Arena Service Manual
14.3 FUNCTIONAL DESCRIPTION
The Instrument constantly monitors the photocell signal. If blood
leaks through the dialyzer membrane, the blood passing between
the LED and photocell in the BLD will absorb a portion of the
light from the LED, preventing it from reaching the photocell.
This change in the amount being received by the photocell will
cause the Instrument to trigger a blood leak alarm stopping the
blood pump, clamping the venous line, and warning the operator.
Bubbles in the BLD could cause the same response creating false
alarms.
Both the LED and the photocell connect to the I/O Hydraulic
Power board. The LED is connected to a voltage-to-current
converter on the I/O Hydraulic Power board. The input to this
circuitry comes from the I/O Controller board. The photocell is
tied to the +5 V reference supply through a 750 kΩ resistor. This
provides a voltage divider which is monitored on the I/O
Controller Board.
The current through the LED is adjustable and controlled via a
Digital-to-Analog output from the I/O Controller board. The light
intensity of the LED is adjusted to illuminate the photocell to a
point where its resistance is below the alarm threshold. During a
blood leak, the presence of blood in the housing attenuates the
light striking the photocell, which causes an increase in both the
photocell resistance and voltage. The increase in voltage
(monitored by the microcontroller on the I/O Controller board)
results in a blood leak alarm.
14-6 157-1278-921 Rev A
April 2004
To Next Page
Loading...
