7. Dialysate Preparation
Using the dialysate flow rate, sodium and bicarbonate profiles,
sodium button and the proportioning ratios, the SBC determines
the associated concentrate flow rates and stores the two
concentrate pump speeds in the UF–Proportioning controller.
The 4-phase, 100-step/rev stepper motors used in the concentrate
pumps are driven in a 2-phase bidirectional mode. This drive was
implemented on the UF-Proportioning Power board utilizing a dual
full bridge driver for each motor. Since two inverters are also
included on the board, only two clock signals are required to step
each motor. These motors are driven from an independent power
supply developed on the UF-Proportioning Power board so that
various motor voltages can be accommodated by adjusting the
power supply level. Inductive currents developed by the motors
are dissipated through a Shottky diode array IC on the UF-
Proportioning Power board.
7.5 DIALYSATE FLOW CONTROL
The Instrument flow path contains three pumps: a deaeration
pump, a dialysate pump, and a supply pump.
7.5.1 Deaeration Pump
The deaeration (or air removal) pump runs at a constant speed for
all flow rates. This speed is set in the calibration mode so that a
deaeration pressure of approximately –500 to -650 mmHg in the
air removal sprayer chamber (deaeration sprayer) is achieved. The
deaeration pump is situated in the flow loop with the deaeration
sprayer (directly after the "A" mix point) to produce a low pressure
that removes the dissolved air in the water being prepared as
dialysate.
7.5.2 Dialysate Pump
The dialysate pump operates at a different but constant speed
depending on each of the dialysate flow rates. This pump runs at
about 1500 mL/min, when the dialysate flow is set to 500 mL/min.
The speed at which it operates is proportional to the dialysate flow
rate. Therefore its lowest speed is at the 300 mL/min dialysate
flow rate and its highest speed is at the 1000 mL/min dialysate
flow rate. The dialysate pump is in the postdialyzer flow circuit
and provides the pressure, which draws the fluid from the dialyzer
and delivers it to the flow equalizer. This pump has a recirculation
flow path around it so that the flow through the pump remains
constant even though the flow equalizer flow is pulsatile.
157-1278-914 Rev A 7-11
January 2004
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