A High Precision Fast Capacitive Sensor for Respiratory Moisture Measurement

Humidity sensing in human breath is important for analyzing pulmonary function. The humidity present in the breath of a healthy person during inhalation and exhalation varies from 49% RH to 90% RH. The value below 49% RH indicates unhealthy conditions such as dehydration, irritation in the nasal passage, dry coughing, asthma, and other respiratory issues. Over the years efforts have been made to develop a user-friendly cost-effective point-of-care sensing technology working on different principles including resistive, capacitive, ultrasonic, and optical for respiratory humidity measurement. However, these devices have some demerits like low sensitivity, long response and recovery time, and inconvenience for use. This paper reports the development of a flexible semi-cylindrical capacitive cigarette-like sensor for respiration humidity monitoring. The sensor works on the principle of change in capacitance value because of the humidity in the human breath during inhalation and exhalation. The sensitivity of the sensor is 11.4 fF /10% RH in the 50% - 90% RH range and the recovery time of the sensor is 33 ms. The sensor is highly precise, fast $(\lt1\mathrm{s})$, convenient to use and inexpensive.