Datex-Ohmeda Aestiva 7900 SmartVent, Service Manual

Page: 26 / 181

2/Theory of Operation
2-8 1503-0151-000 3/27/97
Figure 2-10
Bleed Resistor, Pressure Switch and Free Breathing Valve
The Bleed Resistor is, by design, a "controlled leak" from 0 to 10 L/min. in re-
sponse to circuit pressures from 0 to 100 cmH
2
O
.
The small amount of pneumatic
flow exhausting through the bleed resistor allows for control of the exhalation
valve's pilot pressure by modulation of the valve output. The bleed resistor ex-
hausts only clean drive gas and should not be connected to a waste gas scaveng-
ing circuit. The output is directed away from the electrical components to ensure
that systems using oxygen drive gas meet the 10VA requirement for oxygen en-
richment.
The Pressure Switch is an electrical limit which automatically shuts off the GIV
(Gas Inlet Valve) and flow valve if the pressure reaches a level of approximately
104 cmH
2
O
.
When either or both of these valves is shut, the pilot pressure to the
exhalation valve falls rapidly as the flow discharges out the bleed resistor. As the
pilot pressure is relieved, the exhalation valve opens allowing circuit pressure to be
relieved.
The Free Breathing Valve , also shown in figure 2-10, allows the patient to take a
spontaneous breath. The ventilator is programmed to supply a certain number of
breaths per minute to the patient. If, in between one of these programmed cycles,
the patient requires a breath (spontaneous) the free breathing valve allows the pa-
tient to inhale ambient air spontaneously.
Mechanical
Overpressure
Bleed Off
(MOBO)
Pressure
Switch
Drive Gas Check Valve
(DGCV)
Free
Breathing
Valve
Supply
Gas
Hose
Manifold
Pressure
Transducer
Flow
Valve
Pressure
Regulator
Inlet
Filter
Gas
Inlet
Valve
(GIV)
35 - 100 psi
Supply
25 psi
25 mm port 17 mm port
Exhaust
to Scavenging
System Exhaust to
Ambient
Bleed Exhaust
to Ambient
Exhaltion
Manifold