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Reading the oil sight glass — normal vs faulty patterns and stage-by-stage interpretation

Likely causes

Normal patterns — sight glass appearance by operating stage

Check: Observation points (reciprocating reference): ① At rest — oil settled, sight glass at 1/2–3/4 level, no foam. ② Immediately after start (first 30 s) — heavy foaming briefly (suction pressure drop releases dissolved refrigerant from oil). Normal. ③ Stable operation (5–10 min after start) — 1/2–3/4 level + slight micro-foam (small bubbles, slow rise). By compressor type — many scroll/rotary types are hermetic without a sight glass. Inverter systems — foam pattern may shift with operating frequency.

What to do: If the pattern is normal, no further action. Log a sight-glass photo for each operating stage (stop, start, stable) on the R-Pro inspection card to serve as a baseline for future comparison. To avoid camera glare, illuminate from the side with a flashlight and shoot from the front — capture both bubble shape and oil color clearly.

Faulty patterns — what abnormal foam, color, and level mean

Check: Mapping abnormal patterns: ① Heavy foaming and cloudy glass even at stable run — excess refrigerant dissolved in oil (low load + flooded back) or large pressure swings from leaks. ② Thick milky, viscous foam — moisture in oil (poor evacuation, leak letting air in). Immediate evacuation and moisture removal needed. ③ Color — normal is golden / light amber. Dark brown = oxidation progressing; black = suspect burnout; milky/cloudy white = moisture. ④ Level — very low (<1/4): oil-return failure or leak. Very high (>3/4 to over the top): heavy refrigerant dissolution (possible flooded back).

What to do: Action: ① Heavy foaming + system pressure swings — apply the diagnosis and procedures from superheat-flooded-emergency or suction-high-flooded. ② Milky foam + moisture — recover → evacuate (≤500 micron + 30-min decay) → install fresh filter-drier + add a suction-line drier → fresh oil (discard old oil, match type/viscosity) → recharge refrigerant. ③ Dark color + acid suspected — apply the compressor-oil-acid procedure. ④ Very low level — apply pump-down from oil-return-recovery, or fix leaks then top up. ⑤ Very high level — check charge (possible overcharge) and look for flooded-back symptoms. In all cases, save sight-glass photos with time labels in the R-Pro inspection card.

Field tip

Sight glass reading is not a one-second glance — observe for at least one minute. How foam changes over time (rising, falling, steady) is the diagnostic signal. Capture 15–30-second videos at each operating stage and attach them in the R-Pro inspection card; they are very useful for later expert consultation. Record all three stages: stop, start, stable.

All 565 cases work offline in R-Pro, with error codes for 62 brands and P-T data for 108 refrigerants.

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