Covidien Puritan Bennett 700 Series, Service Manual

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Theory of operation
2-28 10070389 Rev. A Puritan Bennett™ 700 Series Ventilator System Service Manual
2.2.5 Exhalation system
The exhalation system, shown in Figure 2-16 and Figure 2-17, controls and monitors the flow
of the patient’s exhaled gas.
Figure 2-16. Exhalation system pneumatic diagram
2.2.5.1 Component descriptions
The exhalation assembly includes these components; these are shown in Figure 2-18.
• The expiratory filter housing retains the expiratory filter.
• The exhalation check valve is a one-way device that prevents the patient from
rebreathing exhaled gas. It is opened by the pressure of exhaled gas, resulting in exhaled
gas flowing through the exhalation system. When the patient stops exhaling, the valve
closes.
• The exhalation pressure transducer (on the pressure solenoid PCB), a gauge type,
measures the exhalation system pressure. The ventilator uses these exhalation pressure
readings in its breath delivery calculations. The transducer is autozeroed via an autozero
solenoid (see below).
• The exhalation pressure transducer autozero solenoid is a two-way valve. It autozeroes
the transducer at power-on, once a minute for the first 10 minutes, and hourly thereafter.
The transducer is autozeroed by venting to atmosphere. A muffler reduces noise and
filters the venting gas.
• The blanket-type exhalation heater assembly contains two 10 W heaters that wrap
around exhalation system tubing. One is located just before the exhalation valve. The
other is located at the exhalation flow sensor. The heaters maintain the temperature of
the exhalation assembly above the condensation point to prevent “rain-out” in the
exhalation assembly.
• The exhalation thermistor assembly provides the electronics with temperature feedback.
An analog circuit regulates the current to the heater assembly accordingly to maintain the
heater at 50 °C, thereby eliminating condensation during exhalation. Feedback from one
thermistor, which is in contact with the cross tube heater, is used to control the
temperature to both heaters. The second thermistor, which is in contact with the
exhalation flow sensor housing, provides a backup to monitor the heater temperature.
7-00098
Exhalation pressure
transducer autozero
solenoid
From exhalation
solenoid
From
expiratory
filter
Exhalation
check valve
Heater
assembly
Exhalation
flow sensor
Exhalation
valve
Thermistor
assembly