How to correctly measure conductivity (EC)?

The subject of this article is best practices for measuring the electrical conductivity of a solution, a value that plays a very important role especially in indoor plant growing. If you want to know how to use EC meters, and more generally EC/TDS probes, or how to care for them so your measuring instruments last as long as possible, you’re in the right place.

If you first want to learn what EC actually is, you can start with the separate article: Electrical Conductivity of the Solution – EC. In practice it is used to determine the amount of mineral (“nutrient”) salts dissolved in the nutrient solution.

Measuring EC frequently and accurately is an indispensable part of growing plants not only in a growbox. Many beginning growers then wonder about the difference between EC (measured value given in μS/cm) and TDS (Total Dissolved Solids) (measured value given in ppm). The difference is simply that from the TDS (Total Dissolved Solids) value you can read the total amount of dissolved solids per unit in ppm.

EC meters and their calibration

EC calibration solutions are available in variants of 1.413 mS and 2.76 mS. Don’t be confused by variants of 2.8 mS or 2.77 mS, which are equivalent to 2.76 mS solutions and differ only by rounding.

How often should you calibrate an EC meter?

Here we have a simple answer. To ensure maximum measurement accuracy, electrodes should ideally be calibrated with the calibration solutions mentioned above before each use. So if the probe is used daily, it should be calibrated every day.

And now we come to other proven practices for measuring conductivity:

• Always use a fresh calibration solution! In other words, calibration standards are easily contaminated and have no buffering memory, so it makes no sense to try to reuse them.
• Before calibrating, rinse the probe with deionized water (see the separate article: Reverse osmosis). Only then can you safely immerse the probe into the calibration standard. For even more accurate results, prepare a second separate beaker with calibration solution or your sample for rinsing. Proper rinsing of the electrode with deionized water is very important. Accumulated salts and other residues can contaminate samples and cause inaccurate readings. Therefore rinse the probe every time you remove it, before storage, or when measuring conductivity of different samples in multiple beakers.
• Plastic beakers are generally recommended because other materials, such as glass or metal, can cause electromagnetic interference with the measured electrical conductivity.
• Correct placement of the electrode in the beaker means an equal distance from the beaker wall in all directions.
• Also make sure the electrode is sufficiently submerged. Otherwise you will get distorted conductivity readings. Check that all openings are actually below the surface and that there are no bubbles between the sensors.

Example of replacement electrodes for an EC meter.

• Check for air bubbles, because air can remain in the body or on the surface of the electrode even when it is submerged, causing inaccurate measurement. If you notice air bubbles, submerge the probe further and move it up and down. Gently tapping will help release any bubbles from the openings.
• Although electrodes typically have temperature compensation, allow time for them to reach thermal equilibrium; always wait until the measured conductivity or temperature stabilizes.
• Choose the appropriate type of electrode, because not all electrodes are suitable for every range or sample. Sometimes selecting the measuring device depends on the correct range as well as chemical compatibility. For certainty you can always consult us (see below).
• To ensure the longest possible life and correct functioning of any EC electrode, frequent calibration should be complemented by regular cleaning, which is often underestimated. Deposits can accumulate in the electrode area or on vent holes and are a common cause of calibration problems and inaccurate results. How to clean the electrode properly? A proven method is to rinse the electrode thoroughly with warm soapy water. Do not use chemical cleaners or solvents.

Some multifunction measuring instruments use so‑called combined electrodes; proper storage of an EC electrode is important. For correct storage of an EC electrode, it is important to note that in the case of a combined EC/pH electrode, improper storage can lead to unwanted drying out of its electrolyte. Sometimes a dried probe can be regenerated by soaking it in a storage solution for pH. For correct storage of storage of a combined electrode, see the article on storage solutions.

If you are interested in working with pH meters, we have also prepared a separate article: How to use calibration solutions and pH meters?

If you would like to ask us anything, not only about correct conductivity measurement, we will be happy to advise you at the well-known e‑mail address info@higarden.cz.

We also recommend these products:

• Measurement kits
• Replacement probes for pH and EC meters
• Aqua Master Tools P160 PRO, pH and EC/TDS/PPM meter and thermometer

Further reading:

• Hydroponics – what it involves and how to start?
• Do you know what VPD is? Discover the key to perfect conditions for plant development
• ONA Block: solid odor absorber