Amongst positive displacement type, reciprocating pump is commonly used. This system can deliver very high pressure since the cylinder piston surface does not allow back flow; whence the name. However, capacity, which is defined as mass flow rate of the device per minute, is low because of limited size of the cylinder. Plunger-barrel arrangement is also a reciprocating system like piston cylinder. It is used for diesel fuel pumps as quantity of fuel required is very low but discharge pressure is quite high. Plunger has length comparatively large with respect to its diameter; cylinder is called barrel. Other techniques include gear, lobe, vane, and screw pumps, which are rotary devices, in which back flow does not occur. The rotary type gear pump has wide application in industry mainly for raising pressure of lubricating oil. This pump can be used for extremely large pressure ratio. For this reason, these are used extensively in hydraulic controls.