Relative pump calibration for ultrafiltration control in a dialysis apparatus

A dialysis apparatus, comprising:

• a dialyzer;
• a dialysis fluid distribution system connected for fluid communication with the dialyzer and comprising a first pump and a second pump, the dialysis fluid distribution system being operable to selectively establish a main flow path that extends between the first and second pumps via the dialyzer, and a bypass flow path that bypasses the dialyzer and extends between the first and second pumps;
• and
• a control unit electrically connected to the dialysis fluid distribution system and configured to control a respective frequency of the first and second pumps, wherein the first and second pumps are configured to generate a respective flow rate by repeatedly discharging, at the respective frequency, a respective stroke volume of dialysis fluid;
• wherein the control unit is configured to, in a calibration mode,
• (i) operate the dialysis fluid distribution system to establish the bypass flow path,
• (ii) control the first pump to operate at a first calibration frequency and the second pump to operate at a second calibration frequency so as to balance the flow rates generated by the first and second pumps, and
• (iii) determine relative calibration data that represents a relation between the stroke volumes of the first and second pumps based on the first and second calibration frequencies;
• and
• wherein the control unit is configured to, in a treatment mode,
• (i) operate the dialysis fluid distribution system to establish the main flow path,
• (ii) assign a predefined nominal stroke volume to the first pump,
• (iii) set a respective treatment frequency of the first pump to generate, based on the predefined nominal stroke volume of the first pump, a first flow rate of dialysis fluid,
• (iv) set a respective treatment frequency of the second pump to generate a second flow rate of dialysis fluid that differs from the first flow rate by a selected ultrafiltration rate, and
• (v) control the first and second pumps, based on the relative calibration data, to operate at the respective treatment frequencies of the first and second pumps to generate the selected ultrafiltration rate in the dialyzer.