Airborne Water Droplets Detector

If you try to google the term “projected capacitance” you'll mostly find about the technology used to make touch screens. I've found another application that exploits this phenomena: droplets detection.

2020

During the fall of 2019 I started a project aimed to detect the state of condensation of water from its gaseous state, by the mean of an electronic sensor, exploiting the phenomena of the projected capacitance.
A few months later something unexpected happened, and during the Pandemic I found myself involved in an interesting project about testing the efficacy of face masks by exploiting the characteristics of my sensor. This led to a research work that resulted in paper that is also available though this site.

Particles of water Detected by exploiting Projected Electric Fields

A sensor of particles of water is presented, and the effects that water droplets can cause on wood and iron or steel. Invisible water particles can cause rust and mould. Tiny particles mostly made of water are also ejected from our mouth and nose and in some cases they can carry infectious diseases. This sensor proved to be sensitive enough to detect those tiny particles, and has been used in a research to assess the effectiveness of face masks. Also it is used to detect water condensation in buildings to prevent mould and structural damages. The sensor uses projected electric fields to detect the proximity of water particles thanks to the fact that water has eighty times the dielectric coefficient of air. Projected electric fields are also used in touch screens, in that case it is known as projected capacitance.

The sensor detects this variation in dielectric and provides a signal proportional to the amount of the total volume of water the particles would form if joined together. This quantitative information is different from the one that comes from humidity sensors, that sense gaseous water, while this sensor detects actual liquid particles that are formed when condensation occurs and such particles can build up on cold surfaces.

Possible applications are mould control in buildings, warehouses, farms, in crop storage, and other food control quality. Also it can be used to detect condensation to prevent rust or damages in electronic components.
Finally it can be used to gauge aerosols based on water particles, such as in mist generators and atmosphere; or as an extreme application it can be used to gauge the amount of water-based droplets ejected by our breath.

How it works

The sensor is integrated into the transducer circuit that has a constant current source that test the capacitance of the sensitive area, and a fixed threshold comparator. This is combined with a circuit that derives power to supply the circuit from the same cable that carries the signal. This is to minimize the interactions with external elements and limit the self heating. The output is a frequency that is proportional to the amount of particles detected. This signal is managed by an external microprocessing unit that keep track of other physical properties relevant for the specific application, such as temperature.