A method of maintaining a predefined ratio of flow rates in first and second channels connected to a first pump and a second pump, respectively, of an extracorporeal circuit, comprising: connecting the first and second channels temporarily to create a continuous flow between them; measuring a static pressure of a channel carrying the continuous flow temporarily generated in the connecting; adjusting calibration data of one or more respective pumps, that include the first pump and the second pump, that generate at least one of the second flows responsively to a result of the measuring, wherein the connecting, measuring, and adjusting are done automatically by a programmable controller, the method further comprising adjusting a speed of said one or more respective pumps in response to said calibration data, and limiting low frequency pressure fluctuations in said continuous flow, wherein said limiting includes selecting the one or more respective pumps whose pulsation frequencies at a rate of said continuous flow produce beats due to superposition of the pulsations that are either multiple times shorter than, or multiple times longer than a period of said measuring, the measuring including sampling said static pressure multiple times over said period of said measuring, or wherein said limiting includes adjusting the one or more respective pumps to a rate of said continuous flow that produces beats due to superposition of the pulsations thereof that are either multiple times shorter than, or multiple times longer than a period of said measuring, the measuring including sampling said static pressure multiple times over said period of said measuring, the one or more respective pumps having different ratios of flow rate to pulsation frequencies. connecting the first and second channels temporarily to create a continuous flow between them; measuring a static pressure of a channel carrying the continuous flow temporarily generated in the connecting; adjusting calibration data of one or more respective pumps, that include the first pump and the second pump, that generate at least one of the second flows responsively to a result of the measuring, wherein the connecting, measuring, and adjusting are done automatically by a programmable controller, the method further comprising adjusting a speed of said one or more respective pumps in response to said calibration data, and limiting low frequency pressure fluctuations in said continuous flow, wherein said limiting includes selecting the one or more respective pumps whose pulsation frequencies at a rate of said continuous flow produce beats due to superposition of the pulsations that are either multiple times shorter than, or multiple times longer than a period of said measuring, the measuring including sampling said static pressure multiple times over said period of said measuring, or wherein said limiting includes adjusting the one or more respective pumps to a rate of said continuous flow that produces beats due to superposition of the pulsations thereof that are either multiple times shorter than, or multiple times longer than a period of said measuring, the measuring including sampling said static pressure multiple times over said period of said measuring, the one or more respective pumps having different ratios of flow rate to pulsation frequencies.