The error output from a control valve can come from a variety of sources: uncontrolled input pressure, uncontrolled input flow rate, inaccurate orifice size or turbulent flow through the system which causes irregularity of flow rates through the system. Inaccurate orifice size can result from dirt or scale in the system or from the operator having insufficient ability to set the orifice size. A common bathroom faucet, for example, is very difficult to set to an exact size due to imprecise setting mechanisms and also varying amounts of washer compression.
While progress has been made in creating valves with accurate orifice sizes, many control valves still require electronic control loops to ensure outputs are within desired limits. However, use of these electronic controls is not always practical. They can be expensive. They need regulated electricity which is not always available and which is often inconvenient to connect with. Electronic controls can also be vulnerable to damage, which not only reduces their lifespan, it reduces their reliability. They can require training to use as well. Particularly in foreign, underdeveloped markets or for some home markets, these issues can prohibit use of electronic controls for some applications.