how does soil moisture sensor work?
soil moisture sensor types
Soil moisture sensor work based on different physical principles to measure the amount of water present in the soil. The following are the working mechanisms of some common types:
Capacitive Soil Moisture Sensors
Basic Principle
These sensors work because water has a much higher dielectric constant than air and most soil materials. A capacitive sensor has two or more electrodes that form a capacitor. When soil is in contact with the sensor, the water in the soil affects the capacitance between the electrodes.
Measurement Process
The sensor measures capacitance. It uses the known link between capacitance and soil moisture to find the water content in the soil.
When soil moisture increases, the capacitance value also goes up. Someone then changes this value into a digital or analog signal. A connected device can read and understand this signal.
Resistance Soil Moisture Sensors
Basic Principle
The electrical resistance of soil changes with its moisture content. Dry soil has a high electrical resistance because there are fewer ions available to conduct electricity. As the soil gets wetter, water helps electrical current flow more easily. This reduces resistance.
Measurement Process
The sensor sends a small electrical current through the soil. It does this between two electrodes and measures the resistance that results. Researchers then correlate the resistance value with the soil moisture level. Lower resistance values indicate higher soil moisture, and researchers use this relationship to determine the moisture content.
Time Domain Reflectometry (TDR) Sensors
Basic Principle
TDR sensors send an electromagnetic pulse through a waveguide. This waveguide is often a pair of metal rods or a coaxial cable. They place it into the soil.
The pulse moves through the waveguide. It reflects back when it hits a change in the medium’s electrical properties. This change can happen at the boundary between air and soil or between soil with different moisture levels.
Measurement Process
We measure the time it takes for the pulse to travel and reflect back. The speed of electromagnetic waves in soil depends on the soil’s dielectric constant. This constant relates to the moisture content. By measuring the time delay of the reflected pulse, the sensor can calculate the soil moisture content.
Frequency Domain Soil Sensors
Basic Principle
These sensors emit an electromagnetic signal at a specific frequency into the soil. The signal interacts with the soil and the water molecules present. The water molecules in the soil cause a shift in the frequency of the reflected signal.
Measurement Process
The sensor measures the change in the frequency of the reflected signal. Researchers relate this frequency shift to the soil moisture content through calibration curves. Different frequencies can suit various types of soil and the desired measurement range.
Gravimetric Soil Sensors
Basic Principle
Gravimetric soil moisture sensors measure the actual weight of water in a given volume of soil. The basic concept relies on the fact that the weight of the soil sample changes as the amount of water in it changes.
Measurement Process
A technician collects a known volume of soil and measures its wet weight. Researchers then dry the soil sample, usually in an oven, until they remove all the water and measure the dry weight. The difference between the wet and dry weights gives the weight of the water in the soil sample. Researchers calculate the soil moisture content as the ratio of the weight of water to the weight of the dry soil.
how to install an automatic soil moisture sensor?
Installing an automatic soil moisture sensor involves several key steps, from preparation to calibration. Here is a detailed guide:
Preparation
Gather Materials and Tools: You’ll need the soil moisture sensor, appropriate cables or connectors, a data logger or control unit if required, a power source, and tools like a shovel, screwdriver, and multimeter.
Select the Installation Location: Choose an area that is representative of the soil you want to monitor. Avoid areas near water sources, drains, or where there is frequent human or machinery traffic. For example, in a garden, select a spot away from the edge and not directly under a downspout.
Ground Installation
Dig a Hole: Use a shovel to dig a hole to the appropriate depth for your sensor. The depth depends on the type of soil and the plants you are monitoring. For most garden plants, a depth of 10-20 cm is often suitable.
Insert the Sensor: Gently place the sensor in the hole, ensuring that it is vertical and in good contact with the soil. Some sensors may come with a protective casing or mounting bracket to help with this.
Backfill the Hole: Fill the hole with the dirt you dug out. Press it down gently to remove air pockets. This helps the soil-sensor make good contact.
Above-Ground Installation
Mount the Sensor: If the sensor is designed for above-ground use, attach it to a stake or pole using the provided mounting hardware. Position the sensor so that the sensing element remains close to the soil surface without touching it.
Connect Cables: If the sensor needs outside power or data, connect the right cables as the manufacturer says. Use waterproof connectors if necessary to protect against moisture.
Connect to Power and Data Logging Device
Power Connection: Connect the sensor to the power source. This could be a battery, solar panel, or mains power. Follow the wiring diagram provided. Use a multimeter to check the voltage and ensure the power supply is correct.
Data Logging Connection: If you use a data logger or control unit, connect the sensor with the right cables. Use the correct communication method, like USB, RS485, or wireless. Configure the data logger to set the sampling interval and other parameters.
Testing and Calibration
Initial Testing: Before you bury or secure the sensor, do a quick test. This will ensure it is working properly. Check the readings on the data logger or connected device and ensure they are within the expected range.
Calibration: Many soil moisture sensors require calibration to ensure accurate readings. Follow the manufacturer’s instructions for calibration. This may include exposing the sensor to known moisture levels. Then, adjust the calibration settings as needed.
Final Checks
Check Connections: Once installed, double-check all connections to ensure they are secure and free from damage.
Test Data Transmission: Send a test signal or sample data. This checks if the sensor is working well with the data logger or control unit.
CDT-22B Soil Moisture & Temperature Sensor
CDT-22B Soil Moisture & Temperature Sensor to integrate the moisture and temperature measurement. The stainless steel probe is inserted into soil surface or soil profile to test soil moisture and temperature quickly.
