Method for determining the validity of a measured concentration of urea in a fluid, the method comprising: measuring a temperature of a fluid comprising urea; and measuring a concentration of urea in said fluid; characterized in that the method further comprises: after the step of measuring the temperature, determining an interval of validity of a concentration of urea in said fluid for a given predetermined physical phase of the fluid at the measured temperature; and determining whether the measured concentration of urea falls in the determined interval of validity for said predetermined physical phase; wherein the predetermined physical phase is the liquid phase; wherein the fluid is a urea solution comprising deionized water, in particular diesel exhaust fluid (DEF); wherein determining the interval of validity comprises using a phase diagram for said urea solution, comprising information regarding the concentration of urea at a given temperature in the solid and liquid phases; wherein the phase diagram comprises a phase boundary representing where the transition between the liquid and solid phases of said urea solution occurs; and wherein the interval of validity is defined by a minimum and maximum urea concentration in the liquid phase determined by using the phase boundary of the phase diagram at a given temperature. measuring a temperature of a fluid comprising urea; and measuring a concentration of urea in said fluid; characterized in that the method further comprises: after the step of measuring the temperature, determining an interval of validity of a concentration of urea in said fluid for a given predetermined physical phase of the fluid at the measured temperature; and determining whether the measured concentration of urea falls in the determined interval of validity for said predetermined physical phase; wherein the predetermined physical phase is the liquid phase; wherein the fluid is a urea solution comprising deionized water, in particular diesel exhaust fluid (DEF); wherein determining the interval of validity comprises using a phase diagram for said urea solution, comprising information regarding the concentration of urea at a given temperature in the solid and liquid phases; wherein the phase diagram comprises a phase boundary representing where the transition between the liquid and solid phases of said urea solution occurs; and wherein the interval of validity is defined by a minimum and maximum urea concentration in the liquid phase determined by using the phase boundary of the phase diagram at a given temperature. after the step of measuring the temperature, determining an interval of validity of a concentration of urea in said fluid for a given predetermined physical phase of the fluid at the measured temperature; and determining whether the measured concentration of urea falls in the determined interval of validity for said predetermined physical phase; wherein the predetermined physical phase is the liquid phase; wherein the fluid is a urea solution comprising deionized water, in particular diesel exhaust fluid (DEF); wherein determining the interval of validity comprises using a phase diagram for said urea solution, comprising information regarding the concentration of urea at a given temperature in the solid and liquid phases; wherein the phase diagram comprises a phase boundary representing where the transition between the liquid and solid phases of said urea solution occurs; and wherein the interval of validity is defined by a minimum and maximum urea concentration in the liquid phase determined by using the phase boundary of the phase diagram at a given temperature. <EOS>