EC or TDS - What should I use?

Electrical Conductivity or 'EC' measures the potential for a material to conduct electricity – or how well the material can carry an electric current through it.

Pure water (H2O) has no EC, while rain water and tap water have a very low EC. Their ability to conduct electricity is low, as there is a lack of salt for an electrical current. At the other end of the scale, salt water (which has high amounts of dissolved salts) has a high EC.

When a Hydroponics Nutrient is added to water the EC rises as the salts in the nutrient blend with the water. Generally the more nutrient in the water, the higher the EC will be.

Electrical conductivity increases as the temperature of the solution increases. So it’s important when taking measurements, to use a meter which has automatic temperature compensation.

What is TDS?
Total Dissolved Solids (TDS) refers to the amount of substances (organic and inorganic) that are dissolved in a liquid. TDS is generally measured in parts per million (ppm).

Digital meters determine TDS by measuring the EC, and then using a conversion factor - which is generally incorporated within the meter - to determine the TDS. So even when using a digital meter, this method only estimates the TDS levels. To get a true TDS measurement, the sample would need to go to a laboratory.

TDS Conversion Factors??
EC to TDS conversion factors differ depending on the sample being tested. The conversion factor makes an assumption about the TDS in a solution, and makes allowances for particles that have low or no electrical charge. When a TDS meter takes a reading it actually tests the EC of the solution, then compensates the result by multiplying by the conversion factor and displays the result as TDS.

As hydroponic solutions vary enormously in their compositions,
the most accurate way to take consistent measurements is always in EC.