Humidity sensors transform the moisture content of the air, gases, bulk materials, or soil into an electric output signal.
More people are looking for humidity sensors with improved accuracy, higher repeatability, and longer lifespans. The usage of wireless humidity sensors is expanding for industrial, home, and even greenhouse humidity monitoring.
Every time the rain falls on the earth, the air immediately around us becomes damp. It appears as though the rainwater is floating everywhere. In certain cutting-edge ACs, all you have to do is press a few unique buttons, and after a little while, everything is crystal clear. What causes everything to happen, and how? Because of the formation of moisture in the air, our surroundings feel humid to us. Your AC system's humidity sensor catches up to this moisture and completely removes it. Okay, yeah! Let's now discuss how this sensor functions.
Humidity Sensor: What Is It?
An electronic device called a humidity sensor can measure the amount of evaporated water present in any medium and turn it into an analog signal. On the one hand, these sensors can be found in larger embedded systems and come in a variety of sizes and functionalities similar to handheld devices.
What Kinds of Humidity Sensors Are There?
There are three primary kinds of humidity sensors.
- Sensing Capacitive Humidity
- Sensor for Resistive Humidity
- Sensor for Thermal Humidity
Sensing Capacitive Humidity
A tiny bit of metal oxide is sandwiched between two electrodes in a capacitive humidity sensor to measure relative humidity. The electrical limit of the metal oxide varies with the relative humidity of the atmosphere. Water that has been vaporized can enter this layer, affecting the capacitance.
The direct, 5%–95% relative humidity range is measured by capacitive type sensors. Placing the sensor in the path of moving air enables it to respond more quickly since the sensor uses evaporated water from all around. The sensor features a thermistor that is used for adjusting as the relative humidity is dependent on the temperature.
The Way a Capacitive Humidity Sensor Works
An air-filled capacitor can be converted into a straightforward capacitive humidity sensor by altering its permittivity in response to atmospheric moisture. However, using air as a dielectric for real-world applications is impossible.
Because of this, the space between the capacitor's plates is typically filled with a suitable dielectric material, whose dielectric constant changes depending on the humidity.
A hygroscopic polymer film is often used as the dielectric and two layers of electrodes are deposited on either side to create a capacitive humidity sensor.
Observing changes in the frequency of an oscillator built using a capacitor with a humidity-sensitive test subject as the dielectric is another technique to employ capacitive humidity sensors. Pharmaceutical items frequently use this configuration.
In the LC oscillator circuit, the test samples, such as pharmaceutical tablets, are sandwiched between two plates (which serve as the capacitor electrodes). Concerning the humidity around the test sample, the oscillator's frequency fluctuates.
Let's examine the design of a thin capacitive humidity sensor based on a thermostat polymer film. On a silicon substrate, it is constructed. Two metal electrodes—made of aluminum, platinum, or chromium—are placed on this substrate. These electrodes have been cut out such that they form an interdigitated pattern.
A dielectric layer is then put on top of this layer. The capacitive humidity sensor is shown in the following image from the top and in the cross-section. To offer temperature compensation, observe that two temperature-sensitive resistors are placed on the same substrate.
Capacitive humidity sensors have advantages.
- Nearly linear voltage is produced.
- Over extended use, they deliver consistent outcomes.
- It can identify a variety of RH.
Using capacitive humidity sensors
- Systematic HVAC
- A weather station
- Processing Food
- Ovens, refrigerators, etc.
Sensor for Resistive Humidity
An electrical impedance change in a hygroscopic media, such as a conductive polymer, salt, or treated substrate, is measured by a resistive humidity sensor.
Interdigitated windings serve as the foundation for resistive sensors. After a coating made of hygroscopic polymers is applied, their resistance to humidity changes in an opposite manner. The relationship between the impedance change and humidity is typically inverse exponential.
Its other name is RH sensor.
The Function of a Resistive RH Sensor
The materials that make up the Resistive RH Sensor typically have low resistivities, and these resistivities change dramatically as humidity levels change. The relation between resistance and humidity is inverse exponential. Over two electrodes, the low resistivity material is deposited.
To maximize the contact area, the electrodes are arranged in an interdigitated pattern. When the top layer absorbs water, the resistivity between the electrodes varies, and this change may be detected using a straightforward electric circuit.
Salt, specially treated substrates, solid polyelectrolytes, and conductive polymers are a few of the materials that are frequently employed. Modern resistive RH sensors have a ceramic coating to add an extra layer of protection. Typically, noble metals like gold, silver, or platinum are used to create the electrodes for the sensor.
Resistive RH Sensor Benefits
- Low price
- Little Size
- There may be a significant distance between the sensor and the signal circuit.
- Since there are no calibration standards, they are very interchangeable.
Resistive RH Sensor Benefits
Chemical vapors and other pollutants can be detected by resistive RH sensors.
When utilized with goods that are water soluble, the output measurements could change.
Applications of Resistive RH Sensor
Sensors for measuring humidity that is electrically conductive or resistive are inexpensive and have a compact design. They are frequently employed in a variety of commercial, home, and industrial applications.
Sensor for Thermal Humidity
Since they detect absolute humidity, thermal humidity sensors are sometimes referred to as absolute humidity (AH) sensors. Sensors that detect thermal humidity gauge the thermal conductivity of both dry and water-vaporized air. Absolute humidity can influence how the different thermal conductivities differ from one another.
The Thermal Humidity Sensor's Operation
A thermistor is the best component to use for thermal conductivity-based sensors. As a result, a bridge circuit is created using two small thermistors with a negative temperature coefficient.
In that, one thermistor is hermetically sealed inside a space that is dry nitrogen-filled, and the other is exposed to the outside world via tiny venting holes. The resistance of the two thermistors is computed when the circuit is turned on, and the difference between those two values is exactly proportional to the absolute humidity (AH).
Thermal Humidity Sensor Benefits
- Suitable for highly corrosive and high-temperature conditions.
- It is very robust.
- Higher resolution than other types
Thermal humidity sensor applications
- Dry-bulb kilns
- Plants that make medicines
- Dryers and machines for drying clothes
- Dehydration of food
Parameters for Basic Humidity Sensors
Now that you are familiar with the many types of humidity sensors, their operating principles, and their uses, let's speak about some of their characteristics.
Each sensor's calibration curve is unique and is based on a 9-point scheme. In essence, it contrasts the advantages and disadvantages of the specific sensor.
It displays the observed output voltage value converted to relative humidity as well as the voltage divergence from the BFSL value.
The sensor frequently loses sync as a result of the measurements. However, a sensor must deliver accurate readings to be useful.
A sensor's measurements must be accurate enough to prevent drift. The measurement of drift between measurements of a particular quantity is known as repeatability.
The response time of a sensor is often measured as the amount of time it takes for it to increase to 66% or fall to 33% of its maximum output voltage.