Which instrument is used to monitor the water level in a boiler?

The instrument specifically designed to monitor the water level in a boiler is the water level gauge. This device provides a visual indication of the water level within the boiler, ensuring that the operator can easily see if the water level is within the safe operating range. Maintaining the proper water level is crucial for the safe and efficient operation of a boiler, as low water levels can lead to overheating and potential damage, while excessively high levels can lead to flooding.

A thermometer measures temperature, which is important for understanding the thermal conditions within the boiler but does not provide information on water levels. The pressure gauge measures the pressure of the steam or water within the boiler system, which is also critical for operation but does not indicate how much water is in the boiler. A pump indicator shows the status of pumps in terms of operation, flow, or pressure, but it does not directly relate to monitoring the water level in the boiler. Thus, the water level gauge is the most relevant and accurate instrument for this specific task.