Before revision: Originally the gas flow computer was a mechanical ( 1920's technology) or later a pneumatic or hydraulic computing module( 1940's technology used to the early 1990's but still available from a number of suppliers), subsequently superceded in most applications with an electronic module, as the primary elements switched from transmitting the measured variables from pneumatic or hydraulic pressure signals to electric current as explosion proof (1960's technology to present)) and then intrinsic safe (1970's to present) transmitters ( driven by low power transistor applications ) became available, that simply provided a dedicated gas flow computer function.
After revision: Originally the gas flow computer was a mechanical ( 1920s technology) or later a pneumatic or hydraulic computing module( 1940s technology used to the early 1990s but still available from a number of suppliers), subsequently superseded in most applications by an electronic module, as the primary elements switched from transmitting the measured variables from pneumatic or hydraulic pressure signals to electric current as explosion-proof (1960s technology to present)) and then intrinsically-safe (1970s to present) transmitters ( with low power transistor circuitry ) became available, that simply provided a dedicated gas flow computer function.