Method for determining the error of a flow meter arranged along a duct in which a base gas flows, said method comprising the operation of connecting to said duct a measuring system for measuring the flow rate comprising: - a feeding unit suited to supply a flow rate of a secondary gas under specific thermodynamic supply conditions and containing a tracer gas; - a feeding duct interposed between said feeding unit and said duct, suited to introduce said flow rate of secondary gas in said duct at the level of an injection point and comprising a nozzle provided with a converging portion that ends with a throat section having a predefined minimum surface area; - a measuring unit for measuring the flow rate of said secondary gas that flows in said feeding duct, comprising a sensor unit (12a, 12b) suited to detect said thermodynamic supply conditions and means for calculating said flow rate of secondary gas according to said thermodynamic supply conditions and to said predefined minimum surface area in the event that said secondary gas reaches sound speed at the level of said throat section; - an extraction duct communicating with said duct at the level of an extraction point arranged downstream of said injection point with respect to said flow direction, suited to extract from said duct a portion of the mixture comprising said base gas and said secondary gas; - a detector device suited to measure the concentration of said tracer gas in said mixture, connected to said extraction duct; - a processing unit operatively connected to said measuring unit and to said detector device and configured so as to calculate said flow rate of said base gas according to said concentration of said tracer gas and to said flow rate of said secondary gas; wherein said feeding unit is configured so that said secondary gas reaches sound speed at the level of said throat section; wherein said method further comprises the following operations: - measuring said parameter representative of the flow rate of said base gas by means of said measuring system; - comparing said parameter representative of the flow rate with the corresponding parameter measured by said flow meter in order to determine said error. - a feeding unit suited to supply a flow rate of a secondary gas under specific thermodynamic supply conditions and containing a tracer gas; - a feeding duct interposed between said feeding unit and said duct, suited to introduce said flow rate of secondary gas in said duct at the level of an injection point and comprising a nozzle provided with a converging portion that ends with a throat section having a predefined minimum surface area; - a measuring unit for measuring the flow rate of said secondary gas that flows in said feeding duct, comprising a sensor unit (12a, 12b) suited to detect said thermodynamic supply conditions and means for calculating said flow rate of secondary gas according to said thermodynamic supply conditions and to said predefined minimum surface area in the event that said secondary gas reaches sound speed at the level of said throat section; - an extraction duct communicating with said duct at the level of an extraction point arranged downstream of said injection point with respect to said flow direction, suited to extract from said duct a portion of the mixture comprising said base gas and said secondary gas; - a detector device suited to measure the concentration of said tracer gas in said mixture, connected to said extraction duct; - a processing unit operatively connected to said measuring unit and to said detector device and configured so as to calculate said flow rate of said base gas according to said concentration of said tracer gas and to said flow rate of said secondary gas; - measuring said parameter representative of the flow rate of said base gas by means of said measuring system; - comparing said parameter representative of the flow rate with the corresponding parameter measured by said flow meter in order to determine said error.