Blood treatment machine comprising: - at least a filtration unit with a first compartment for the circulation of blood and a second compartment for the circulation of the treatment fluid, said first and second compartments being separated by interposing at least a semi-permeable membrane; - means for changing the conductivity of the treatment fluid upstream from the filtration unit; - at least a first and second sensor installed upstream and downstream from the filtration unit for measuring, respectively, the conductivity of the process fluid upstream and downstream from the unit; - a control unit governing said devices in order to change the conductivity of the process fluid and which is able to receive conductivity signals from the aforementioned first and second sensors, wherein the control unit carries out a method of determining conductivity of a treatment fluid downstream from the filtration unit, said method comprising the steps of: - creating a flow of treatment fluid through the second compartment of the filtration unit; - imposing, for a predetermined time interval, a change in the conductivity of the treatment fluid at the inlet of the filtration unit in order thereby to cause an induced conductivity change in the fluid at the outlet of said filtration unit; - measuring over time a predetermined number of conductivity values Cdo downstream from the filtration unit and belonging to a conductivity curve of the treatment fluid downstream from the filtration unit, characterized in that the method further comprises the steps of: - defining at least one interpolating mathematical function for the purpose of estimating the pattern of the conductivity curve Cdo downstream from the filtration unit in an interval of time after the occurrence of the induced conductivity change; - determining a characteristic measuring time tcalc clr; the determination of the characteristic measuring time tcalc clr comprises the following sub steps: - estimation of an intermediate time tA clr representative of a transient due to the induced conductivity change in the fluid; - correction of the intermediate time tA clr through the addition of a second time term tcbf, which is a function of blood flow; - calculating the value of the interpolating mathematical function at said characteristic measuring time tcalc clr, said value representing the conductivity value Cdo2 of the process fluid downstream from the filtration unit after the induced conductivity change. - at least a filtration unit with a first compartment for the circulation of blood and a second compartment for the circulation of the treatment fluid, said first and second compartments being separated by interposing at least a semi-permeable membrane; - means for changing the conductivity of the treatment fluid upstream from the filtration unit; - at least a first and second sensor installed upstream and downstream from the filtration unit for measuring, respectively, the conductivity of the process fluid upstream and downstream from the unit; - a control unit governing said devices in order to change the conductivity of the process fluid and which is able to receive conductivity signals from the aforementioned first and second sensors, wherein the control unit carries out a method of determining conductivity of a treatment fluid downstream from the filtration unit, said method comprising the steps of: - creating a flow of treatment fluid through the second compartment of the filtration unit; - imposing, for a predetermined time interval, a change in the conductivity of the treatment fluid at the inlet of the filtration unit in order thereby to cause an induced conductivity change in the fluid at the outlet of said filtration unit; - measuring over time a predetermined number of conductivity values Cdo downstream from the filtration unit and belonging to a conductivity curve of the treatment fluid downstream from the filtration unit, - defining at least one interpolating mathematical function for the purpose of estimating the pattern of the conductivity curve Cdo downstream from the filtration unit in an interval of time after the occurrence of the induced conductivity change; - determining a characteristic measuring time tcalc clr; the determination of the characteristic measuring time tcalc clr comprises the following sub steps: - estimation of an intermediate time tA clr representative of a transient due to the induced conductivity change in the fluid; - correction of the intermediate time tA clr through the addition of a second time term tcbf, which is a function of blood flow; - estimation of an intermediate time tA clr representative of a transient due to the induced conductivity change in the fluid; - correction of the intermediate time tA clr through the addition of a second time term tcbf, which is a function of blood flow; - calculating the value of the interpolating mathematical function at said characteristic measuring time tcalc clr, said value representing the conductivity value Cdo2 of the process fluid downstream from the filtration unit after the induced conductivity change.