Datex-Ohmeda Anesthesia Delivery Unit S/5, Technical Reference Manual

Page: 232 / 494

Datex-Ohmeda ADU
B-6
Document No. 8502854
(no voltage), the sensor ports are connected to opposite sides of the restriction. The valve
compensates pressure sensor zero drift and is connected to two lines. One line is upstream the flow
restrictor, the other line is downstream. The valve can pneumatically short-circuit the pressure sensor
ports, thus giving the zero pressure (= zero flow) signal. Both the zeroing valves are connected to zero
1 circuit.
Figure 5 Electrical Flow Measurement Unit
N
2
O limiting control
The N
2
O limiting control of the A-FGC1 calculates continuously the maximum allowed N
2
O flow using
the current O
2
flow as input. If the N
2
O needle valve, see (4) Figure 4 on page B-4, is opened to a
position that would cause a flow higher than the maximum calculated, a proportional valve (5)
reduces the N
2
O flow so that a minimum of 25% O
2
is achieved. The sw uses the proportional valve to
control the O
2
concentration in the fresh gas.
The proportional valve (5) is a linear current driven proportional valve located after the N
2
O needle
valve before the N
2
O/Air selection valve. The current is controlled with the 12 bit DAC on the CPU
board. The valve control is calibrated. The calibration procedure is described later in this section.
N
2
O/Air selection
With the N
2
O/Air switch (7) the user may select either N
2
O or Air to be mixed with O
2
. The selector
switch (7) transmits the switching information (N
2
O /Air) to the CPU board of the fresh gas control
unit. The N
2
O/Air control valve (9) selects N
2
O or Air to be mixed with O
2
according to the information
from the CPU of the fresh gas control unit. In power off situation, the N
2
O/Air valve (8) N is al-ways in
Air position.
When the N
2
O/Air selector switch is in N
2
O position the CPU opens the N
2
O/Air control valve and
thereby switches N
2
O gas pressure to the N
2
O/Air valve, which is pneumatically driven by the N
2
O gas
pressure (2). The N
2
O gas flow can then be mixed with O
2
flow according to the needle valve settings
and the N
2
O proportional valve.
The N
2
O/Air selection valve (8) is a 5/2 pneumatically driven piston valve. The N
2
O/Air control valve
(9) pilot valve is an electrical driven 2/2 solenoid valve that drives the selection valve (8). Both valves
is integrated into the N
2
O/Air selection valve