Abstract: An automatic drainage system is described for draining one of two non-miscible liquids which are of different densities from at least one tank (1) via a drain valve (3,10,60) including a system for monitoring the relative proportions of the non-miscible liquids in the liquid flow along the drain line (2). According to the invention the monitoring system includes a recirculating path (2A, 26 FIGS. 1 to 5: 2A FIG. 6) for liquid from the drain line (2) back to the tank (1), a pump (5) in the recirculating path for moving liquid around the path, the pump being arranged to maintain a substantially constant liquid flow around the path substantially independently of the head of liquid in the tank, a detector (7) in the path having at least one sensing head (8) for sensing the relative proportions of the two non-misible liquids flowing in said path, and control means (9) for controlling the system in dependence upon said relative proportions.