Labs Idea #19: Inhalation Application Benefits Using Sensirion's Differential Pressure Sensor
Some respiratory applications use diaphragm based barometric absolute pressure sensors to estimate flow. Sensirion's thermal flow sensor is often selected for flow measurement applications, which demand virtually noise free pressure output signals or require precise measurements of ultra low, medium and large flows. The benefits of Sensirion's thermal flow sensor in differential pressure mode compared to a diaphragm absolute pressure sensor are demonstrated in the following section.
Sensirion's miniaturized differential pressure sensor SDP3x is a semiconductor MEMS version of the traditional hot wire anemometer. The sensing principle is based on thermal conductivity of the gas flowing over the sensor surface (Figure 1).
As opposed to diaphragm based differential pressure sensors, Sensirion's CMOSens® differential pressure (aka flow) sensor characteristic shows high sensitivity especially at low differential pressures (Figure 2). This allows to reliably measure ultra-low differential pressures and therewith accurately measure inhalation or exhalation in medical or consumer respiratory devices.
In order to directly compare the performance of Sensirion's differential pressure sensor with a diaphragm absolute pressure sensor both devices were attached to the same flow tube (Figure 3) and subjected to different real life situations presented in Figure 4 to 10.
A small flow rate measurement demonstrates that Sensirion’s SDP3x differential pressure sensor reliably measures also small inhalation flows, while the diaphragm absolute pressure sensor output lacks stability (Figure 5).
Closing a door generates pressure waves (Figure 6). The signal of an absolute pressure sensor mounted in a respiratory device might confuse such pressure waves with an inhalation or exhalation event. The SDP3x differential pressure sensor output signal remains unaffected by such disturbances.
Opening and closing of car doors results in high pressure peaks (Figure 7). The signal of an absolute pressure sensor mounted in a respiratory device carried by the user in real life situations might mislead to believe such pressure peaks are inhalation or exhalation events. The SDP3x differential pressure sensor output signal remains unaffected by such disturbances.
During a train ride a lot of rapid pressure changes from tunnels and crossing trains occur (Figure 8). The SDP3x differential pressure sensor is insensitive to ambient pressure changes, which reach both ports simultaneously. The diaphragm absolute pressure sensor signal is more noisy than the SDP3x sensor output. Furthermore its pressure output varies with altitude changes.
During an elevator ride the absolute pressure sensor output changes, while the SDP3x differential pressure sensor output signal remains unaffected (Figure 9).
Sensirion’s SDP3x thermal pressure sensor output signal is more stable than a diaphragm pressure sensor (Figure 10).
The measurement results demonstrate that Sensirion's SDP3x differential pressure sensor is well suited to measure the inspired air flow of respiratory devices. Inhalation duration, flow profile, volume (by integrating over the inhalation profile) as well as many other quantities such as the peak inhalation rate can easily be extracted from the curves shown in Figure 4 and 5. Sensirion’s SDP3x differential pressure sensor measures pressure with less noise than a diaphragm pressure sensor allowing faster and simpler signal analysis.
Unlike an absolute pressure sensor the SDP3x does not give false inhalation detection alerts in the following real life use cases:
Sensirion’s SDP3x differential pressure sensor is not affected …
- by rapid pressure changes, such as closing doors in a room.
- by rapid pressure changes, such as closing doors in a car.
- by rapid pressure changes, such as vertical movement in elevators, escalators or stairs.
- by rapid pressure changes, when entering a tunnel with a car or in a train.
To integrate the small flow sensor (Figure 11) used inside of Sensirion's differential pressure sensor SDP3x series (Figure 12) into inhalation or exhalation devices please contact Sensirion. For first feasibility tests we propose to use the flow and differential pressure sensor evaluation kit EK-P4 (Figure 13).
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