Which device is crucial for ensuring a boiler does not operate without sufficient water?

The low-water cutoff is essential for ensuring that a boiler does not operate without sufficient water. This device automatically detects the water level within the boiler. If the water drops below a predetermined level, the low-water cutoff will shut down the boiler to prevent it from running dry. Operating a boiler without adequate water can lead to severe damage, including overheating, and can potentially cause a boiler explosion.

The other devices listed serve different functions. A pressure gauge monitors the pressure within the boiler system but does not protect against low water levels. A blowoff valve is used for discharging water and sediment from the boiler, while an air separator removes air from the boiler water, but neither of these directly addresses the critical concern of low water levels. Thus, the low-water cutoff is specifically designed to prevent unsafe operating conditions that arise from insufficient water in the boiler.