Respiration Monitor (piezo crystal) for use with iWorx interfaces

The RM-204 (Human) Respiration Monitor for use with iWorx computer interfaces employs piezo technology to follow the relative depth and frequency of breathing. The measuring element is mounted in a belt that straps around the chest of the subject.

Overview

The RM-204 Respiration Monitor is a transducer used to measure the relative
depth and frequency of breathing in a human subject during experiments where it
is impractical to monitor breathing with a spirometer. For example, the use of a
spirometer to monitor breathing during a psychological test could be a distraction
that affects the results of the test. If the subject is not familiar or comfortable with
breathing through a spirometer, the subject cannot focus on completing the test.
The RM-204 permits breathing rates and relative amplitudes to be measured
easily, accurately, and unobtrusively while the subject performs another task.

How It Works

The RM-204 Respiration Monitor has two components: a piezo-electric sensor that
produces a voltage in response to movement; and an elastic belt that is placed
around the chest to hold the sensor in place.
The primary driving force for pulmonary ventilation is the diaphragm, the large
muscle between the thoracic and abdominal cavities. During inhalation, the
diaphragm contracts, moves downward, and forces the abdominal wall to move
outward. The process is reversed during exhalation. The piezo-electric sensor in
the RM-204 detects the cyclic movement of the ventral body wall and generates a
voltage that is proportional to the amount of movement.
Piezo-electric sensors are devices that generate a voltage in response to motion.
When a piezo-electric sensor stops moving or is moving slowly, the voltage output
of the sensor returns to its baseline level within milliseconds. To make this motion
detector suitable for breath monitoring, the sensor of the RM-204 is fitted with a
filter that slows the return of the voltage to its baseline level. Because of the filter,
the RM-204 is able to record breath rates from 4 to 100 breaths per minute.
However, when the subject is holding his or her breath, the voltage output of the
sensor decreases very slowly. This decrease occurs because the filter cannot hold
its voltage level indefinitely and still respond to subtle changes in breathing.