Which device in a boiler indicates the level of water within the boiler?

The gauge glass is the device that indicates the level of water within a boiler. This is crucial for safe boiler operation, as it allows operators to visually assess the water level. Maintaining the appropriate water level is vital; too low, and there is a risk of the boiler running dry, which can lead to overheating and damage. Conversely, too high a water level can also cause problems, such as flooding the boiler and reducing efficiency or causing damage.

By providing a clear view of the water level, the gauge glass helps ensure that the boiler operates efficiently and safely. The other devices mentioned serve different purposes: the pressure gauge measures the pressure within the boiler, the thermometer tracks the temperature of the fluid, and the flow meter indicates the rate of water flow, but none directly show the water level as effectively as the gauge glass does.