Conductivity and understanding the measuring scales
Conductivity tells you how much food is in your solution.
Getting a grip on the different measuring scales
All these scales measure the concentration of dissolved solids in a solution. They just
use different processes and scales to do so.
EC
is the only absolute measure of conductivity for nutrient solutions. EC is measured
in millisiemens mS/cm
2
. It’s the standard everywhere outside North America. It
also gives the most accurate conductivity measure. This is because different tester
manufacturers use different standards to convert from EC to ppm - often resulting in
inaccurate and confusing ppm readings (use our conversion chart on page 6).
CF
is closely related to EC. It just doesn’t have the decimal point.
ppm verses TDS.
The ppm 500 scale is the same as TDS. TDS - Total Dissolved Solids
- is represented as ppm by some meters. A TDS reading of 50 means that there are 50
milligrams of dissolved solids in each litre of water. 50ppm.
The ppm 700 scale as no relation to TDS, so 70 milligrams of dissolved solids in a
litre of water is 70 ppm. However, the actual ppm of a solution can only be measured
accurately by chemical analysis.
Understanding these scales
We saw that EC measures are the most accurate. Most ppm meters measure in EC. They
then convert this to a ppm value.
The conversion factor from EC to ppm can vary, but those most widely used for measuring
nutrient solutions are the ‘EC x 500’ scale. It is worth remembering the ppm 500 scale is
sometimes called TDS.
No ifs or buts!
Measure conductivity every day.
• If you don’t know which ppm scale to use, ask your nutrient
manufacturer or use EC
• Avoid over-diluting or cover-concentrating your nutrient solution
• Start with a quality nutrient
• Completely change the reservoir solution every 7 days
• Top up the nutrient level between changes - so your plants have
the right amount of food.
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