A basic resistive humidity sensor, also known as a resistive humidity element or a humidity-dependent resistor (HDR), is a type of sensor used to measure relative humidity in the surrounding environment. It operates on the principle that the electrical resistance of certain materials changes with variations in humidity levels.
The fundamental function of a basic resistive humidity sensor can be summarized as follows:
Sensing Material: The sensor is constructed using a humidity-sensitive material that has the property of changing its electrical resistance in response to changes in humidity. Typically, this material is a moisture-absorbing or moisture-releasing substance, such as a polymer or a ceramic compound.
Resistance Change: As humidity levels increase, the humidity-sensitive material absorbs moisture from the surrounding air, causing it to expand. This expansion leads to a corresponding increase in the material's electrical conductivity and a decrease in its resistance. Conversely, when humidity levels decrease, the material releases moisture, causing it to contract. This contraction results in a decrease in conductivity and an increase in resistance.
Measurement Circuit: The sensor is integrated into an electrical circuit that measures its resistance. This circuit often consists of a voltage source and a known resistor (often referred to as a "reference resistor") in a configuration known as a voltage divider. The output voltage of the circuit is then proportional to the resistance of the humidity sensor.
Calibration: To convert the sensor's resistance into a meaningful humidity measurement, the sensor is calibrated using known humidity values and corresponding resistance measurements. A calibration curve is established to relate resistance to humidity. This curve can vary from sensor to sensor based on factors like the specific humidity-sensitive material used and the sensor's design.
Output: The humidity sensor provides an analog output, usually a voltage or current signal, that corresponds to the relative humidity of the environment being measured. This output can then be further processed by microcontrollers, analog-to-digital converters (ADCs), or other electronic components for interpretation and display.
It's important to note that while basic resistive humidity sensors are simple and cost-effective, they might have limitations in terms of accuracy, drift over time, and sensitivity to temperature changes. More advanced humidity sensors, such as capacitive humidity sensors, are often employed in applications where higher accuracy and stability are required.