Dräger Babylog 8000SC, Technical & Service Manual, Page: 56 / 155

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Dräger Medical AG & Co. KGaA 6173.3
Function description Babylog 8000
11.2.1 Measuring Principle of the Flow Measuring Bridge
The flow sensor incorporates two hot-wire flowmeters. A separate wire is used for direction recognition.
Both wires are evaluated on the Flow PCB by two separate measuring channels.
Fig. 29: Principle diagram of the flow measuring bridge
The measuring bridge comprises the resistors R 54, R 72, R 70, and the measuring wire of the sensor.
The measuring wire is kept at a constant resistance. This means that the amount of heat dissipated by the
gas flow has to be readjusted. As system deviation the voltage passes through the node R 54/R72 and
the measuring wire R 70. If the voltage is adjusted to "0", the current flowing through the measuring wire
and the R70 is a measure for the heat dissipated. The controllable power source is made with V7. N7 is a
controller with PI behavior. To adjust the bridge a current can be injected via the D/A converter D 24 into
the node R54/R72. The current through the bridge is changed such that the wire begins to glow. Burning
out of the wire is prevented by a current limiter. The current limiter compares the voltages and switches
the current source off, if required. Current limitation is effected through the comparator which is connected
to the input of V7. The current through the flow measuring bridge is changed for test purposes. R 70 is a
current measuring resistor (four-wire measurement) which measures the heating current (proportional
voltage).
+
-
U
B
Flow test
12 Bit
R 54
Hot wire
glowing
R 72
R 70
Umeß
PI regulator
Hot wire
Comparator
RDR
D 24
V 7
N 7