Which appliance is used to warn of low water levels in a boiler?

The appliance used to warn of low water levels in a boiler is the water sight glass. A water sight glass is a transparent tube or glass that allows operators to visually monitor the water level within the boiler. It provides a direct and clear indication of the water level, enabling operators to ensure that the water level remains within safe operating limits. If the water level drops below a certain point, it can signal potential issues such as low water conditions, which can lead to overheating or damage to the boiler.

Other options like the pressure gauge, temperature sensor, and flow meter serve different functions. The pressure gauge measures the pressure of the steam or water in the boiler, which is critical for operation but does not directly indicate water level. The temperature sensor measures the temperature of the water or steam, which is also important for assessing boiler performance but does not provide information about water volume. The flow meter measures the amount of water entering or exiting the boiler, which can indicate flow rates but does not help in directly monitoring water level.