Which tool can provide a quick visual indication of refrigerant levels?

The sight glass is the tool that provides a quick visual indication of refrigerant levels in a refrigeration or air conditioning system. It allows technicians to easily observe the state and amount of refrigerant flowing through the system. When viewed, the sight glass shows whether the refrigerant is in liquid or gas form, and can provide additional insights into system performance, such as whether there might be a refrigerant leak or if the system is properly charged.

While the manifold gauge set can measure and display pressure in the system, it does not provide an immediate visual representation of refrigerant levels. A digital thermometer measures temperature but does not indicate refrigerant levels or states. The pressure relief valve is designed to release excess pressure and protect the system but does not provide information about refrigerant levels either. Therefore, the sight glass is the most effective tool for quickly assessing refrigerant levels visually.