Document Metadata & Authorization
- Document ID:
SOP-LEAK-2021-18(Rev 2.0) - Lead Specialist: Kadeem Manning (Tier II Vacuum Technician)
- Compliance Standards: DIN EN 1779 / ISO 20485 / ALCOA+ Data Integrity
- Target Leak Detectors: Leybold PHOENIX, Agilent HLD, Pfeiffer ASM 340
- Official PDF: Download Official SOP PDF
1. Operational Scope & Quantitative Thresholds
This standard operating procedure defines the quantitative helium leak detection protocols for high-vacuum chambers, cryogenic vessels, and gas delivery manifolds.
| Vacuum Boundary Section | Testing Method | Maximum Allowable Leak Rate ($Q_L$) | Required Corrective Action on Failure |
|---|---|---|---|
| UHV Chamber Flanges (CF/ConFlat) | Method A1 (Local Spray) | $< 1.0 \times 10^{-9}\text{ mbar}\cdot\text{L/s}$ | Replace OFHC copper gasket; re-torque in star pattern |
| HV Processing Bay (KF Viton) | Method A1 (Local Spray) | $< 1.0 \times 10^{-7}\text{ mbar}\cdot\text{L/s}$ | Clean Viton O-ring, inspect for micro-cracks, re-grease LVO 100 |
| Foreline / Roughing Piping | Method A1 Spray / B1 Sniff | $< 1.0 \times 10^{-5}\text{ mbar}\cdot\text{L/s}$ | Retighten KF clamps, inspect centered ring assembly |
| Process Gas Manifolds | Method B1 Sniffer (1.2 bar He) | $< 1.0 \times 10^{-6}\text{ mbar}\cdot\text{L/s}$ | Re-torque VCR fitting; replace silver-plated nickel gasket |
2. Systematic Local Spray Protocol (Method A1)
[Helium Probe Nozzle] ──(Bottom-to-Top Sequence)──> [Flange Joint Under Test]
│ (Helium Infiltration)
▼
[Vacuum Chamber] ──> [Turbomolecular Pump] ──> [Mass Spectrometer Detector (Mass 4)]Step-by-Step Execution:
- Calibrated Baseline Verification:
- Connect internal/external calibrated leak reference standard ($Q_{ref} \approx 2.4 \times 10^{-7}\text{ mbar}\cdot\text{L/s}$).
- Allow 15-minute filament stabilization time.
- Record baseline zero signal: verify background noise $< 5.0 \times 10^{-10}\text{ mbar}\cdot\text{L/s}$.
- Sequential Spraying Sequence:
- Always commence spraying at the lowest physical elevation of the vacuum vessel and work upwards.
- Rationale: Helium is lighter than air ($M = 4\text{ g/mol}$) and rises. Spraying top flanges first creates ambient clouds that trigger false leaks on lower joints.
- Dwell Time Calculation:
- Maintain probe dwell time equal to $3 \times \tau$ (system time constant) where: $$\tau = \frac{V_{chamber}}{S_{eff}}$$
- Typical dwell: 5 to 8 seconds per flange circumference segment.
3. Hood Enclosure Total Integral Test (Method A2)
When individual joint spraying passes but base pressure stalls above $1.0 \times 10^{-6}\text{ mbar}$:
- Enclose entire vacuum vessel in an impermeable polyethylene shroud.
- Evacuate shroud and backfill with $100%$ helium gas at $1.0\text{ atm}$.
- Record peak signal deflection over 10 minutes. If signal rises $> 5.0 \times 10^{-8}\text{ mbar}\cdot\text{L/s}$, a distributed through-wall porosity or virtual weld leak is present.