| name | leak-testing |
| description | Leak testing — methods (pressure/vacuum decay, helium mass spectrometer, bubble, tracer gas, ultrasonic), leak rate units (Pa·m³/s, scc/s, mbar·l/s), sensitivity comparison, ASTM/ISO standards. |
| metadata | {"priority":7,"promptSignals":{"phrases":["leak testing","leak detection","leak rate","helium leak test","pressure decay test","vacuum leak test","tracer gas leak"],"minScore":3}} |
Leak Testing — Complete Skill
Leak Rate Units and Conversions
SI: Pa·m³/s = [pressure × volume / time]
Practical: mbar·L/s; scc/s (standard cc/s at 0°C, 1 atm); cc/min; atm·cc/s
| Unit | Multiply by to get Pa·m³/s |
|---|
| 1 mbar·L/s | 0.1 |
| 1 scc/s (std cc/s) | 0.1013 |
| 1 atm·cc/s | 0.1013 |
| 1 cc/min | 1.69 × 10⁻³ |
| 1 oz/yr | ~8.9 × 10⁻⁸ |
Leak rate to equivalent hole size:
L [Pa·m³/s] = C_d × A × P × √(2γ/(γ-1) × 2/(RT) × ...) (choked flow for large leaks; viscous flow for small pores)
For viscous flow through capillary:
Q = π r⁴ ΔP / (8 η L_cap) [Hagen-Poiseuille; r = pore radius]
Sensitivity of Leak Test Methods
| Method | Sensitivity [Pa·m³/s] | Notes |
|---|
| Bubble immersion | 10⁻² to 10⁻³ | Gross leaks only |
| Pressure gauge decay | 10⁻² to 10⁻⁴ | Volume dependent |
| Flow meter | 10⁻³ to 10⁻⁵ | Large volumes |
| Thermal conductivity (Pirani) | 10⁻⁴ to 10⁻⁵ | Rare gas tracers |
| Ultrasonic detector | 10⁻³ to 10⁻⁴ | Qualitative |
| Halogen detector | 10⁻⁵ to 10⁻⁷ | Refrigerant tracer |
| Hydrogen detector (H₂/N₂) | 10⁻⁶ to 10⁻⁸ | Safe H₂ mixture |
| Helium mass spectrometer | 10⁻⁸ to 10⁻¹³ | Most sensitive |
| Radioactive tracer (Kr-85) | 10⁻¹³ | Special applications |
Pressure / Vacuum Decay Testing
Principle: pressurize or evacuate part; measure pressure change vs. time
Sensitivity:
ΔP = Q × t / V
Q_detectable = V × ΔP_resolution / t_test
Example: V = 1 L, ΔP_resolution = 0.01 mbar, t = 30 s:
Q_min = 0.001 L × 0.01 mbar / 30 s = 3.3 × 10⁻⁷ mbar·L/s = 3.3 × 10⁻⁸ Pa·m³/s
Differential pressure method: test part vs. reference volume → eliminates temperature drift
Temperature stability: 1°C change in 1 L vessel causes 3.4 mbar pressure change at 1 bar
→ stabilize ≥ 2× thermal time constant before measurement
Masking leaks from connections: differential measurement subtracts known leak paths
Helium Mass Spectrometer Leak Testing
Methods
Bombing (Gross Leak): pressurize sealed part with He; place in vacuum chamber; MSLD measures He escaping through internal He pressure
Hard Vacuum (Fine Leak): evacuate part; apply He externally; MSLD detects He entering
Sniffer (Pressurized): fill part with He at positive pressure; scan exterior with sniffer probe
Accumulation: seal part in bag with He; accumulate for time t; sample bag gas with sniffer
MSLD (Mass Spectrometer Leak Detector):
Ionizes gas mixture; magnetic/quadrupole sector separates by m/z; detects He at m/z = 4
Background He in atmosphere: 5.2 ppm → always present as background
Calibrated leak standard:
Permeation leak: known Q_ref [Pa·m³/s]; use to calibrate detector response
ASTM E498 specifies calibration
Hard Vacuum Test Procedure (ASTM E493)
- Evacuate test part to P < 1 Pa
- Connect MSLD; evacuate detector
- Background reading I_bg [A]
- Apply He externally to suspected areas
- Reading rises to I_He
- Q_part = Q_cal × (I_He - I_bg) / I_cal
Sensitivity: Q_min = 10⁻¹² to 10⁻¹³ Pa·m³/s
Limitation: part must withstand vacuum; large volumes slow response
Bubble Leak Testing
Immersion method: pressurize part with air/N₂; submerge in liquid; observe bubbles
Sensitivity: ~10⁻³ Pa·m³/s (bubble size ≥ 1 mm/s)
Minimum bubble rate: 1 bubble per minute is detectable
Liquid soap application: apply to exterior of pressurized part; watch for bubbles
Less sensitive than immersion; used for field checks
Standard: ASME V Article 10; ASTM E515
Tracer Gas Methods
Halogen (Refrigerant) Detection
Heated sensor (Krytox) reacts with halogen → current signal
Tracer: HFCs (R-134a, R-1234yf), HCFCs; pressurize part
Sensitivity: 10⁻⁵ to 10⁻⁷ Pa·m³/s
Applications: refrigeration systems, air conditioning
Hydrogen (H₂/N₂ mixture)
5% H₂ in N₂ — below LEL, non-flammable
Semiconductor sensor or thermal conductivity sensor
Sensitivity: 10⁻⁶ to 10⁻⁸ Pa·m³/s
Applications: automotive cooling circuits, heat exchangers
SF₆ (Sulfur Hexafluoride)
Electron capture detector; extremely sensitive to SF₆
Sensitivity: 10⁻⁸ Pa·m³/s; GWP = 23,500 → restricted under F-Gas Regulation EU 517/2014
Ultrasonic Leak Detection
Turbulent gas flow through leak generates broadband high-frequency noise (> 20 kHz)
Ultrasonic detector (Superphonic, SDT270) detects turbulent noise
Range: 20–100 kHz; directional probe; 1 m detection distance
Sensitivity: ~10⁻³ Pa·m³/s (gross leaks only); use to locate, then quantify with other method
Application: compressed air systems, steam traps, valves (during operation — no shutdown required)
Leak Classification (Acceptance Criteria)
Hermetic (electronics, medical implants):
MIL-STD-883 Method 1014: fine leak Q < 10⁻⁸ atm·cc/s per unit volume; gross < 1 bubble/30s
ASTM F2854: microelectronic packages
Pressure vessels (ASME):
No specific numeric leakage; proof test + visual typically
For gas service: hydrostatic test at 1.3× MAWP + NDE
Refrigeration (UL, EPA):
Annual leakage < 10% of charge (commercial A/C); 30% (industrial)
ASHRAE 15: emergency pressure relief required
Medical devices: varies; implants require hermetic (< 10⁻⁹ Pa·m³/s per ISO 20857)
Locating vs. Quantifying
Locating: helium sniffer, soap bubbles, ultrasonic — pinpoints where
Quantifying: pressure/vacuum decay, helium MSLD in accumulation mode — total rate
Combine both: locate with sniffer, quantify with MSLD chamber
Output
Provide: required leak rate [Pa·m³/s] per spec, recommended test method (with sensitivity), test pressure [bar], stabilization time [s], detection limit at given volume and test time, calibration standard required, applicable standard (ASTM E493/498/515, MIL-STD-883, ASME V), and pass/fail criterion.
