What Is a Moisture Sensor?
Moisture sensors are sensors used in a wide range of fields to measure the amount of moisture in wood, coal, food, fiber, and other materials.
The measurement methods used include electric capacitance, electric resistance, microwave, near-infrared, and neutron. Electricity, neutrons, and near-infrared rays are applied to the object to be measured, and the moisture sensor detects and determines the amount of moisture.
Although it depends on the method, there are two main types: those that directly contact the sample for measurement and those that allow non-contact measurement.
Applications of Moisture Sensors
As mentioned above, moisture sensors are used to measure the moisture content of various materials and are utilized by manufacturers and factories to measure the moisture content of wood, paper, coal, food, fiber, and other materials. It is often used to check the quality of products before they are shipped.
Soil moisture sensors are especially used to check the amount of moisture necessary for the growth of agricultural crops. They are also sometimes used by cosmetics manufacturers to measure the moisture content of skin.
Moisture sensors can thus be used to measure the moisture content of various materials, but it is important to select the right sensor and model for the application and object.
Principles of Moisture Sensors
The main measurement methods include the following
1. Electric Capacitance Type
In the electric capacitance method, an alternating current is applied to the measurement target from an electrode, and the change in capacitance, or electrical capacitance, is converted into the amount of moisture. In the oscillation circuit, which is the detection circuit, the change in electrode capacitance causes the oscillation frequency to change.
The larger and thicker the object being measured, the greater the change in electrode capacitance. The advantage of this method is that it is easy to miniaturize and does not damage the object to be measured. Since it is affected by thickness and density, it is necessary to measure under the same conditions.
2. Electrical Resistance Type
Moisture sensors of the electrical resistance type conduct electricity to the object to be measured and convert its resistance value into the amount of moisture. Since it is not easily affected by the density of the object to be measured, it is suitable for measuring objects whose density easily changes.
3. Microwave Type
This method measures the moisture content based on the attenuation of microwaves, a type of electromagnetic wave when they are irradiated onto the object to be measured. It can quickly measure the moisture contained inside the object but requires the installation of a large device.
4. Near-Infrared Type
Near-infrared rays are light with wavelengths longer than visible light and outside of red. Near-infrared moisture sensors irradiate near-infrared rays, which are light with wavelengths longer than visible light and outside of red, to the object being measured. Since moisture absorbs near-infrared light well, the greater the moisture content, the less light is reflected. Using this principle, the amount of moisture is determined from the reflectance of the irradiated near-infrared light.
The advantage of this method is that it is easy to install and does not damage the object to be measured. The disadvantages are that only a small portion of the surface can be measured and that it is affected by the color of the object being measured. Wide-area irradiation type, fiber type, and other forms are available.
5. Neutron Type
Neutron-type instruments use a source of neutrons, a type of radiation. Fast neutrons lose energy when they repeatedly collide with hydrogen atoms and are transformed into slow thermal neutrons. In other words, the amount of fast neutrons converted to thermal neutrons is proportional to the amount of hydrogen, so it is possible to calculate water content using a neutron source that emits fast neutrons and a detector sensitive to thermal neutrons.
This method can also measure the water content inside. On the other hand, it should be noted that the equipment has a short life span and is fragile. Also, since it handles radiation, it requires specialized knowledge.
6. Dry Weight Loss Method
In the loss-on-drying method, a sample of the measured material is taken and dried using a convection dryer or a heat source (halogen lamp, microwave, etc.), and the moisture content is determined from the weight of the sample before and after drying. This method is said to be highly accurate, but it should be noted that sample collection is required and that this is not a nondestructive test.
Types of Moisture Sensors
Moisture sensors, as mentioned above, use a variety of principles. Many of the actual products, such as “Soil Moisture Sensors” and “Concrete and Mortar Moisture Sensors,” have limited applications envisioned in advance and should be selected according to the intended use.
In addition, there are a variety of shapes and sizes, from tabletop and portable machines to relatively large machines, so there is a wide range of types to choose from depending on the situation in which they are used.