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System vacuum test with manifold gauge — procedure and pass criteria

Likely causes

Step 1 — connect vacuum pump and run standard evacuation

Check: Tools: vacuum pump (≥2-stage, ≥6 CFM), manifold gauge, digital micron gauge, vacuum-rated hoses (short and large diameter preferred). Connect: manifold center hose → vacuum pump; micron gauge directly at the system service port or on a manifold tee. Open both low-side and high-side manifold valves. If refrigerant remains in system, recover it first — never use the vacuum pump for refrigerant (pump damage + atmospheric release violation).

What to do: Procedure: ① Start the pump → both manifold gauges drop quickly below 0 PSIG. ② Watch micron gauge. Measure time to first 1000 micron target (15 min to several hours depending on system). ③ Continue past 1000 down to 250–500 micron (typically another 30 min–2 h). ④ For large commercial/industrial systems use triple evacuation: vacuum → 0.5 bar nitrogen → re-vacuum → nitrogen → re-vacuum.

Step 2 — isolate vacuum and run decay test

Check: Procedure: ① At 250–500 micron stop the pump (close manifold center hose or disconnect pump). ② Leave only the micron gauge connected → observe 30 minutes. ③ Result: stable within 500 micron = pass (no leak, no moisture). 1000–2000 stable = residual moisture (more vacuum needed). 5000+ rising continuously = leak or hose/joint leaking. ④ Manifold hoses themselves can leak slightly — disconnect them and retest with only the micron gauge on the system.

What to do: Moisture remediation: restart pump, extend time (2–4 h) + warm suction line (30–40°C with mild heat gun). If insufficient, run triple evacuation. Leak remediation: pressurize system with 150 PSIG nitrogen + soap or leak-detection fluid → bubbles mark the leak point. Repair and re-evacuate. After pass: close manifold and start refrigerant charging (liquid charge for glide blends; vapor acceptable for pure refrigerants). After charging, tighten service caps firmly when removing the manifold and re-check with leak detector.

Field tip

Common mistakes: ① treating 30 inHg on the manifold scale as 'vacuum complete' (it is ~25,400 micron — moisture remains). ② Using long, narrow vacuum hoses (standard manifold hoses) → far longer to reach target. ③ Mounting micron gauge next to manifold instead of at the system → manifold hose leaks hidden, false pass. Solve all by using a digital micron gauge, direct system connection, and short large-bore hoses.

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