How soil responds to electricity

How soil responds to electricity

Soil Moisture Sensor Project

How soil responds to electricity

To understand the sensors I read about when reviewing previous projects, we need to know how soil responds to electricity. Soil can either resist the flow of electricity or can “store” electrical energy.

Note: I have written more about the dielectric properties of soil compared to how soil resists the flow of electricity. This is because I am not going to use resistive sensors.

Resistance

We all know that dry things don’t conduct electricity as well as wet things. This means that resistance to the flow of electricity decreases as you add moisture to the soil.

You could take advantage of the fact that soil resists the flow of electricity by using soil as a resistor. But in order to do this you need to leave parts of your circuit uninsulated. They will corrode.

Dielectric properties

The other way soil responds to electricity is to “store” electrical energy. Meet your new friend: relative permittivity, also known as the dielectric constant.

It isn’t dangerous. In fact, if you are using a touch screen to read this there are sensors in or just below the glass of your screen. These sensors respond to the presence of your finger. It is probably not responding to the pressure your finger exerts on the screen. To prove this, take the blunt end of a pencil and see if you can get your touch screen to respond. I take no responsibility for scratched or broken screens. You can’t, unless you have a very old touch screen, something like the screens at an old ATM.

What I gathered from reading about touchscreens is that there are multiple technologies behind the touch screen which I won’t go into in this blog. All of these technologies rely on the fact that your finger responds much more to the presence of electricity compared to air or the pencil you just used to scratch your screen. This is because your body (being mostly made of water) has a higher permittivity or dielectric constant than air or pencils. Here are some of the sites I read: this one, another one, that one, yet another one, and the last one.

The great bit about measuring relative permittivity is that the sensors don’t need to be in direct contact with the material being measured. This means that the probes that corroded until they were useless in the resistive sensor were sacrificed in vain. The sensors can be insulated.

If you understand the phrase “permittivity describes the amount of charge needed to generate one unit of electric flux in a particular medium”, perhaps skip the rest of this post and jump to the methods that I rejected. If this term doesn’t make sense, read on!

Permittivity

Permittivity (also known as the dielectric constant) is a measure of how much the particles in your finger, the air, or a pencil respond to the presence of electricity. Strictly speaking, they are responding to an electrical field. But I never studied electrical engineering, I am building a network of garden sensors to control watering.

The Wikipedia pages on “Dielectric” and “Permittivity” are very technical. After a few paragraphs of Greek mathematical symbols and equations I was hopelessly lost. If you want to know more but don’t want to read pages of equations, have a read of Britannica ¬†and physics.info. The bit that I found important was the fact that the relative permittivity (also known as the “dielectric constant”) is a measure of how much a material responds to the presence of electrical energy compared to air (well, actually compared to a vacuum). If the relative permittivity is 2, then the material is twice as reactive as air.

So how is permittivity useful?

Permittivity is important to an electronic component called a capacitor. A capacitor is a gap filled with a “dielectric medium” that separates two pieces of metal. A “dielectric medium” is just material that responds to the presence of electricity, such as soil. This can be used to store electric energy. The amount of electric energy that can be stored is determined by the permittivity of the dielectric medium. The medium can store more energy if it has a higher permittivity. The amount of energy that can be stored is measured in farads, named after Michael Farady.

Every substance has a different dielectric constant (or relative permittivity). The relative permittivity of air is 1, dry soil is somewhere between 3 and 5 depending upon the soil, and water is 80.

What does this mean for soil moisture?

What this means for soil is that as you add water, you increase the relative permittivity of that soil. I found two references that provide some examples of how this changes as you increase the water content.

  • http://pe2bz.philpem.me.uk/Comm/-%20Antenna/Info-905-Misc/soildiel.htm
    • Each section is a different soil type and different moisture content. The first column for each section is the frequency, the second column labelled E/Eo is the dielectric constant, and I have no idea what the last column means.
    • If you look for the same frequency across different soil moisture levels you will notice that the dielectric constant changes. For example, looking at the bottom row (the one with the frequency of 1.0E10 (which is a fancy way of saying 1.0 * 10 to the power of 10 which equals 10,000,000,000) of each section for Sandy Soil
      • 2.18% water = 2.500
      • 3.88% water = 3.600
      • 18.8% water = 13.00
  • https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19790010347.pdf
    • Page 24 of the document (page 28 of the PDF) shows a graph of how the dielectric constant increases as the soil moisture content increases.

How do you measure this in soil?

As I was researching methods for measuring the dielectric properties of the soil, I found out that there are two sets of methods; time domain reflectometry and capacitance measurements.

Note: I haven’t written much about time domain reflectometry because a) it is very complicated, and b) it is expensive. I ended up choosing to use a method that is based on using soil as a capacitor.

What is a capacitor?

A capacitor is a simple electrical component made from two plates of metal (electrodes) that are near each other but are not touching. When you connect one piece of metal to one side of a battery and the other piece of metal to the opposite side of the battery, and bring them close together, the flow of electricity will want to jump across the gap but can’t. Since they can’t jump across the gap but are very close to each other:

  1. one piece of metal will become positively charged while the other becomes negatively charged, and
  2. an electric field is created between the pieces of metal. An electric field can be thought of as the attraction that is causing the electricity to want to jump across that gap.

What this means is that you can store an electric charge in the capacitor to be released at a later time. The amount of charge you can store depends upon:

  1. the area of the electrodes (more area means more charge can be stored between the plates),
  2. the permittivity of the stuff in between the electrodes (higher permittivity means the electric field can pass through more easily, which means more charge), and
  3. the distance between the electrodes (further apart means that the field has to travel further, which means less charge)

If all of this seems like voodoo, let’s use our kitchen magnet as an analogy. Electricity and magnetism are so closely related, you can’t have one without the other. If you have two magnets near each other, there will be an magnetic field between the opposing poles of the magnets. So:

  1. larger magnets will pull together more strongly than smaller magnets
  2. pieces of iron between the magnets will transmit the magnetic field better than air, and
  3. the further apart the magnets are, the less they pull on each other.

Recall that adding water to soil increases the soil’s relative permittivity (a.k.a dielectric constant). Also recall that a higher relative permittivity means that you can store more electrical energy. This means you can use soil as the dielectric medium of a capacitor. If you make a capacitor out of two metal plates with soil in between them, the value of that capacitor will change depending upon how moist the soil is.

If you hadn’t already guessed, I decided to use soil as a capacitor.

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