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The conductivity of a solution is an expression of the capacity of that solution to conduct an electric current. Distilled or de-ionised water will conduct virtually no electricity at all and therefore will have a conductivity reading of zero. As salts are dissolved in the water so the conductivity of the solution increases. The conductivity of a nutrient solution is therefore a measurement of its “strength” as indicated by the actual amount of salts dissolved.

What is electrical conductivity?

In hydroponics all nutrients are made of mineral salts, which are dissolved into water and the strength of the nutrient solutions can be detected by an electrical conductivity (EC) meter. The higher the ionic (salt) concentration, the higher the level of EC, hence EC can be used as an indicator of the strength of a solution. EC meters consist of two electrodes spaced 1 centimetre apart, which are placed in the solution to be measured.

When power is supplied to the meter, an electrical current passes from one electrode through the solution to the other electrode and the meter displays how much electrical current is flowing between the electrodes and shows it as EC. These meters work on the principle that an electrical current is conducted more easily as the concentration of nutrient salts in a solution increases. Finding the right tool for the job all depends on your setup, have a browse of our product range to find the right one for you or get in touch if you have any questions.

Knowing the EC of the feed solution is important because it gives the grower an indication of the strength of the solution and how plant growth is affected by the nutrient level. Different plants grow well at different levels of nutrient strength and it is important to control this nutrient strength to provide the optimal conditions in the root zone. This in turn allows the maximum uptake of the nutrients into the rest of the plant’s cellular structure.

Electrical conductivity can be expressed using a number of different units but the international standard is EC and the unit of measurement is usually milliSiemens or microSiemens. The difference between these two units of measurement is the placement of the decimal point, so 1 milliSiemens is equal to 1000 microSiemen. The important thing to remember is that a nutrient solution at “full strength” (according to the manufacturer’s directions) has an EC between 2-2.5 milliSiemens and is expressed as 2mS/cm.

Sometimes EC is expressed in other units of measurement, such as CF and TDS/ppm. CF (conductivity factor) is basically EC multiplied by 10 and is used in some places because it eliminates the need for a decimal point (e.g. an EC of 0.1 is equal to a CF of 1). TDS (total dissolved salts) and PPM (parts per million) are commonly used in the United States and the meter used for this is actually an EC meter, which has an internal correction factor that converts the EC to the TDS readout. Unfortunately, this is where it becomes complicated as different meters have different correction factors, usually dependent upon the industry in which they are used.

  • The first problem is that some meters use a factor or 500 ppm per mS/cm and others use 700 ppm per mS/cm
  • The second problem and more importantly, is that different salts have a different capacity to conduct electricity (e.g. two solutions made up at 1,000 ppm with different salts would have quite different readings on an EC meter or a ppm meter)

For these reasons it is recommended that growers use an EC meter and avoid using TDS meters. It is important to know that EC will measure the strength of a nutrient solution, but it cannot measure the nutrient balance of that solution. Also, EC does not measure any non-ionic components in the solution and when using organic fertilisers, the solution strength will be higher than indicated by its EC. This is because most carbon-based compounds are not ionic and will not register on an EC meter. The Bluelab Truncheon is the most widely used EC meter on the market.

Tap water contains mineral salts which will produce an EC, but EC alone does not necessarily mean that the water contains nutrient salts that will help plants to grow. For example, tap water contains sodium and chloramine, which have an EC value, but these minerals are not beneficial to plants. Therefore it is important to check the EC of the tap water before making the feed solution and add the appropriate amount of nutrient concentrate to achieve the desired EC.

Active (recirculating) systems
These systems include: NFT; Flood and Drain; Aeroponic; and Deep Water Culture. In these systems, where the nutrient solution is re-circulated and re-fed to the plant over a period of time, the EC of the recirculating solution will usually rise with time due to evaporation and by nutrient uptake of the plants. How quickly this happens depends upon the size of your plants, the environmental conditions and especially the volume of solution you have in the system. If the EC gets too high, the solution becomes toxic and the plants will suffer, eventually dying. It is recommended that the tank should be topped up with water when needed and completely changed on a weekly basis.

Passive and run to waste systems
These systems include hand-feeding or dripper-fed, media-based systems, where a small amount of the feed solution is run off from each container. It is critical to measure the EC and pH of this run-off solution, because this is what needs to be controlled. Similar to an active system, the EC will usually rise through the system and this depends upon the same factors (that is - size of plants, etc.). Again, if the EC gets too high the plants will die. The controls that you have are the EC of the feed solution and the proportion of run-off.  Therefore, if the run-off EC is too high you can lower the EC of the feed solution and/or increase the amount of run-off by increasing the volume and/or frequency of irrigation. This will help restore the nutrient balance needed for healthy plant growth.

EC meters need to be maintained by keeping them cleaned and calibrated at all times. This will insure that the meter always delivers an accurate reading so that managing the feed tank is consistent and the plants always get the required nutrients. The surface of the probe is where the instrument takes the reading of the solution. Thus, if the surface of the probe is dirty or contaminated, the reading may be inaccurate. Proper care and maintenance of the EC meter will ensure accurate readings as well as maximising the life of the device, so make sure that you follow these easy steps:

  • Follow the directions that come with the unit
  • Clean the probe tip after each use with fresh water
  • Periodically deep-clean the probe tip using an EC probe care kit, following the instructions carefully
  • Check and calibrate the EC meter monthly if required, using the Growth Technology EC Standard
  • Before long-term storage, remove the batteries

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