Page 143 - Biomedical Engineering and Design Handbook Volume 2, Applications

122 MEDICAL DEVICE DESIGN

The third group of flow sensors are the turbines, devices that are based on a rotating vane placed
in the gas stream, much like a wind speed indicator on a simple weather station. The rotation of the
vane is proportional to the velocity and density of the gas stream. As with the Pitot-effect devices,
gas velocity is related to volumetric flow by the cross-sectional area of the apparatus. Turbine flow
sensors can be purely mechanical, with precision gearings and a display dial, making for a very
portable device requiring no electricity. Battery-powered models using an LED and a light sensor to
count the rotations of the vane are also common. Turbine sensors must overcome several challenges
to remain accurate, however. First, the moving parts must be lightweight to reduce the effects of iner-
tia, which tend to impair the frequency response of the device. At the same time, however, the vanes
must be made strong enough to withstand the forces of high flow rates.
The chief advantages of flow sensors are their small size and their ability to measure continuously
in flow-through designs. They all offer good dynamic range, with frequency responses that exceed
that needed for most or all respiratory measurements. They suffer from the need for more complex
calibrations and generally require a computer or processor unit. The Pitot-effect device requires
simultaneous measurement of gas concentrations. The hot-wire anemometer requires external cir-
cuitry capable of heating the wires sufficiently and somewhat sophisticated feedback circuitry. The
turbine sensors can be fragile and susceptible to damage.

4.4.3 Pressure

Although there are a variety of pressure transducer types available, for most respiratory applications
the newer solid-state pressure transducers offer the most attractive alternative. Variable capacitance
transducers generally use the deflection of conductive plates by the input pressure differential to gen-
erate an output signal. Similarly, variable inductance transducers use the input pressure differential
to create a change in inductance. In both cases, the transducer requires an input AC excitation signal
in order to create an output signal. The circuitry used to create the excitation, and to output a usable
pressure signal, is often referred to as a carrier-demodulator. These types of transducers are very pre-
cise, offer a large variety of input ranges, and maintain stable calibrations. However, they can be
somewhat expensive and require dedicated external circuitry.
Solid-state pressure transducers come in a single package, often quite small, and also offer a wide
choice of input pressure ranges. Typically, they use the input pressure differential to deform slightly
a semiconductor plate separating two chambers exposed to each of the two input pressure levels. Any
difference between the two levels will deform the separating plate, changing its conductive proper-
ties. Onboard circuitry usually allows the transducer to accept a DC excitation voltage, and provides
a linear, temperature-corrected DC output voltage. These transducers are offered by a large number
of vendors at affordable pricing.
All of these pressure transducers offer frequency response characteristics acceptable for most or
all respiratory applications. In situations requiring the measurement of a single pressure (e.g., the
pressure at the mouth during breathing), one of the two input ports to the transducer is left open to
atmospheric pressure. In other situations requiring the measurement of a pressure differential (e.g.,
measuring the output of a pressure-drop flow sensor), the two ports of the transducer are attached to
the two taps of the sensor.

4.4.4 Gas Concentration
There are a large number of gas analysis technologies available to measure the concentrations of var-
ious species present in a gas mixture. Some of the more common types used in respiratory applica-
tions are based on principles of thermal conductivity, infrared absorption, zirconium fuel cell
technology, paramagnetism, emission spectroscopy, gas chromatography, and mass spectrometry. In
many cases, some gases in a mixture interfere with the analysis for other gases and must be removed.
Most often, interfering gases to be removed are water vapor and carbon dioxide. Water vapor may