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HOME + GROW INFORMATION + METERS & TESTING FAQ

FAQ: METERS & TESTING

What is PPM or TDS?
What is EC?
How do I convert my readings from TDS to EC and vice-versa?
What is the best way to measure the amount of nutrients in a solution?
What nutrient level should my nutrients be at?
 
What is PPM or TDS?
TDS stands for total dissolved solids and is measured in ppm or parts per million.  One ppm is equal to one part of the total mineral weight in one million parts of solution. In a laboratory, the weight of TDS is found by allowing a liquid to evaporate and weighing the particles left over. In water culture, TDS is estimated using a converstion from the measure of electrical conductivity (EC).  TDS is not exact – it depends on the conversion factor used, and different brand names can vary by as much as 600 ppm.   TDS is currently the standard of measurement that most US growers use. This will probably begin to change in the future as hydroponic manufacturers begin changing their products to EC.
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What is EC?
EC stands for Electrical Conductivity and is measured in mS/cm or miliSiemens per centimeter. An EC meter measures an electrical current in the solution and reads the conductivity produced from the motion of the mineral ions. Low conductivity means low nutrient concentration, often resulting in nutritional deficiencies and slow plant growth. High conductivity means more food for your plants. But be careful of not to get overzealous with the concentration of your nutrient solution; very high conductivity can burn or kill your plants.

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How do I convert my readings from TDS to EC and vice versa?
To get the approximate TDS value, simply multiply the EC reading (in miliSiemens/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 x 1000/2 = 500ppm
And if your ppm is 500:
500 x 2/1000 = 1EC

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What is the best way to measure the amount of nutrients in a solution?
In Europe, EC has been used exclusively. In the US, most growers have been using TDS. The problem is that, as explained earlier, TDS is a conversion from EC, and different manufacturers use different conversion rates. Different TDS meters may show a discrepancy of as much as 600 ppm when reading the same solution. A few years ago,  at the Hydroponic Merchants' Association conference it  was decided that the US industry would begin to switch over to EC. This switch over could take years.

American growers still seem to want to use TDS meters most often and we supply a wide range of quality ones. Be sure when calibrating these meters to use the correct calibrating solution from the correct manufacturer.

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What nutrient level should my nutrient be at?
For seedlings, your nutrient level should be between 400ppm (800ms) and 600ppm (1,200ms). For vegetative growth, the level should be between 800ppm (1,600ms) and 1,100ppm (2,200ms). For bloom, the level should be between 1,000 ppm (2,000ms) and 1,400ppm (2,800ms). These of course are very general levels.

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