Arena Service Manual
7.5.7 End-of-stroke Sensors
Flow detection is accomplished by an optical sensor. For more
information on this sensor, refer to Section 4, Hydraulic Theory.
7.6 CONDUCTIVITY MEASUREMENT AND MONITORING
Conductivity is used as a measurement of the electrolyte
composition of the dialysate. The hardware for measuring
conductivity is always a cell for measuring changing resistance as
the amount of dissolved salt changes and a thermistor for
measuring temperature changes. Electronic circuits are used to
equate these changing resistances to the standard measuring
conditions of a 1 cm cell at 25 Celsius. The information from
these devices can be used to give us conductivity in the units of
milliSiemens/cm (mS/cm) @ 25°C.
The Flow Equalizer and the concentrate pumps control the
dialysate composition, therefore controlling the conductivity.
Conductivity redundancy involves three conductivity probes
communicating with two electronic systems: UF-Proportioning
and I/O systems. Each of these systems involves separate control
and power.
7.6.1 Conductivity Measurement
Conductivity is measured by the Input/Output system.
Measurement consists of a primary conductivity probe positioned
just prior to the bypass valve, in the dialysate manifold. The
primary probe's conductivity is used for primary alarms and the
displayed conductivity.
The primary probe uses an AC signal with local temperature
compensation. The signal from the primary probe is amplified,
rectified and buffered on the I/O Hydraulics Power board. The
resulting signal is sent to the A/D converter on the I/O Controller.
The microprocessor on the I/O Controller applies conductivity and
temperature calibration factors and compares the resulting value to
the primary alarm limits. An alarm message is passed to the SBC
operating software if necessary. The conductivity value is also
passed to the SBC software for display on the LCD.
7.6.2 Conductivity Monitoring
Conductivity is monitored by the UF-Proportioning System. This
monitoring consists of the following:
7-14 157-1278-914 Rev A
January 2004
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