Liquid flow sensing systems

A liquid flow sensing system for sensing the flow of liquid within a supply line of a metering pumping machine, the system including a magnetic member arranged for movement relative to a pneumatic switch operated by the presence or absence of a magnetic field, the magnetic member being contained within a sealed unit located within the supply line and the arrangement being such that, when the liquid flow rate falls below a predetermined level, the magnetic member moves downwardly from a first operative position into a second operative position.