Penlon AV-S, Service Manual, Page: 87 / 94

Altitude Effects
Gas flow measurements are also affected
by atmospheric pressure, in a linear
relationship.
To compensate for altitude effects an
ambient pressure sensor is available. When
the spirometers are calibrated for zero flow
the ambient pressure is recorded so that
the measured volume may be adjusted.
The measured volume is multiplied by the
ratio of Pamb to Pcal; where Pamb is the
latest ambient pressure and Pcal is the
ambient pressure recorded when the
spirometers were calibrated at zero flow.
Carrier Gas Effects
The effect of air as the dilutent gas is
different to that of nitrous oxide and as the
ventilator includes only an oxygen monitor,
the additional information of gas being
ventilated is included to increase available
accuracy. In the event of the wrong gas
selection being made by the user, the error
in delivered volume could reach up to
approximately 7%. This possible variation is
of no known clinical disadvantage.
Anaesthetic Agent Effects
The addition of anaesthetic agent is known
also to increase the spirometry readings
depending on the agent and its
concentration by up to approximately 2%.
Again this minor volume measurement
variation is of no known clinical
disadvantage and is therefore not
compensated for other than that due to
oxygen variation due to the percentage
change.
Water Vapour Effects
Water vapour volumes in the breathing gas
are not detectable in normal breathing
system dynamics.
Additional Features
Additional spirometry features available for
selection by the user are the ability to turn
off the automatic compliance and fresh gas
compensation and also the feedback
provided by the oxygen monitor.
In this event, the ventilator relies on the
basic delivery look up table and the internal
flow sensor to confirm delivery volumes as
near as possible, under the circumstances.
Accuracies for spirometry measurement are
>300 ml ± 10%
>100 ml <300 ml ± 20%
<100 ml ± 50%.
Flow sensor description
The microbridge mass airflow sensor
operates on the theory of heat transfer.
Mass airflow is directed across the surface
of the sensing elements.
Output voltage varies in proportion to the
mass air or other gas flow through the inlet
and outlet ports of the package.
The specially designed housing precisely
directs and controls the airflow across the
microstructure sense element.
The microbridge mass airflow sensor has a
unique silicon chip based on advanced
microstructure technology. It consists of a
thin-film, thermally isolated bridge structure
containing heater and temperature sensing
elements. The bridge structure provides a
sensitive and fast response to the flow of
air or other gas over the chip.
Dual sensing elements positioned on both
sides of a central heating element indicate
flow direction as well as flow rate.
Laser trimmed thick film and thin film
resistors provide consistent
interchangeability from one device to the
next.
The microbridge mass airflow sensor uses
temperature-sensitive resistors deposited
within a thin film of silicon nitride. They are
suspended in the form of two +bridges over
an etched cavity in the silicon.
The chip is located in a precisely
dimensioned airflow channel to provide a
repeatable flow response.
Highly effective thermal isolation for the
heater and sensing resistors is attained by
etching the cavity space beneath the flow
sensor bridges. The small size and thermal
isolation of the microbridge mass airflow
sensor are responsible for the extremely
fast response and high sensitivity to flows.
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