Chapter 5: Operating Principles
The Spirometry Source
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The Astra 300 spirometer is an instrument that acquires
mechanical signals and processes the information provided
by the signal related to the pulmonary function. For
processing purposes, mechanical must be changed to
electrical. The devices responsible for this change are
called transducers. The ASTRA 300 has a Turbine-type
transducer.
The turbine transducer performs transduction in two
stages: The volume to be measured crosses the turbine
vane and records its rotation that is proportional to that
volume. This rotation is detected by the interrupting of
a beam of infrared light, the sensor of which converts
the light received into a digital-type electrical signal.
5.1 TURBINE
The turbine is axial with two stators in the form of a
propeller and a rotor made up of a flat, rectangular blade.
The shape of the stators means that the air flow passing
turns, which makes the blade turn. The turbine operates
based on the Fluid Mechanics theory and, more
specifically, the Machine Turbo theory. Applied to this
case, the angle at which the rotor turns is directly
proportional to the fluid volume crossing the turbine and
the proportionality constant depends on its shape.
5.2 TURBINE ROTATION SENSOR
The turbine rotation sensor consists of three pairs of
emitting diodes and an infrared (invisible) photo-transistor
that detect the rotation and direction of the blade. The
number of times the beam is interrupted is equivalent to
an accumulated angle of rotations and, therefore, the
volume of air to have crossed the turbine. The photo-
transistor provides a digital electrical signal that
represents the times the beam of light is interrupted and
is directly acquired by the microprocessor.