2 Theory of Operation
1505-1018-000 08/07
2-9
2.1.2 Expiratory circuit
At the expiratory side of the ventilator, a solenoid powered exhalation valve
{16} controls exhaust from the breathing circuit. The valve contains an
elastomer diaphragm that is held against a rigid seat by a solenoid (voice coil)
driven piston. The valve achieves a balance between the pressure generated
within its 21-mm diameter seat area and the force applied by the piston,
releasing exhalation flow as necessary to maintain balance. The proportional
solenoid controls the exhalation sealing pressure within a range of 0 to 100
cm H
2
O. Software control provides continuous oscillatory movement
(dithering) of the exhalation valve to minimize static friction effects.
Immediately upstream of the exhalation valve is a tap for the expiratory
pressure transducer {17} and its associated zeroing valve. The expiratory
pressure tap is continuously purged with 35 ml/min of air to ensure that
exhaled condensate does not occlude the tap. The air flow is established from
the regulated Air supply using a fixed orifice (pneumatic resistor) {18}.
Downstream of the exhalation valve is the expiratory flow transducer {19}. In
principle, the transducer is similar to a hot-wire anemometer. A wire having a
large “temperature to electrical resistance” relationship is placed in the
flowstream. The wire is kept at a constant temperature using a Wheatstone
bridge circuit. The current necessary to maintain the resistance of the sensor
portion of the bridge is a function of the flow through the sensor.
At the output of the flow sensor is a flapper type check valve {20} that
prevents gas from being drawn in through the expiratory valve and minimizes
patient rebreathing in the event of a ventilator failure.
Located downstream on the patient wye is an optional Neonatal Flow Sensor
{28}. The sensing elements for the Neonatal Flow Sensor Subsystem consist
of two hot wires each with its own drive circuitry and Wheatstone bridge. The
hot wires reside off-board in a flow tube situated at the patient wye and
O
2
Safety
Relief Valve
Flow
Sensor
Flow
Sensor
Total Flow
Inspiratory
Effort
Valve
Free Breathing
Check Valve
H
T
H
T
P
22 / 15 mm
Male / Female
Pneumatic
Resistor
O2 Sensor
Inspiratory Zero
Valve
Auxiliary Pressure
Inspiratory Pressure
Flow
Valve
Flow
Valve
Micropump
Nebulizer
Patient
H
30 mm Male Cone
22 / 15 mm
Male / Female
Exhalation
Valve
Exhalation
Flow Sensor
Pneumatic
Resistor
Expiratory Zero
Valve
Expiratory Pressure
EXP
INSP
P
H
T
H
Neonatal Flow Sensor - Optional
{16}
Air
{17}
{18}
{19}
{20}
Patient
From
To
{28}
Summary of Contents for Engstrom Carestation
Page 1: ...Engström Carestation Technical Reference Manual ...
Page 12: ...Engström Carestation x 08 07 1505 1018 000 ...
Page 52: ...Engström Carestation 2 26 08 07 1505 1018 000 ...
Page 100: ...Engström Carestation 5 26 08 07 1505 1018 000 ...
Page 124: ...Engström Carestation 7 16 08 07 1505 1018 000 ...
Page 188: ...Engström Carestation 8 26 08 07 1505 1018 000 ...
