About EC and TDS

EC: Electrical Conductivity (of an aqueous solution) is its ability to conduct electricity based on ionic activity and content.

TDS: Total concentration of Dissolved Solids – mostly inorganic salts/minerals and small amounts of organic matter – in water.

Why Measure EC-TDS?

Knowing and controlling EC-TDS is important in agriculture, gardening, and hydroponics for determining nutrient concentration; in aquariums and aquaculture operations for simulating a natural environment; in water/wastewater treatment; and in keeping plumbing, pools and spas in good condition. TDS is one of the oldest known water measurements and is generally considered to be a measure of water purity.

EC and TDS? EC or TDS?

Digital meters determine Total Dissolved Solids (TDS) by measuring Electrical Conductivity (EC), that is, how much current is passing from one of the probe’s electrodes to the other as a gauge of the ionic activity in a solution. Some industries use the EC measurement directly, expressed in microsiemens (µS/cm) or millisiemens (mS/cm), while others use readings that have been converted to TDS, which are expressed in parts per million (ppm) or parts per thousand (ppt). While ppm is more popular, TDS is sometimes given as mg/L. 

Getting to TDS

TDS can be measured directly by weighing a sample of water, letting the water evaporate, then comparing the weight of the residue with the original sample weight. Modern meters use another method; it’s not perfect, but much quicker and easier: mathematically converting the EC to TDS. There are three different conversion factors to get from EC to TDS; NaCl, 442™, and KCl. These conversions are done automatically by your meter, so you need to determine which one works best for your particular application. (For instance, NaCl, with a .5 TDS conversion factor, is designed to correlate closely with nutrient solutions.)

Temperature Matters. A LOT.

Temperature of the solution when measured can alter measurements up to 4% for every degree Celsius. However, most meters automatically compensate for temperature (ATC) and bullseye on 25° Celsius, 77° Fahrenheit, adjusting for temps above and below. Less expensive models require users to compensate manually, utilizing a conversion table.

Buying the Right Meter

It’s simple advice, but very important. If your industry uses EC, buy an EC meter. If it uses TDS, buy a TDS meter. Combo meters are available, but generally reserved for lab use where many different types of readings may be required. In addition, be sure to buy the meter with the range most tightly centered on your anticipated readings. Common sense might tell you to buy the meter with the widest range, but this will decrease the accuracy of your measurements. So… you can utilize conversion tables until your face turns blue, but why not buy the meter that displays the reading you want? No math required.

Caring for Your Meter

After each use, rinse the probe in tap water or soapy water, and store dry. Most EC-TDS units hold calibration well. If used every day, calibrate once a week. If readings become suspect, you should calibrate.