The humidity value is detected from the varying electrical quantities associated with the sensor type.
In capacitive humidity sensors, the relative humidity is measured by sandwiching a thin metal oxide layer between two electrodes.
Humidity sensor circuit design includes the design of an analog sensor conditioning circuit, analog-to-digital converter, transmitter, 4-20mA current loop, and 4-20mA receiver circuit.
Humidity sensors can be employed for measuring the moisture or water content in an environment
In certain applications—such as air conditioning, food processing, chemical manufacturing, and mining—ambient characteristics (temperature, humidity, pressure, airflow, etc.) need to be monitored for the safety of personnel and engineering equipment systems. Among the atmospheric quantities, humidity is the most difficult one to measure. Humidity influences physical, biological, and chemical processes, and it is a significant parameter to measure in industrial engineering systems. Depending on the harshness and hostility of the environment, wired or wireless sensor networks can be used to do this.
Humidity sensors are used to measure the humidity level of the environment under test. Humidity sensor circuits may be part of a subsystem of a larger project or process. These sensor designs must provide accurate results with high resolution.
Let’s explore the basics of humidity sensors and their design.
Humidity sensors are employed for measuring the moisture or water content in an environment. Usually, measurements are read as changes in electrical properties such as capacitance, resistance, etc. The measured analog electrical quantity is processed digitally using microprocessors, and thus the actual humidity value is evaluated.
Applications of Humidity Sensors
Humidity sensors are important in the following industries:
- Food processing
- Paper production
- Chemical manufacturing
- Weather monitoring
- Air conditioning systems
Types of Humidity Sensors
Unfortunately, there are only a few design-oriented humidity measurement circuits available and they tend to be expensive and complex. The humidity value is detected from the varying electrical quantities associated with the sensor type. The measured quantity can be absolute humidity or relative humidity. Humidity sensors can be classified into different types based on the parameter used for measuring humidity:
Capacitive humidity sensors - In capacitive humidity sensors, the relative humidity is measured by sandwiching a thin metal oxide layer between two electrodes. The electrical capacity of the metal oxide varies with the humidity and the variation in the capacitance corresponds to the humidity measurement. Capacitive humidity sensors are commonly used in weather applications.
Resistive humidity sensors - Resistive humidity sensors utilize a salt medium between the electrodes. With the humidity, the electrical impedance of the salt atoms changes and so does the resistance of the electrodes. The variation in the resistance of the electrodes represents the humidity value.
Thermal humidity sensors - In thermal humidity sensors, two thermal sensors are used to measure the thermal conductivity in dry air and moist air. One thermal sensor is immersed in dry nitrogen and the other in ambient air. The difference between the two thermal sensor measurements gives the absolute humidity value.
Humidity Sensor Circuit Design
The most essential element of humidity sensor circuit design is the humidity sensor. The humidity sensor interfaces the environment to the electrical system. The humidity sensor measures the physical quantity of humidity in the form of electrical properties such as capacitance, resistance, etc. Other circuits associated with humidity sensor circuit design are:
Analog sensor conditioning circuit: The conditioning circuit converts the variation of electrical properties in the humidity sensor into quantities such as voltage, current, frequency, etc.
Analog to digital converter (ADC): The analog signals from the sensor conditioning circuit are converted into digital signals to make them compatible with the transmitters in the downstream circuit.
Transmitters: Most humidity sensor circuits rely on microcontroller-based transmitters. The transmitter measures the digital signals from the ADC, makes necessary calculations on it, and outputs analog current signals within the range of 4-20mA. In certain cases, the ADC is integrated into the transmitter, which can handle mixed-signal inputs.
4-20mA current loop: The 4-20mA current loop forms a robust signaling standard. It connects the transmitter to the 4-20mA receiver. The 4-20mA current loop is formed by the series connection of the transmitter, receiver, and power supply. The accuracy of the measurement is maintained by choosing the 4-20mA current standard instead of the voltage standard.
Power supply: The transmitter is powered by a DC source. The DC source voltage is determined by the voltage drop in the 4-20mA current loop, especially the transmitter voltage requirement and receiver voltage drop.
4-20mA receiver: The 4-20mA receiver converts the 4-20mA analog current signal into a voltage signal for easy measurement. The receiver can be a simple resistor in the 4-20mA current loop. The voltage drop across the receiver circuit can be easily converted into humidity measurements by the controllers.
In chemical, medical, or ventilation systems, humidity sensor circuit design is critical, as humidity is vital in altering process flow. To design the circuits associated with humidity sensor networks, Cadence’s PCB design software can be utilized. With Cadence’s PCB layout tools, you can quickly finish the design of a high accuracy humidity sensor circuit.
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