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A method and system for determining change in a fluid's flow rate within a line. The pressure variation in a second fluid, separated from the first by a pumping membrane, is measured in response to energy applied in a time-varying manner to the second fluid. From the response of the second fluid to

A method and system for determining change in a fluid's flow rate within a line. The pressure variation in a second fluid, separated from the first by a pumping membrane, is measured in response to energy applied in a time-varying manner to the second fluid. From the response of the second fluid to the applied energy, changes in the flow rate of the first fluid are determined.

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1. A computer program product for determining a change in a first fluid's flow rate within a line, the computer program product comprising a computer usable medium having computer readable program code thereon, the computer readable program code comprising:program code for applying a time varying am

1. A computer program product for determining a change in a first fluid's flow rate within a line, the computer program product comprising a computer usable medium having computer readable program code thereon, the computer readable program code comprising:program code for applying a time varying amount of energy to a second fluid separated from the first fluid by a membrane;program code for measuring a pressure of the second fluid in response to the applied energy; andprogram code for determining a change in the first fluid's flow rate based at least on the pressure of the second fluid.2. A computer program product as defined by claim 1, wherein the second fluid is a gas.3. A computer program product as defined by claim 1, wherein the second fluid is air.4. A computer program product as defined by claim 1, wherein the first fluid is blood.5. A computer program product as defined by claim 1, wherein the first fluid is dialysis fluid.6. A computer program product as defined by claim 1, wherein the program code for determining a change in the first fluid's flow rate includes:program code for determining a value corresponding to the derivative with respect to a timing period of the pressure of the second fluid creating a derivative value;program code for determining a value corresponding to the magnitude of the derivative value creating a magnitude derivative;program code for low pass filtering the magnitude derivative creating a low pass output; andprogram code for comparing the low pass output to a threshold value for determining a change in the first fluid's flow rate.7. A computer program product as defined by claim 6, the computer program product further comprising:program code for taking the difference between the pressure of the second fluid and a target value; andprogram code for varying an inlet valve in response to the difference between the pressure of the second fluid and the target value for changing the pressure of the second fluid toward the target value.8. A computer program product method as defined by claim 7, wherein the target value comprises a time varying component having an amplitude and a DC component, the amplitude of the time varying component being less than the DC component.