A V-notch tank is used to monitor liquid flow rate, and features a barrier between the inflow and outflow sides of the tank, with a highly accurate V-shaped notch cut out of the top of the barrier. An ultrasonic sensor positioned above the centre of the notch detects the height of the water flow, which is an extremely precise method of calculating flow rate – the faster the flow, the higher the water will be in relation to the lowest point of the notch.