The absorbed power of the screw compressor will be at its optimum only when the working pressure ratio corresponds to the built-in volume ratio. The over and under compression losses can be visualized as additional areas on an indicator diagram . This results in an IE characteristic having a strongly defined peak. To the left of the peak, over-compression of the gas results in loss of efficiency, whilst to the right, there is under-compression with back flow of gas into the compression pocket when the discharge port is uncovered. Changing the size of the discharge port changes the position of the peak . A screw compressor should be chosen to have a volume ratio suitable for the application. Leakage also contributes towards efficiency loss, but friction effects are quite small. Capacity reduction of the screw compressor is effected by a sliding block covering part of the barrel wall, which permits gas to pass back to the suction, so varying the working stroke . It is usual for the sliding part of the barrel to adjust the size of the discharge port at the same time, so that the volume ratio is at least approximately maintained at part load. Many design variations and control methods exist. The single screw type will generally have two sliding valves; lifting valves are sometimes used instead of slides. Reduction down to 10% of maximum capacity is usual. The oil separation, cooling and filtering for a screw compressor add to the complexity of an otherwise simple machine. Liquid injection is sometimes used instead of an external oil cooler. Some commercial screw compressors have the oil-handling circuit built into the assembly.