Hi, I'm new to this forum, so mind me if I'm posting in the wrong area. I am currently in the process of trying to build a conductivity sensor. I have found this tutorial, but am questioning if they are actually finding conductivity. This is because the "conductivity" is being measured through an analog pin. To my knowledge that only measures digital voltage and not conductivity. So, if I am right, what would be a correct way of obtaining this measurement and if I am wrong, I would appreciate any sort of explanation why.
Yes, the analog in does indeed measure voltage. It takes the tiniest, most minute amount of current from the "thing" it's connected to and determines the voltage of that thing. But that's how electricity works. Unless that thing is very, very, very weak (a high-impedance node), the value it senses is a fairly accurate reading of the voltage at that pin. Usually to within 5 milli-volts or less because of it's resolution.
The thing is, conductivity is a property of a material. In order to determine the conductivity of that material, we need to attempt to push some current through it. It's not much different than determining the color of an apple. We have to bounce light off of it to see what color it is. We don't have the technology to simply know what the color is by some magical telepathy machine.
So conductivity is a property that states how easily or difficult it is to allow for the flow of electrical current. It's kind of like saying the resistance of the material. Somewhere, somehow, for a current to flow, there needs to be a voltage. That voltage provides the "push" that electrons need to make them move, to flow, to create a current. Voltage is a quantity of electrical pressure, so that's what provides a push.
So somewhere in your project there must be another lead connected to that material. There will be a voltage difference, and that voltage - also called an electromotive force - will cause some amount of electron current flow to or from that voltage lead and the other lead. The more conductive that material is, the more current will flow. And that's how we determine conductivity. We examine the amount of current flow. On the Arduino we measure voltage on an analog pin. When we connect an analog input pin to that material, the voltage basically appears on all parts of the conductor. From that voltage, the Arduino sips a tiny, tiny amount of charge off of that as a tiny, tiny amount of current flows into the Arduino.
The more conductive the material, the more voltage appears on the measurement lead. The more voltage appears on the measurement lead, the more it appears on the analog input pin. The more voltage appears on the input pin, the more current flows into the Arduino's microprocessor. So there is a direct link, or chain, from the material's conductivity and the input pin of the Arduino.
Thanks for responding. So, I am supposed to find conductivity in order to find concentration of a liquid solution(NaCl + Water). From what you said I have to have another lead somewhere. So if i have a negative and positive lead (analog attached to positive lead) and i put it in the solution, i will get a result in digital volts. The conductivity of that solution will determine the returned voltage, but in order to get concentration(using an equation), I need a number corresponding to conductivity. From there, where should I go. Sorry, if that answer is in what you said. I am pretty new to this.
Here is one way to look at this. You want to measure conductance and the SI (System International) unit of conductance is Siemens. If we want to get fancy Conductance is symbolised by the letter G and represented in mhos or Siemens units. Conductance equals the inverse of resistance mathematically: G = 1/R . Conductance is the extrinsic property, whereas the inherent property is conductivity. When all is said and done you want to measure conductance and the unit of measure is Siemens.
Conductance and Resistance are reciprocals so with that in mind G = 1/R and R = 1/G
Keep a few things in mind. The Arduino Uno is a 10 bit ADC device. The example code linked to uses a known resistor value of 1.0 K Ohm and compares it to an Unknown value using a simple voltage divider circuit. In the example we have a known Resistance and a known Voltage applied.
Just remember the Arduino limitations when setting up your circuit.