Arena Service Manual
averaged over a 400 millisecond time period and reduced by
multiplying it by 15/16 and subtracting 50 mV (for noise
immunity). This new value is then converted back to an analog
level to be used as an alarm limit. This software-generated limit is
compared to the rectified DC signal from the detector. The output
state of this comparator is monitored by the I/O Controller Board.
The voltage from this comparator is sampled every 0.96 ms. When
the unaveraged signal falls below the software-generated limit a
counter is incremented by 16. The counter limit is set in
customizing. When the counter reaches the count limit, an alarm
occurs. If the voltage is above the software-generated limit, the
counter is decreased by one. The lower limit of the counter is zero.
This allows many small bubbles equivalent to a large bubble to be
detected. Sensitivity of the software alarm can be calibrated
according to Section 17.3.1.2.
13.2.1.2 Hardware Alarm Detection (Secondary Alarm)
The hardware alarm is redundant to the software-generated alarm.
This alarm uses two comparators (U7A & U7B) on the I/O
Electronics Power Board. One comparator looks for a minimum
DC level from the rectified detector signal. The second
comparator is AC-coupled to react to a large air bubble in the
tubing. Sensitivity of this detector is approximately 300 microliter
air bubble. Both comparator outputs are ORed (U4B& U4C)
together so that either comparator will generate an alarm.
The hardware alarm provides a signal to the SBC, and the
following occur:
the air alarm information is displayed on the video display •
•
•
the main alarm lamp and 3-Light Status lamp flash
an audio alarm is generated.
The alarm sends a signal to the I/O Controller Board which
provides a hardware line to shut off the blood pump. It also causes
the input on the line clamp driver board to go LOW so the venous
line clamp driver is de-energized, closing the clamp and sealing off
the venous blood line.
13.2.2 Air Detector Tests
There are two tests that can be performed on the Air Detector:
Voltage
Bubble
13-6 157-1278-920 Rev A
April 2004
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