What device is used to visually detect vapor in the liquid line of a refrigerant?

A sight glass is the correct device for visually detecting vapor in the liquid line of a refrigerant. This tool is typically installed in the liquid line after the condenser, allowing service technicians to observe the condition of the refrigerant. When the refrigerant is in a fully liquid state, it appears clear in the sight glass. However, if there is vapor present, it will cause bubbles or may obscure the view, indicating that there is not enough liquid refrigerant in the system, which could lead to compressor damage or operational inefficiencies.

The other devices mentioned do not serve this purpose. A pressure gauge measures the pressure within the system but does not provide visual feedback about the phase of the refrigerant. A thermometer measures temperature but cannot indicate whether there is vapor or liquid present. A manometer measures pressure in various contexts but is not specifically designed to evaluate refrigerant flow or condition visually. Hence, the use of a sight glass provides direct visual confirmation of the refrigerant's state, making it an essential component for proper diagnostics and maintenance in refrigeration and air conditioning systems.