….the electrical conductivity measurement on a nutritious solution has a direct relationship with the amount of dissociated solid materials that there are dissolved in it.

         Conductivity is the measurement of the capacity that a material has to lead the electrical current. Nutritious solutions contain loaded ionic particles and therefore they have this ability. Whichever is greater the conductivity of the resulting solution, the amount of these ions dissolved in the water is greater. Therefore the measurement of the electrical conductivity on a nutritious solution has a direct relationship with the amount of dissociated solid materials that there are dissolved in it.

      Conductimeters are apparatuses used to measure conductivity. Basically this conductimeters are instruments made up of two plates of a special material (platinum, titanium, nickel covered with gold, graphite, etc.), a nourishing power source and a measurement scale. Applied a tension between the conductimeter's plates, it measures the amount of current it pass through them.

     With the values of the applied voltage and the electrical intensity of the current that pass through the plates, conductimeters determine, according to their previous calibration, the conductivity of the tried sample.

     There are many types of conductimeters and the conductivity values are related to the cell geometry of each apparatus. Each one really measures a specific conductivity which is the product of the really measured conductivity multiplied by the cell's constant of the same one. This constant is a relationship between the surface of the plates and the distance to which are.

     Conductivity measurements also is dependent on the sample's temperature during the test. It is for that reason that most of these conductimeters have automatic devices to compensate temperature changes. Other electrical conductivity dependencies are not important for their consideration in hydroponics.

     The well known Ohm's law tell us that when somebody apply a tension between two points of a conductor a circulation of electricity is produced being this current directly proportional to the applied voltage and inversely proportional to the conductor's resistance. Thus:

where
I is the current intensity measured in Amperes
E potential tension in Volts, and
R the resistance in Ohms.

      Purest water doesn't carry electrical current. However, when water contains an electrolyte dissolved in it becomes a conductor where the resistance is proportional to the distance between electrodes and inversely proportional to the  area of them. Thus:

R = r.L/A 

where
r is the specific resistívity measured in Ohm/cm 
L is the distance between electrodes in cm, and
A is the area of these in cm2.

The inverse of specific resistivity is the specific conductivity (k). This is:

 k=1/r 

and therefore its unit of measurement stays defined as Ohm-1.cm-1. 

     Hystoricaly being Ohm-1 the inverse of resistance unit it was called mho, the original word written other way around. Thus,the specific conductivity unit was left to mho/cm. 

     In order to simplify, during the last decades of the XX century, the mho unit was changed to be denominated Siemens assigning the capital letter S as a symbol to it. Finally the specific conductivity unit stay as S/cm.

     For many this page's readers everything described here can seem a great tangle. But this explanation, complex for most people, has been necessary since there are lots of calibrated measuring instruments using in different form all these units and submultiples of them. There are a lot of confusion between the users. As corolario the following summary goes: 

î    when electrical conductivity of a nutritious solution is measured,  the specific conductivity of that dissolution is determined.

î    the unit of measurement of this specific conductivity is the Siemens/cm or S/cm. 

î    equivalences between units and the forms in which frequently they are expressed are the following ones:

     To practical effects the four emphasized units in the red line above are those that hydroponics fans can find in their apparatuses. Most common they are EC (electrical conductivity) and ppm (parts by million). The relationship with this last one is empirical and approximate since there are certain interferences on the ions loads transport of their charges. Explanation is much more complex than this one but it is unnecessary to the practical effects. In order to express equivalence in parts per millon of disolved solids, diverse authors agree in assigning a number between 0.600 and 0.650, being 0.640 sufficiently accepted, by each 1 EC. Calibration apparatus in ppm use these empirical factors.

    CF unit spread mainly in Australia and Nueva Zelanda is 100 times greater that the EC unit. 

    On the other hand, the unit dS/m often can be seen mainly in papers from USA researchers.