at 1e-8 scc/s, the helium sniffer at 1e-5 with localization, and bubble at 1e-4 with localization and both sides accessible.
• Confirm disposition margins: accept at 1.0 vs 2.0 with margin 3.01 dB, reject at 3.0 vs 2.0, review at 2.4 vs 2.0.
• Confirm every non-positive volume, time, temperature, target, allowable, every negative pressure drop, and every unknown method string raises ValueError.
• Run the contract test offline: python3 scripts/test_leak_testing.py (34 tests, deterministic).

Related leaves

• manufacturing-quality/ndt/ndt-method-selection: the RT, UT, ET, PT and MT screening context that stops short of leak testing.
• manufacturing-quality/as9100/calibration-control: gauge resolution and calibration control for the test instrumentation.
• manufacturing-quality/as9100/risk-management: process risk context for the leak test acceptance decisions.

Pitfalls

• Running a test shorter than the gauge resolution time: the 600 s test in the worked example cannot catch a 0.05 scc/s leak because gauge_resolution_time is 986.9 s - a short test that shows no drop is a false pass, not a clean part.
• Converting helium to air with the molecular factor unconditionally: q_air = q_he * sqrt(M_HE / M_AIR) = 0.3717 applies to molecular flow; a viscous flow leak sits closer to the viscosity ratio, which the approved procedure must resolve before the conversion is used.
• Dispositioning straight accept/reject without the review band: measured values between the maximum allowable and 1.25 times it (ratio in band, e.g. 2.4 vs 2.0) return review, not accept or reject - only a ratio above 1.25 rejects.
• Picking the method on sensitivity alone: the threshold chain (helium MS hood at or below 1e-6 scc/s, sniffer at or below 1e-5 when localization is needed, bubble at or below 1e-2 with localization) also weighs access - a one-sided, pressure-capable part routes to pressure or vacuum decay even at low sensitivity.
• Forgetting the standard-temperature basis: leak rates are scc/s at 293.15 K and the decay equation temperature-corrects the measured drop, so an uncorrected rate quoted at test temperature is not comparable to the max allowable.
• Treating the gauge as trustworthy without calibration control: gauge resolution drives both the test-time sizing and the decay reading, so the instrumentation must sit inside the calibration-control frame before any disposition is issued.

Behavior contract (gate 3)

Run the deterministic contract test (stdlib unittest, offline):

python3 scripts/test_leak_testing.py

The test covers the pressure and vacuum decay worked example (50 L, 0.02 bar, 600 s to 1.645 scc/s), the gauge resolution time anchor (986.9 s), the helium to air conversion and both round trips, the bubble geometry anchor (3 mm at 1 bubble/s to 0.01414 scc/s), every method recommendation branch of the threshold chain, the disposition accept, reject and review bands including the 1.25 ratio edges, the helium mass spectrometer verdict, the summarize dict, and ValueError rejection of all non-physical inputs.

Compliance

• Standards referenced, not reproduced: AS9100 is named as the special process control frame by id only, per standards-map.yaml; all relations above are standard engineering methodology, summary-only.
• compliance: STANDARDS-REF, gated: false.