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Fig. 6.1 Overview of the Bathonify system
Figure 6.2 shows the output data from a water pressure sensor (model KH0578-30
by Hamamatsu Photonics) for an actual bather based on the movements described
above. The measurement data for a water heater contains circuit noise that has a
higher frequency than both bather movements and water movements in the bathtub.
As a result, the noise can easily be eliminated with a low-pass filter. A comparison
of Figs. 6.2 a and b shows that stirring the water strongly causes a large change in
the pressure of the water. Figure 6.2 c shows how the water pressure slowly rises as
the bather enters the bath, starting from the feet and submerging the body up to the
shoulders. Conversely, Fig. 6.2 f shows how the water pressure is suddenly lowered
as the bather rapidly exits the bath. Figure 6.2 e shows that the pressure changes
only a little compared to when the water is strongly stirred without a bather in the
bathtub. This difference may be because the sensor cannot easily detect large stirring
motions on the surface, and thus changes in water pressure, when a bather is in close
proximity to the opening of the hot water supply. These measurement results draw
attention to the importance of considering differences in ripple amplitude when a
bathtub is occupied and unoccupied.
6.2.2.2
Measuring Vital Signs
We use the water pressure sensor to take breathing measurements. The act of breathing
causes the water level to fluctuate by a few millimeters in a bathtub. Therefore, the
sensor extracts information about each breath from variations in the water level.
Preparatory tests showed that low-pass filter processing with a cutoff at 0.35 to 0.4 Hz
gives virtually the same results as a specialized breathing measurement instrument
(model AE-280 S by Minato Medical Science). Figure 6.3 compares the water volume
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