Datex-Ohmeda 7900 Service Manual page 26

2/Theory of Operation
35 - 100 psi
Supply
Inlet
Filter
Supply
Gas
Hose
Mechanical
Overpressure
Bleed Off
(MOBO)
Exhaust to
Ambient
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
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 O When either or both of these valves is shut, the pilot pressure to the
.
2
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.
2-8 1503-0151-000
Gas
Inlet
Pressure
Valve Regulator
(GIV)
Manifold
Pressure
Transducer
Exhaltion
Manifold
Exhaust
17 mm port
to Scavenging
System
3/27/97
25 psi
Flow
Valve
Pressure
Drive Gas Check Valve
Switch
(DGCV)
Free
25 mm port
Breathing
Valve
O The small amount of pneumatic
.
2
Bleed Exhaust
to Ambient