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ISO 23551-3:2005 specifies the safety, construction and performance requirements for gas/air ratio controls, pneumatic type, for inlet pressures up to and including 50 kPa, of a nominal connection size up to and including DN 250, intended for use with gas appliances using fuel gases as natural gas, manufactures gas or liquefied petroleum gas (LPG). It also describes the test procedures for evaluating these requirements and specifies information necessary for installation and use. It is applicable to gas/air ratio controls for gas-burning appliances that can be tested independently of the appliance. It is applicable to gas/air ratio controls which function by controlling a gas pressure (or differential pressure) output in response to an air pressure (or differential pressure) and to a furnace back pressure signal input; but gas/air ratio controls which change the air pressure in response to the gas pressure are not excluded. It is not applicable to mechanically linked valves and electronic systems.