Controlling Irrigation Systems with a Soil Humidity Sensor

Photo by Fokko Muller.

When irrigating gardens or lawns, there is a possibility of over-watering. However, there are means of ensuring that plants get the right amount of moisture using a soil humidity sensor. The sensors are designed to use an electrical charge to measure soil moisture and send a signal to the irrigation controller that will either turn it on or off. The sensors can be installed with the irrigation systems or even later.


Before installing the sensor:

  • Determine the moisture level that the plants and soil in the garden require. Most soil sensors come with a guide providing such information. Another way of knowing the required soil humidity level is through trial and error.
  • Select the location for the soil humidity sensor. The soil sensor is normally positioned at the center of the irrigated area. However, one should ensure that the sensor is installed at the right height; it should not be too high or low. These areas will make the sensor record false readings which will lead to improper watering. Installing the soil humidity sensor in a high place will lead to over watering while a low area will lead to under watering.
  • While installing the humidity sensor, consider that multiple sensors can be installed in different parts of the garden but they will still control the entire irrigation system.
  • When installing multiple sensors, ensure that they are at least 12 meters ~ 40 feet apart.
  • In most cases, one sensor is suitable for every 20,000 square meters ~ 5 acre of land.
  • Wires that connect the sensors to the control panel should be buried below the ground. However, the sensor should not be buried deeper than 10 cm ~ 4 inches.

Types of soil humidity sensors

Modern humidity sensors measure the difference between electrical charge and temperature between moist and dry air.

Capacitive humidity sensors are made of a thin polymer dipped in a glass or a ceramic substrate placed between two electrodes. Changes in the capacitance of the film are proportional to the relative humidity. This type of sensors can calculate dew points and produce the results between 30 to 60 seconds. They can operate at temperatures of up to 200 degrees Celsius ~ 392 degrees Fahrenheit.

Resistive humidity sensors work on the fact that when substances get wet, they undergo changes in their electrical resistance. This type of sensors may be made of silver, platinum or gold wound around plastic or ceramic tube. The response time of this sensor is between 10-30 seconds and can operate at temperatures of up to 100 degrees Celsius ~ 212 degrees Fahrenheit.

Thermal conductivity humidity sensors are designed to work on the principle that dry gas mixture has higher heat conductivity than gas that is water saturated. This type of sensors has a quick response time and can operate at temperatures of up to 302 degrees Celsius ~ 575 degrees Fahrenheit.

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