What device in a steam gauge keeps the tube filled with water?

The device that keeps the tube of a steam gauge filled with water is indeed a siphon. In steam boiler applications, a siphon is used to connect the pressure gauge to the boiler while preventing high-temperature steam from directly entering the gauge. This is crucial because if steam were to enter the gauge directly, it could produce inaccurate readings and potentially damage the gauge due to the high temperatures and pressures involved.

The siphon functions by utilizing the principle of water trap, which effectively uses the hydrostatic pressure to keep a column of water in the gauge tube. This column of water serves as a barrier that protects the gauge from the steam while allowing it to measure the pressure accurately.

Other devices like a water pump or pressure regulator are not directly responsible for maintaining the water level in the steam gauge tube, and a float valve is typically used in applications for controlling water levels rather than in conjunction with gauges meant for pressure measurement.