What device is used to indicate the water level in a steam boiler?

The gauge glass is the correct device for indicating the water level in a steam boiler because it provides a direct visual reference for operators to monitor the water level. It consists of a transparent tube typically mounted on the side of the boiler, filled with water that matches the water level in the boiler itself. This allows for immediate and easy assessment of the water level, which is crucial for safe boiler operation since the water level must be maintained within specific limits to prevent overheating or damage to the boiler.

In contrast, the pressure gauge measures the pressure of the steam within the boiler, which does not provide information about the actual water level. The temperature sensor, while also important for monitoring the system, is used to measure the temperature of the water or the steam, rather than the water level itself. A flow meter, on the other hand, measures the rate of flow of water or steam, but it does not indicate the static water level present in the boiler. Therefore, the gauge glass is specifically suited to the task of indicating the water level, making it the correct answer.