Understanding What PPM Scale To Use

CF, EC, PPM 500, PPM 700, TDS... What's the difference?

CF and EC
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 nutrient 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. This represents the units measured - CF or EC.

PPM measures parts per million. ppm is known as dimensionless quantities; that is, 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? The most widely used scales in Hydroponics are the 500 scale, 650 scale and the 700 scale.

What's the difference?
 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 and is also referred to as TDS - total dissolved solids. Individual nutrient ions have different electrical effects! The true ppm of a solution can only be determined by a chemical analysis. ppm cannot be accurately measured by a CF or EC meter. They are present on Bluelab products as a conversion guide only. The conversion is as follows:

2.4EC x 500 = 1200ppm (500 scale) or 1200ppm / 500 = 2.4EC

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? 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 three!

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

  • What ppm scale is the book referring to?
  • What ppm scale is your meter using?
  • Which standard or calibration solution should you use for your meter?
  • What ppm scale is the nutrient formula referring to?
(Courtesy of BlueLab)
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At GYOstuff, we do the research so you don't have to. So let's get to it... What PPM scale is your nutrient formula referring to?

Manufacturer Scale
Advanced Nutrients 700 scale

Botanicare 700 scale

CES/Cutting Edge Solutions 500 scale

Dutch Master 500 scale
Dyna-Gro 500 scale

FoxFarm 700 scale using dechlorinated tap water

General Hydroponics 500 scale using reverse osmosis water

General Organics 500 scale using reverse osmosis water

House & Garden 700 scale

Humboldt Nutrients 500 scale
Hydro Organics/Earth Juice  500 scale
Nectar for the Gods 700 scale
Rock Nutrients 700 scale
Roots Organics 500 scale
Rx Green Solutions 500 scale  using distilled water
Soul 500 scale

Technaflora 500 scale

Now that you know which scale to use, check out our selection of EC/TDS meters.

Also handy is this PPM conversion chart:


Want to learn more? Check out our 1 hour classes or other topics in the GYOstuff Learning Center.

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