P.c.b. Lp 493 Blood Leak Detector; Description - Fresenius Medical Care 4008 E Technical Manual

8.8

P.C.B. LP 493 Blood leak detector

8.8.1

Description

Operational principle
A 2-color LED alternately emits green and red light through a measuring cuvette. A photoelectric
cell converts the light intensity received into a voltage logarithmic to the light intensity, so that the
square-wave amplitude generated from the alternating green/red illumination is proportional to
the logarithm of the green/red quotient and that non-specific types of dimness, which equally
attenuate both colors, do not affect the signal.
Circuit description
Blood leak channel
Together with crystal Q1 and the capacitive voltage divider C8/C9, IC 6 generates a frequency of
32768 Hz in a three-point circuit. Via R23/R22, the binary divider IC 6 supplies transistor T1 with
pulses of 32 Hz from Q10 for energizing the green LED and, inverted via the joint emitter resistor
R34 and transistor T2, for energizing the red LED.
The output voltage of the photoelectric cell is amplified via C2 and OP IC 1a and then inverted via
OP IC 3a. The channel switch IC 5 scans the 2nd half (from IC 6-13 to IC 5-10) of the transient
output amplitude of OP IC 1a and OP IC 3a respectively in phases synchronous to the green/red
energization (from IC 6-15 to IC 5-9). The voltage is filtered via R12/C3 and amplified via OP IC
4b.
Temperature and aging regulation
A second photoelectric cell, which evaluates a reference ray of light not passing through the
cuvette, readjusts the current of the red LED (via C10, OP IC 1b, OP IC 3b, IC 5, OP IC 4a, and
T3) so that the different temperature coefficients or agings of the green and the red LED are
compensated. The Zener diode raises the potential so that T3 can act as current-decreasing
element. Since the difference is measured, the temperature coefficient of the photoelectric cells
has no effects whatsoever.
Calcium warning
During bicarbonate dialysis, calcium is deposited in the measuring cuvette. Due to color-specific
attenuation, this calcium delivers a signal opposite to that of a blood leak. This is the reason why
the difference between cuvette channel and reference channel is evaluated via OP IC 2a/2b for
calcium warning.
Test
By means of H level, transistor T4 becomes conductive via R5, thus detuning, by means of
voltage division at R21/R28, the control loop for the green/red ratio, so that a blood leak signal is
simulated.
Fresenius Medical Care 4008 4/09.03 (TM)
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