Which device in a steam heating system enables observation of the system's operation without direct contact?

The sight glass is an essential component in a steam heating system that allows for the visual observation of the system's operation without the need for direct contact. This device provides an indication of the liquid level within a boiler or steam system. By peering into the sight glass, one can monitor the water level and ensure it remains within optimal ranges, which is crucial for safe and efficient operation. Maintaining appropriate water levels is key in preventing issues such as overheating or system failure.

The other devices mentioned serve different purposes within the heating system. A pressure gauge measures the pressure of the steam but does not provide a visual indication of the system's operation. A flow meter indicates the volume of steam or water flowing through the system but does not visually display any operational characteristics. A steam trap is designed to remove condensate and non-condensable gases but does not allow for observational access to the system's conditions. The sight glass stands out because it directly aids operators in visual assessments, which is critical for effective system management.