Patent ID: 7815809

Claim:
A method for determining a conductivity of a treatment fluid downstream from a filtration unit in a blood processing machine, said unit comprising a first compartment for the circulation of blood and a second compartment for the circulation of the treatment fluid, said second compartment being separated from the first compartment by interposing at least a semi-permeable membrane; said method comprising the steps of: creating a flow of blood through the first compartment of the filtration unit; 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 an inlet of the filtration unit to cause an induced conductivity change in the fluid at an outlet of said filtration unit; measuring a predetermined number of conductivity values Cdo downstream from the filtration unit, said conductivity values Cdo defining a downstream conductivity curve; defining at least one interpolating mathematical function for estimating a pattern of said downstream conductivity curve in an interval of time after causing the induced conductivity change; wherein the mathematical function has the form: Cdo _ln cir =Cdi step,mean −sign*exp( c (1)* t+c (2)) where c(1) and c(2) are coefficients derived from the least squares interpolation of the mathematical function and Cdi step,mean =an inlet conductivity value after the change in conductivity; determining a characteristic measuring time tCalc clr , wherein said determination of said characteristic measuring time tCalc clr comprises the sub steps of estimating an intermediate time tA clr representative of a transient due to the induced conductivity change in the fluid, and correcting the intermediate time tA clr through an addition of a second time term tcbf, which is a function of the blood flow; and calculating by means of a control unit a value of the interpolating mathematical function at said characteristic measuring time tCalc clr , said value representing a conductivity value Cdo2 of the fluid downstream from the filtration unit after the induced conductivity change, wherein the determination of the characteristic measuring time tcalc clr comprises the following sub steps: estimation of the intermediate time tA clr representative of a transient due to the induced conductivity change in the fluid, said time tA clr depending on at least a filtration unit volume, blood and treatment fluid flows, and the change in the conductivity of the treatment fluid at the inlet of the filtration unit, and correction of the intermediate time tA clr through the addition of the second time term tcbf, which is a function of blood flow rate, tcalc clr being defined as tcalc clr =tA clr +tcbf, the Cdo2 value being the value of the equation at t=tcalc clr .