Design Of Capacitive Soil Moisture Sensor

ABSTRACT

There are techniques available for the determination of soil moisture content as a tension or a volume is offered. The method of determining soil moisture content by the different techniques is described with attention given to the neutron probe (NP), time-domain reflectometry (TDR) and frequency domain (Capacitance) techniques in particular. The choice of instrumentation for soil moisture determination will depend on the consideration of factors such as: physical limitations of different techniques; the level of information required (either an absolute or relative moisture measurement); the amount of data needed to objectively decide upon an irrigation regime (with consideration to spatial and temporal problems); the initial cost of the instrument and sampling; the reliability of the instrument and the collected data; and, the ease of use of the instrument in the field. However, this work focuses on capacitive sensor in particular.

CHAPTER ONE

1.0                                                        INTRODUCTION

Soil moisture sensors measure the volumetric water content in soil. Since the direct gravimetric measurement of free soil moisture requires removing, drying, and weighting of a sample, soil moisture sensors measure the volumetric water content indirectly by using some other property of the soil, such as electrical resistance, dielectric constant, or interaction with neutrons, as a proxy for the moisture content. The relation between the measured property and soil moisture must be calibrated and may vary depending on environmental factors such as soil type, temperature, or electric conductivity. Reflected microwave radiation is affected by the soil moisture and is used for remote sensing in hydrology and agriculture. Portable probe instruments can be used by farmers or gardeners.

This soil moisture sensor measures soil moisture levels by capacitive sensing rather than resistive sensing like other sensors in the market. It is made of corrosion resistant material which gives it an excellent service life.
Insert it in to the soil around your plants and impress your friends with real-time soil moisture data! This module includes an on-board voltage regulator which gives it an operating voltage range of 3.3 ~ 5.5V. It is perfect for low-voltage MCUs, both 3.3V and 5V. For compatibility with a Raspberry Pi it will need an ADC converter. This sensor is compatible with our 3-pin “Gravity” interface, which can be directly connected to the Gravity I/O expansion shield.

1.2                                             OBJECTIVE OF THE PROJECT

Objective soil moisture measurement can be undertaken with simple tools, such as a shovel, or complex tools that record measurement of soil moisture on a volumetric basis. The method of measurement is simply a device allowing moisture determination in an objective fashion. It is important that measurements are made regularly and recorded systematically to allow improvement in irrigation scheduling and soil/plant management decisions. The main objective of work is to design a capacitive type of soil moisture sensor.

1.3                                         SIGNIFICANCE OF THE PROJECT

Capacitive measuring has some advantages:  it possible to avoid corrosion of the probe, capacitive measuring also gives a better reading of the moisture content of the soil as opposed to resistance measuring. In the latter one doesn’t really measure moisture (as water is a bad conductor of current), but in fact one measures the ions that are dissolved in the moisture. Adding fertilizer for instance will decrease the resistance of the soil, even though no water is added. Capacitive measuring basically measures the dielectrum that is formed by the soil and the water is the most important factor that forms that dielectrum. Capacitance determines by measuring the charge time of a capacitor, which uses soil as a dielectric medium. The capacitance sensor forms a pair of electrodes and the soil acts as a dielectric. The capacitor charge time is a linear function of the dielectric permittivity of the soil

1.4                                             APPLICATION of the project

Agriculture

Measuring soil moisture is important for agricultural applications to help farmers manage their irrigation systems more efficiently. Knowing the exact soil moisture conditions on their fields, not only are farmers able to generally use less water to grow a crop, they are also able to increase yields and the quality of the crop by improved management of soil moisture during critical plant growth stages.

Landscape irrigation

In urban and suburban areas, landscapes and residential lawns are using soil moisture sensors to interface with an irrigation controller. Connecting a soil moisture sensor to a simple irrigation clock will convert it into a “smart” irrigation controller that prevents irrigation cycles when the soil is already wet, e.g. following a recent rainfall event.

Golf courses are using soil moisture sensors to increase the efficiency of their irrigation systems to prevent over-watering and leaching of fertilizers and other chemicals into the ground.

Research

Soil moisture sensors are used in numerous research applications, e.g. in agricultural science and horticulture including irrigation planning, climate research, or environmental science including solute transport studies and as auxiliary sensors for soil respiration measurements.

Simple sensors for gardeners

Relatively cheap and simple devices that do not require a power source are available for checking whether plants have sufficient moisture to thrive. After inserting a probe into the soil for approximately 60 seconds, a meter indicates if the soil is too dry, moist or wet for plants.

1.5                                           LIMITATION OF THE PROJECT

Capacitive sensors don’t measure earth’s resistance. Also, there must be a resistor between the driving pin and the capacitor. One way is to use an RC circuit, and to measure the voltage on the positive terminal of the capacitor. Other limitation includes:

• Operating Voltage: 3.3 ~ 5.5 VDC
• Output Voltage: 0 ~ 3.0VDC
• Operating Current: 5mA
• Interface: PH2.0-3P
• Dimensions: 3.86 x 0.905 inches (L x W)
• Weight: 15g

1.6                                     ADVANTAGES OF WATER SENSORS

• Maintaining adequate soil moisture is needed, especially during critical crop growth stages to help achieve maximum yield potential.
• Soil water content measurements with water sensors can be used as a guide to determine the timing and amount of irrigation.
• Soil moisture measurements can be used in combination with ET-based irrigation scheduling.

The goal of irrigation management is to use water in the most profitable way, while maintaining yield potential at sustainable production levels. Irrigation needs to be applied only when soil moisture measurements and crop growth stage warrant water inputs, and with a method that limits any waste or excess.