Part 3 of this European Standard specifies the requirements and tests for the construction, performance, safety and conformity of flow computers (FCs) used to meet the metrological and technical requirements of a high accuracy volume conversion device. They are used to determine volume of fuel gases, including those of the first and second families according to EN 437. For the purpose of this European Standard, only flow computers that are intended to operate with ultrasonic meters according to ISO 17089-1 or gas turbine meters conforming to EN 12261 are considered. For the purpose of this European Standard only classification classes E2 and M1 are considered for the flow computer calculator. FCs are equipped with external separate transducers for pressure and temperature which may be approved separately. The provisions concerning pressure and temperature transducers are given in Annex B and C. Requirements and tests are given for energy calculator in EN 12405-2.