A Diagnostic Sequence for Residual Gas Analysis

The value of residual gas analysis (RGA) is turning "is the vacuum OK?" from a judgment into data. But to get reliable conclusions, the diagnostic sequence matters: confirm the instrument itself, then the system background, then locate the anomaly.

Step one is instrument self-check: filament and ionizer state, quadrupole tuning and mass calibration within tolerance, and detector (Faraday cup or electron multiplier) sensitivity drift. Record the instrument's own background spectrum first so its contribution is not misread as a system problem.

Step two reads the system background. Watch the signature peaks: water at mass 18, nitrogen at 28, oxygen at 32. High water suggests outgassing or insufficient bake-out; N₂/O₂ near the air ratio strongly indicates a leak; hydrocarbon clusters (41, 43, 55, 57…) often point to oil contamination or backstreaming.

Step three excludes leak vs. outgassing vs. contamination one by one. Helium tracer testing locates micron-scale leaks; baking while watching the water peak drop indicates outgassing; comparing spectra before/after replacing parts locates contamination. Keep a spectrum log for side-by-side comparison.

Finally, map conclusions to actions: seal repair, bake-out, seal replacement, or pump oil change. RGA's role is continuous monitoring, not one-shot diagnosis — build a baseline, compare regularly, and catch anomalies before they grow.