1. Reference articles
  2. Conductivity (Electrical Conductivity, Parts Per Million, Total Dissolved Solids)

What are the different conductivity scales? What do they mean?

EC, CF, ppm500, ppm700, TDS... What's the difference?

EC and CF 
are measures of electrically charged nutrient ions in a solution. Pure water will not conduct electricity. Water usually conducts electricity because it is full of impurities, in our case, electrically charged nutrient ions. The two black dots on the end of Bluelab conductivity probes are called electrodes. When these are placed in a solution, an electrical current passes from one electrode, through the water to the other electrode and counts the number of electrically charged ions present. The reading is then presented as an Electrical Conductivity (EC) reading or Conductivity Factor (CF) reading. 

PPM measures parts per million. Known as dimensionless quantities, they are pure numbers with no associated units of measurement. A mass concentration of 2mg/kg - 2 parts per million - 2ppm - 2 x 10-6.

There are many different scales used for different industries around the world and for many different reasons! Did you even know there are more than two scales? Widely used ppm scales in hydroponics are the ppm500 scale, ppm650 scale and the ppm700 scale.

What's the difference between ppm500 and ppm700 scale? 

  • The ppm 700 scale is based on measuring the KCl or potassium chloride content of a solution.
  • The ppm 500 is based on measuring the NaCl or sodium chloride content of a solution.
  • The ppm 500 scale is also referred to as TDS - total dissolved solids.

It is worth noting that while this are what each scale is based on, in reality these scales do not measure only the KCl content or NaCl content of the solution, but rather the overall conductivity of all electrically charged ions in the solution, this reading is then converted to give you a ppm500 or ppm700 representation.

The true ppm of a solution can only be determined by a chemical analysis, ppm cannot be measured by an EC meter. They are present on Bluelab products as a conversion guide only.

Bluelab meters will measure the EC of the solution, and then convert this to a ppm reading if the desired ppm scale is selected on the Bluelab device. The conversion from EC to ppm is as follows:

  • ppm500 = EC x 500
  • ppm700 = EC x 700

Some examples of how this works, using a conductivity reading of 2.4EC:

  • ppm500 scale: 2.4EC x 500 = 1200ppm [500 scale]
    • (or 1200ppm / 500 = 2.4EC)
  • ppm700 scale: 2.4EC x 700 = 1680ppm [700 scale]
    • (or 1680ppm / 700 = 2.4EC)

If you are reading from a book that says you should grow your crop at 1100ppm - how do you know which scale the writer is referring to? It is very important to match the scale on your meter to the scale being referred to in the book. Is the scale on your ppm meter right for the job? If the book was written in the USA, it could be the 650 or 500 scale. If the book is written in the UK, it could be the 700 scale. If it was written in Australia, well it could be any of the 3!

Because of the complications of the ppm scale(s), we highly recommend the use of EC, the international scale for conductivity.

If you must grow using ppm, you will need to know the following;

  • What ppm scale is the book or feeding schedule referring to? Match your meter to this!
  • What ppm scale is your meter using? Ensure it matches the feeding schedule or book being followed.
  • What ppm scale is the nutrient formula referring to? Match your meter to this!

Which ppm scale should you use? Which ever matches the rest of your system !