Water condensation on surfaces is a phenomenon that, depending on circumstances and applications, can be a desired or unwanted effect. While psychrometric devices can provide information about the thermodynamic properties of the water vapour-air mixture, few devices are available to detect and quantify the actual build-up of dew droplets on surfaces. Even less are capable to quantify the concentration of liquid water in an aerosol, that is when relative humidity goes beyond 100%.
This sensor addresses this problem providing a way to detect and quantify the amount of liquid water particles in air at proximity of the sensor's sensitive surface, or detect and quantify the amount of fine water droplets deposited on its sensitive surface.
This project has undergo throughly experimentation to test the capabilities of the sensor and particularly of various design and configuration of the electrodes that form the sensitive area of the sensor.
How it works
It works exploiting the well known capacitive effect (self capacitance) when a body is influenced by an electric field. Ultimately it can be seen as a peculiar version of a capacitive sensor. Electric fields are generated providing charges to the electrodes through a current source, and a comparator detects a specific level of voltage calibrated on the limit of the circuit, and removes the charges to start a new measurement cycle.
Basically it is a constant current source that charges a capacitor created on the PCB and designed in a way to magnify the projection of the electric fields, converting the measured capacitance into a variable pulse width signal that is carried through the same line that powers the sensor itself, to an external processor that reads the signal and controls the power cycle of the sensor to limit the risk of self-heating.
It is a pretty simple circuit, you know, often the challange is making things simple and efficient.
Unlike in a typical capacitor, the generated electric fields are projected toward the space above (and below) the sensitive area thanks to the design of the network/grid of electrodes. These projected fields expand the sensitivity to the air above the sensor's sensitive area.
When exposed to air (or any other gas for the matter) the sensitive area projects the electric fields through the air (or gas) and if any solid particle is floating in proximity of the electrodes, it interacts with the electric fields causing a variation in the mutual capacitance composed by the electrodes, the air/gas gap, and the particle. This variation is related to the dielectric permittivity specific of the material of which the particle is made of. Thus in the case of water (liquid, at 25°C) the difference between the relative dielectric permittivity of air and water is about 80 times larger for the latter.
An experiment was made to test how far the fields are projected. As expected, there is no linearity and a cut off distance has been set to discard non significative variations. It was assessed that the sensitivity significantly starts from about 5 mm above the sensitive area.
Effect of droplets build-up
Even the build up of droplets show a limited linearity. In an other experiment aimed to demonstrate this, resulted that a single drop with smaller surface area, but thicker, produces a larger increase in deviation than the thinner drops that in sum cover a larger surface area.
Comparison with other similar technologies
Capacitive sensing is a well known technology in the state of the art. Proximity and touch sensing have been developed using open plate capacitors, with circuits that are based upon frequency deviation or upon frequency excitation such as the one integrated in the FDC1004 produced by Texas Instruments.
Even though the sensor...Read more »