Theory Of Operations - BIO-MED DEVICES CROSSVENT-3+ Operation & Service Manual

VIII. THEORY OF OPERATIONS
The CROSSVENT-3+ is a time cycled, volume or pressure limited ventilator.
operation is extremely simple. Supply gas, either air, oxygen, or a blended mixture, is connected to the
CROSSVENT-3+ inlet fitting.
NOTE: For blended gas, the Bio-Med Devices blender should be used.
WARNING: In rare instances, when using the Crossvent with an air/oxygen blender,
there may be a reduction in the delivered flow at the higher flow settings.
reduction may occur when the blender is set below 30% or above 90% O
Crossvent is set to flows above 80 lpm. Lower supply pressures to the blender will
tend to decrease the flow further so be sure these supply pressures are maintained at
45- 75 PSI (310-517 kPa). If the Crossvent has the exhaled tidal volume monitoring
feature, it is recommended this be used to ensure proper tidal volumes are being
delivered. If it does not have this feature, then an external spirometer is
recommended.
Gas flows first to an internal pressure regulator that provides output gas regulated to approximately 20 psi.
This is used both for patient gas and to drive the pneumatic signals.
From the regulator the gas flows to a normally closed, 2-way, pilot valve operated by a miniature solenoid
valve (valve A). The gas exits the pilot valve and goes to an electronically encoded flow valve.
encoding is accomplished via a precision potentiometer.
and much greater than the downstream (patient) pressure, changes in downstream pressure may be
neglected. Therefore, since the supply pressure is accurately regulated, the flow rate becomes a function
solely of the flow valve setting. The length of time that gas flows is the inspiratory time. The volume of gas
that flows during the on-time is the Tidal Volume and is equal to the on-time (inspiratory time) multiplied
by the flow rate.
Upon exiting the flow valve, the gas then passes through the Diaphragm Actuated Relief Valve (D.A.R.V.)
manifold. This manifold contains a fixed pressure relief valve to limit the maximum pressure as well as a
variable relief valve that is controlled by the Max Pressure Knob. The gas then passes by the Vacuum Relief
Valve, which allows the patient to draw in ambient air if the entire system becomes inoperative. Finally, the
gas goes into the patient circuit through the patient connector.
During the period of time when valve A is open and gas flows, solenoid valve B is actuated, allowing gas
from the Maximum Pressure valve to pressurize the diaphragm of the exhalation valve. This assures that all
gas will flow to the patient. At the end of inspiration, valve A closes and gas flow ceases. Simultaneously,
valve B is de-energized, connecting the PEEP valve signal to the exhalation valve diaphragm. This allows
the patient to exhale to atmosphere and the pressure in the patient circuit to fall to PEEP or atmospheric
pressure.
A low flow flush system is provided to prevent humidity from traveling back up the pneumotach sensing
lines (if used) and damaging the pressure transducer. This is accomplished with two solenoid valves, D1 &
D3. A third solenoid valve, D2, is used to zero the pressure transducer to compensate for drift. During
inspiratory, these solenoids actuate. A very low flow is passed through solenoids D1 & D3 and out the
pneumotach tubes. At the same time, the transducer ports are shunted through solenoid D2. This zeros the
transducer by equalizing the pressure across it. During expiratory, these solenoids are de-energized and the
pressure differential from the pneumotach is then passed through D1 and D3 to the transducer.
Since the upstream (supply) pressure is constant
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