Datex-Ohmeda 7900 - Page 24

2/Theory of Operation
2-6 1503-0151-000 3/27/97
The autoclavable Exhalation Valve Manifold contains an elastomeric diaphragm
used in association with the flow valve to control the pressures within the breathing
circuit. The manifold contains ports for the bellows drive gas (25 mm), the bellows
return gas (17 mm) and the AGSS (Anesthesia Gas Scavenging System) (30 mm).
Pilot control of the exhalation valve is accomplished through pneumatic connec-
tions internal to the 7900 ventilator main pneumatic manifold. The valve is normal-
ly open, requiring approximately 2 cmH
2
O of pilot pressure to seal the breathing
circuit.
Figure 2-8
Drive Gas Check Valve
In order to generate the required pilot pressure for sealing the exhalation valve dur-
ing inspiratory periods, a DGCV (Drive Gas Check Valve) is used downstream of
the exhalation valve pilot connection. This valve is biased shut using an integral
weight so as to generate approximately 3.5 cmH
2
O of bias pressure prior to allow-
ing flow downstream to the breathing circuit. During periods of operation when the
ventilator is exhausting flow from the breathing circuit, the DGCV allows the exha-
lation valve pilot pressure to be de-coupled from the circuit pressure, thus allowing
the exhalation valve to open and flow to exhaust and the scavenging system.
Bleed to
A
mbient
Supply
Gas
Hose
Fl
ow
Valve
P
ressure
Regulator
I
n
l
et
Filter
G
as
I
nlet
V
alve
(
GIV)
25 psi
Drive Gas Check Valve
(DGCV)
25 mm port
17
mm por
t
Exhaust
to Scavenging
System
E
x
h
a
l
a
ti
on
M
an
if
o
ld
35 - 100 psi
Supply inlet