A sight glass can show two different clues: what the refrigerant looks like as it passes the window and the colour of a moisture-sensitive element. A 313A study problem that says “bubbles” is asking a different question from one that says “yellow indicator.” Keep those observations separate before choosing a next check. Danfoss's SG/SGP data sheet provides a concrete model example; colour thresholds and installation details belong to the actual device and refrigerant.
What does the indicator colour tell you?
For Danfoss SG/SGP indicators, the sensitive element changes from green toward yellow as moisture rises. Its data sheet says that a persistent yellow reading calls for moisture-removal measures and that the indicated moisture limits vary with refrigerant and temperature. A yellow element is therefore a moisture clue for that indicator, not a direct measure of charge quantity. At start-up it may temporarily appear yellow after exposure to air humidity; interpret it in the operating context and according to the model's instructions.
Danfoss describes filter driers as moisture-control components. The bi-flow drier guide explains why direction also matters on some heat-pump circuits.
Do bubbles prove that the system is undercharged?
No. Bubbles describe visible vapour or an unsettled liquid stream at the window; they do not by themselves identify why it formed. Danfoss's walk-in cooler guide lists several possible explanations, including a pressure drop through a filter drier, refrigerant shortage and inadequate subcooling. Treat that list as hypotheses to distinguish, not three interchangeable diagnoses. Check the diagram, operating condition and manufacturer procedure before interpreting a single observation.
For example, if a hypothetical liquid-line sight glass bubbles while the moisture element remains green, the exercise has described a flow-state clue without a yellow moisture warning. It has not supplied enough information to declare a leak or charge level. The undercharge-versus-restriction guide shows why other observations matter.
What is a safe study sequence for the two clues?
Write down the observation exactly: colour, bubbles, both, or neither. Identify the sight glass location and operating state. Then list what additional evidence would separate the possible causes—such as relevant pressure, temperature and subcooling information in a properly specified problem. Do not turn a single window observation into a refrigerant-addition instruction.
Real equipment work requires the model instructions and authorized personnel. Follow CCOHS hazardous-energy guidance for tasks that expose electrical or moving parts. The exam guide explains the current exam basis; the free sample is original, unofficial draft practice awaiting certified-mechanic review and cannot predict a pass.
Sources
- Danfoss: SG/SGP sight glass data sheetMoisture colour, temperature/refrigerant dependence and start-up caveat; July 2025 revision, checked 2026-10-10
- Danfoss: walk-in coolers, filter driers and sight glassesMultiple possible explanations for visible bubbles; checked 2026-10-10
- CCOHS: Lockout/TagoutHazardous-energy control context; checked 2026-10-10
- Red Seal: Refrigeration and Air Conditioning MechanicCurrent versus newer standard status; checked 2026-10-10
Examen Studio is independent and not affiliated with the Red Seal Program or Skilled Trades Ontario. This article is a study aid; on the job, follow current codes, regulations and manufacturer instructions.