Because there have been numerous articles published on this subject, in addition to covering the basics I have included some additional detail below to help increase the understanding of the fundamentals involved.

A “Conductivity Meter” or “EC Meter” (also known as “mS”, “cF” or “TDS” meter) is a device used to help monitor the concentration of nutrient solutions.

This article attempts to provide simple explanations of:

1.  What is conductivity?

2.  How is conductivity measured?

3.  The factors affecting the conductivity value

4.  Operating, calibrating and cleaning conductivity meters

5.  The uses of conductivity in hydroponics.

6.  The limitations of measuring TDS (ppm) from conductivity measurements.

7.  What to look for when purchasing a conductivity meter.

8.  Conclusion – conductivity and hydroponics

1.  What is Conductivity?

‘Conductivity’ represents the ease with which a solution conducts electricity.  Numerically it is measured in units called Siemens. 

Liquids such as petrol and pure water are essentially non-conductors of electricity. Further, many water soluble substances, like sugar, alcohol, etc., when dissolved in water do not conduct electricity.  This is because such solutions contain very few electrically charged particles (ions) to transport the electric current through them. However, solid substances, known as salts (e.g., sodium chloride, potassium nitrate, etc.), when dissolved in water yield such ions which permit the flow of electricity through the solution.  Further, the greater the concentration of ions the greater the ease with which the solution carries a current and the higher the conductivity.

2.  How is conductivity measured?

How conductivity meters work:  A conductivity meter is essentially an “amp meter”.  Two plates made of inert metal (e.g palladium coated platinum) are placed in the sample, an alternating current voltage of around 1,000 cycles per second is applied across them and the current is measured. See Figure 1.

Conductivity (G), is the inverse of resistivity (R) and is determined from the voltage (E) and current (I) values according to Ohm's law i.e. G = 1/R = I/E.  Since the charge on the ions in solution permits the conductance of electrical current, for most solution types the conductivity will increase with concentration.

Thus, an EC meter can be used to detect the presence of salts and their approximate concentration in a solution. 

Units of measure:  EC Meters, nutrient labels and general literature represent conductivity values in several ways.  The more common representations (or ‘units’) are:

+  mS/cm (often abbreviated as “mS”).  Pronounced “milli-siemens per centimeter”.

+  µS/cm (often abbreviated as “µS”).  Pronounced “micro-siemens per centimeter”.

+  cF (conductivity factor).

+  ppm* (parts per million) or mg/L (milligrams per litre) both having the same numeric value.  These are the units for “total dissolved salts” (TDS).  Meters that provide these units have internal software that mathematically converts conductivity readings into a TDS estimate.  However, note that as discussed in later sections, this estimate is prone to many errors and therefore its use should be avoided.

To utilize conductivity recommendations (e.g. from a nutrient label – Fig 2.3), you must first know what ‘units’ (e.g. mS/cm, cF) your meter operates in.  This will be displayed on either the digital display or stamped on the body of the meter.  Some brands offer multiple options e.g. cF, mS and uS.  If this is the case, choose the option that matches that used on the nutrient label – or the recommendation you were given.  For example, if a nutrient label specifies “mS” then switch the meter to read “mS”. 

If your meter is incapable of producing units that match the conductivity recommendation, then manually convert the values to match (see Table 1).  For example, if the label gives a recommendation of 2.0mS but your meter only works in cF, then convert the recommendation to cF by multiplying it by 10.  i.e. 2.0mS x 10 = cF 20.

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