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TDS and pH Explained

If you plan on growing in either a hydroponics or soil system, a must have item will be a TDS or EC meter. After selecting the right nutrients for your grow, making sure you mix the right amount will be imperative to the overall health of your plants. A TDS or EC meter will give you a visual number that directly relates to how “HOT” your nutrient mix is. If the mix is too hot, you risk burning your plants, so keeping good records of how much you have been feeding is key.  Along with your TDS meter, you will also need a good pH meter to ensure that your plants have the ability to uptake the nutrient solution you just mixed for them.


What is TDS/PPM/EC

OAKTON_EcoTDSTestr-smallTDS (or Total Dissolved Solids) is measured in units of PPM (or Parts Per Million). EC (or Electrical Conductivity) is measured in either µS (MicroSiemens) or mS (MilliSiemens). TDS and EC meters will measure your nutrient strength in the same way, but will display them in different ways. A measurement of 1000ppm on a TDS meter will be the same as a measurement of 1430 µS or 1.43 mS on an EC meter.

TDS and EC meters measure the strength of your nutrient solution by measuring the flow of electrical current between two metallic probes. The higher the salt concentration in the solution the better it conducts electricity and display a higher reading on the meter. We recommend storing your meter in probe storage solution instead of calibration solution 7.0, as well as regular calibration of your meter to insure a reliable reading.


PPM Conversion Factors

Most TDS meters these days allow you to adjust the conversion factor from 0.5 to 0.7. Its this factor that will provide a different PPM reading for the same nutrient solution, so its imperative that you know which factor your meter is set to.

A meter that uses a conversion factor of 0.5 will display a reading of 0.5 times that of the 0.7 EC reading.  If the EC reading of 0.7 is 700 or 1000 µS, then the 0.5 reading will be 500 ppm. You can get to this number by multiplying 1000 by the conversion factor of 0.5. If your conversion factor is set to 0.7, then 1000 µS would be 700 ppm (1000 x 0.7). Remember when setting your meter to a specific conversion factor, be sure to purchase the correct calibration solution for that setting. Most meters made by manufacturers in Europe will automatically set their conversion factor to 0.5, and manufacturers in the US tend to use a conversion factor of 0.7.

0.5 conversion 0.7 conversion Adjustable conversion
– Hanna Primo – Hanna HI 9813-0 – Hanna HI98129
– Tri-Meter – Hanna HI 9813-6 – Hanna HI98311
– Hanna HI981504 – Hanna HI 983301N – Hanna HI981404
– Hanna HI 983311
(no calibration required) – Hanna HI981404N
– Blue Lab Truncheon TDS/EC (displays reading in PPM (0.5 and 0.7) and ECM
– Oakton Meters

Below is a Conversion Chart based on your Factor setting (.5 or.7)

PPM Chart