Q1. How does a humidity sensor work? Vasilis
A1. Humidity is a measure of the amount of water vapour present in the air. Humidity sensors are low-cost sensitive devices that are used for three primary measurements of humidity: absolute, relative, and specific. Based on these parameters, humidity sensors are classified as capacitive, resistive, and thermal conductivity humidity sensors.
Relative humidity sensors usually contain a humidity sensing element along with a thermistor to measure temperature. Capacitor is used as sensing element in a capacitive humidity sensor whose change in electrical permittivity of the dielectric material is measured to calculate the relative humidity. Low resistivity materials are used in a resistive humidity sensor.
Basic structure of a capacitive humidity sensor is shown in Fig. 1. As you can see, the lower electrodes with two metals (leads) are deposited on the glass substrate and coated with humidity-sensitive polymer material. The capacitor is formed by the upper electrode. Top of the sensing surface is coated with a thin layer of evaporated gold to protect it from ambient contamination or dust and help ensure better condensation.
The high molecular sensitive polymer’ s die lectric constant changes with relative humidity of the environment.
When the humidity
changes, capacitance of the humidity sensor changes accordingly. That is, when the relative humidity increases, the humidity-sensitive capacitance increases, and vice versa. The capacitance is usually
between 48pF and 56pF. A circuit converts the capacitance
to voltage, which corresponds to a change in relative humidity from 0 to 100% RH, and output of the sensor changes linearly from 0 to 1V. Normally, humidity sensor requires a
chip or microcontroller to analyse, process, and display the measurement results.
Some humidity sensors available in the market are DHT11, DHT22, AM2302, and SHT71.