What provides a visual indication of water level in a boiler?

The water sight glass is a vital component in a boiler system as it offers a clear visual indication of the water level within the boiler. This transparency allows operators to easily monitor the level of water, ensuring that it remains within safe operational limits. Maintaining the correct water level is crucial for the safe functioning of a boiler since too low a level can lead to overheating and potential damage, while too high a level can result in water carryover into the steam system.

In contrast, the pressure gauge measures the pressure of the steam within the boiler and does not provide information about the water level. The temperature sensor is designed to monitor the temperature of the steam or water but also does not indicate the water level. Lastly, a flow meter measures the flow rate of water or steam through the system, providing insight into the quantity of fluid moving, rather than the specific level of water in the boiler.

Therefore, the water sight glass is the only instrument among the choices that visually and directly represents the water level, making it essential for safe boiler operation.