TDS  /  EC  /  PPM

What does a TDS or EC meter measure? 

TDS and EC meters are used to measure nutrient solution strength. Nutrient ions have an electrical charge. Electrical Conductivity "EC" is a measurement of all those charges in the solution that conduct electricity. The greater the quantity of nutrient ions in a solution, the more electricity that will be conducted by that solution. Electrical Conductivity (EC) is expressed in siemens per centimeter (s/cm) or milliseimens per centimeter(ms/cm).

What does the term Parts Per Million (PPM) mean? 
Total dissolved solids (TDS) is typically expressed in parts per million (PPM). PPM is a common unit for measuring the concentration of elements in nutrient solutions. One ppm is one part by weight of the mineral in one million parts of solution. Example: A solution of nutrients dissolved in water at a strength of 500 ppm means that there are 500 milligrams of dissolved solids present for every liter of water.

How do I convert between TDS (PPM) and EC readings? 

Multiply the EC reading (in milliSiemens/cm) by 1000 and divide by 2. 

To get an EC value, multiply the ppm reading by 2 and divide by 1000. 

For example, if your EC is 1: 

1*1000/2= 500 ppm. 

And if your ppm is 500: 

500*2/1000= 1 EC 

All TDS and EC meters measure electrical conductivity and then convert the EC measurement to PPM using a conversion factor. There are three conversion factors which various manufacturers use for displaying ppm's:

USA 1 ms/cm EC 1.0 = 500 ppm 
European 1 ms/cm EC 1.0 = 640 ppm 
Australian 1 ms/cm EC 1.0 = 700 ppm 

For Example: 

Hanna, Milwaukee1 ms/cm EC 1.0 = 500 ppm 
Eutech1 ms/cm EC 1.0 = 640 ppm 
Truncheon1 ms/cm EC 1.0 = 700 ppm

Why is measuring and monitoring nutrient solution strength important? 

Delicate plants, cuttings, and seedlings can experience fertilizer burn if the nutrient solution strength is too high. Once the plants begin growing and have developed a healthy root system, the strength of the nutrient solution can be increased. Some plants prefer a milder nutrient strength, while others grow better and produce better quality fruit with a higher concentration. The stronger your nutrient solution is, then more food is available to your plants. 1500 PPM is the maximum nutrient solution strength that most plants can handle before showing signs of fertilizer burn.

How often should I calibrate my TDS or EC meter? 

Before each use or at least once a week. Use standard reference solutions and follow the calibration instructions that came with your meter.

However, be careful of very high levels as this can burn and or kill the plant.

Interactive 500 Scale EC/PPM Conversion Chart