Apparatus for measuring gas, in particular of the type suitable for measuring the flow of gas present and circulating within a pipe, comprising: - a containment casing provided with a first opening for the gas inlet inside it and a second opening for the gas outlet, - a first module which is housed inside said containment casing and which comprises at least one sensor for detecting a corresponding quantity of the gas flow which passes through and/or circulates in said apparatus, - a second module which is associated externally to said containment casing, - a single and only one processing and/or control unit which is defined by an external electronic processing and/or control unit which is housed in said second module and which is electronically connected to said at least one sensor to thus receive the detections made by said at least one sensor, - at least one memory unit: - which is external and independent of said second module so that said second module is removable and/or dissociable from said containment casing independently of said at least one memory unit - in which parameters useful for metrological purposes are stored in an unchangeable way, - and characterized in that: said at least one memory, that is external and independent of said second module, is electronically connected with the external electronic processing and/or control unit of said second module external to the containment casing, for receiving and storing information processed by said external electronic processing and/or control unit of said second module and relating to the operation of said apparatus, - the external electronic processing and/or control unit of the second module is configured so that, during normal operation of the apparatus, it sends the information relating to the operation of the apparatus to the at least one memory unit, thus allowing the storage of said information in said at least one memory unit. - a containment casing provided with a first opening for the gas inlet inside it and a second opening for the gas outlet, - a first module which is housed inside said containment casing and which comprises at least one sensor for detecting a corresponding quantity of the gas flow which passes through and/or circulates in said apparatus, - a second module which is associated externally to said containment casing, - a single and only one processing and/or control unit which is defined by an external electronic processing and/or control unit which is housed in said second module and which is electronically connected to said at least one sensor to thus receive the detections made by said at least one sensor, - at least one memory unit: - which is external and independent of said second module so that said second module is removable and/or dissociable from said containment casing independently of said at least one memory unit - in which parameters useful for metrological purposes are stored in an unchangeable way, - and characterized in that: said at least one memory, that is external and independent of said second module, is electronically connected with the external electronic processing and/or control unit of said second module external to the containment casing, for receiving and storing information processed by said external electronic processing and/or control unit of said second module and relating to the operation of said apparatus, - the external electronic processing and/or control unit of the second module is configured so that, during normal operation of the apparatus, it sends the information relating to the operation of the apparatus to the at least one memory unit, thus allowing the storage of said information in said at least one memory unit. - which is external and independent of said second module so that said second module is removable and/or dissociable from said containment casing independently of said at least one memory unit - in which parameters useful for metrological purposes are stored in an unchangeable way, - and characterized in that: said at least one memory, that is external and independent of said second module, is electronically connected with the external electronic processing and/or control unit of said second module external to the containment casing, for receiving and storing information processed by said external electronic processing and/or control unit of said second module and relating to the operation of said apparatus, - the external electronic processing and/or control unit of the second module is configured so that, during normal operation of the apparatus, it sends the information relating to the operation of the apparatus to the at least one memory unit, thus allowing the storage of said information in said at least one memory unit.