Abstract: A liquid level detecting device for vapour generators is characterized by a stack tube for connection to the vapour generator and preferably two separate side tubes in the form of loops each having an upper vapour inlet to the stack tube and a lower outlet disposed respectively at a low water level and a high water level. In use the loops fill with cooled condensate. When the water level moves past either of the cooling tube outlets the condition of that tube is disturbed, and the cooled water is displaced from one portion of the loop to another and steam occupies the previously cooled portion of the tube. The temperature change is sensed and used to give an alarm.