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PT Chart — invalid below 0 PSIG (vacuum range), use micron/vacuum gauge instead

Likely causes

Why PT chart is meaningless in vacuum range

Check: The PT chart defines pressure↔temperature only when refrigerant is in liquid-vapor equilibrium (saturated). During evacuation the system has almost no refrigerant + residual moisture/air + oil vapor. Below 0 PSIG (atmospheric) is the vacuum region, shown on manifold gauges as inHg or negative PSI. Converting these pressures via PT chart yields meaningless temperatures.

What to do: Tools for evacuation: ① digital micron gauge (vacuum micrometer) — unit is micron (=0.001 mmHg). ② Standard target: hold ≤500 micron with 30-minute decay test. The inHg scale on manifold gauges is rough reference only (29 inHg ≈ 25,400 micron — not precise). Connect vacuum pump + micron gauge directly at system access port (service valve).

What pressure rise (decay) after evacuation means

Check: After stopping vacuum pump, isolate manifold and leave only the micron gauge connected → observe 30 minutes. ① Holds at 500 micron, stable = evacuation complete (no leak/moisture). ② Rises slowly to 1500–3000 micron then plateaus = residual moisture still evaporating. ③ Keeps rising past 5000 micron = leak or poor vacuum hose connection.

What to do: Case ②(moisture): restart vacuum pump for longer (2–4h) + warm the system (gently heat suction line to 30–40°C). Triple evacuation (vacuum → 0.5 bar nitrogen → re-vacuum, repeated 3×) is also effective. Case ③(leak): pressurize system with 150 PSIG nitrogen, leak-test with soap solution → repair → re-evacuate.

Field tip

Recommend zeroing the digital micron gauge against atmosphere before every use; the gauge itself needs annual calibration. The habit of trusting a manifold gauge reading 30 inHg as 'vacuum complete' lacks precision — that's still about 25,400 microns (significant moisture remaining).

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