7. Dialysate Preparation
the time it expects the end-of-stroke signal and then switches the
flow equalizer valves (secondary flow equalizer time-out).
To ensure a complete volume transfer in the flow equalizer, flow
sensors (end-of-stroke sensors) have been placed in the flow path
to sense when the flow stops (end-of-stroke). The valves will not
switch before the end-of-stroke condition is sensed unless a
secondary time-out condition is encountered. Since the speed of
the supply pump determines the instantaneous dialysate flow rate,
it is controlled so that both end-of-stroke signals are received in a
predefined amount of time before the desired valve switch time.
This time is fixed and is set to 1 second upon initialization.
The average flow equalizer volume is calibrated (measured) in the
Calibration Mode. The time between the switching of the flow
equalizer valves is scaled by the SBC according to the calibration
constant and stored in the UF-Proportioning controller so that the
desired dialysate flow rate is achieved.
7.5.4 Pump Drive Circuitry
The drive circuitry for the three flow pumps is embedded in each
of the associated pumps. Each pump is supplied with unregulated
nominal +24 VDC, ground, and a control voltage. Each pump
produces a tachometer signal that is sent back to the UF-
Proportioning Power board. This signal is used by the UF-
Proportioning controller to control the long-term speed variations
of the flow pumps.
7.5.5 Flow Equalizer Drive Circuitry
The flow equalizer valve drivers on the UF-Proportioning Power
Board conduct the current which powers the eight flow equalizer
valves. The current typically averages about 1.5 A at 50% duty
cycle with a period range of 6.7 to 22 seconds depending on flow.
There are flyback diodes on the same board which conduct current
each time the associated flow equalizer valve set is de-energized.
7.5.6 Dialysate Pressure Relief (Rinse) Valve
This valve prevents a vacuum build-up in the flow path when the
connectors are on the rinse block during Prime and Rinse Mode. It
is also opened during Rinse and Self-Test Modes except during UF
tests.
The dialysate pressure relief valve driver, on the UF-Proportioning
Power Board, conducts the current which powers this valve.
157-1278-914 Rev A 7-13
January 2004
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