EC, TDS, DS, CF, PPM
Pure distilled water has no resistance and conducts no electrical current. When impurities are added to pure distilled water in the form of fertilizer salts, it conducts electricity. A water analysis wil indicate the impurities or dissolved solids found in household tap water. These impurities conduct electricity.
Nutrient (salt) concentration are measured by thier ability to conduct electricity through a solution. Dissolved ionic salts create electrical current in solution.
The main constituent of hydroponic solutions is ionic salts. Several scales are currently used to measure how much electricity is conducted by nutrients including: Electrical Conductivity (EC
), Conductivity Factor (CF
), Parts Per Million (ppm
), Total Dissolved Solids (TDS
), and Dissolved Solids (DS
). Most American growers use ppm to measure overall fertilizer concentration. European, Australian, and New Zeland growers use EC, however they still use CF in parts of Australia and New Zeland. Parts per million is not as accurate or consistant as EC to measure nutrient solution strength.
The difference between EC, CF, ppm, TDS, and DS is more complex than originally meets the eye.
Different measurement systems all use the same base, but interpret the information differently. Let's start with EC, the most accurate and consistent scale.
Electrical conductivity is measured in
milliSiemens per centimeter (mS/cm) or
microSiemens per centimeter (ÁS/cm).
One ÁS/cm = 1000 mS/cm.Parts per million
testers actually measure in EC and convert to ppm. Unfortunately, the two scales (EC and ppm) are not directly related. Each nutrient or salt gives a different electronic discharge reading. To overcome this obstacle, an arbitrary standard was implemented which assumes "a specific EC equates to a specific amount of nutrient solution."
Consequently, the ppm reading is not precise; it is only an approximation, a ball park figure! Nutrient tester manufacturers use different standards to convert from EC to ppm.
Every salt in a multi-element solution has a different conductivity factor. Pure water will not conduct electrical current, but when elemental salts/metals are added, electrical conductivity increases proportionately. Simple electronic meters measure this value and interpret it as total dissolved solids (TDS). Nutrient solutions used to grow Cannabis generally range between 500 and 2000 ppm. If the solution concentration is too high, the internal osmotic systems can reverse and actually dehydrate the plant. In general, try to maintain a moderate value of approximately 800 to 1200 ppm.
Nutrient solution concentration levels are affected by nutrient absorption by roots and by water evaporation. The solution weakens as plants use nutrients, but water also evaporates from the solution, which increases the concentration.
Adjust the concentration of the solution by adding fertilizer or diluting with more water.
Many factors can alter the EC balance of a solution. For example, if under-watered or allowed to dry completly, the EC reading will rise. In fact, the EC may increase to 2 or 3 times as high as the imput solution when too little water is applied to rockwool. This increase in slab EC causes some nutrients to build up faster than others. When the EC doubles, the amount of sodium can increase as much as four- to six-fold under the right conditions! There should not be any sodium present in your garden unless it is in the water supply, and it should not be in excess of 50 ppm.
Let 10-20% of the nutrient solution drain from the growing medium after each irrigation cycle to help maintain EC stability. The runoff carries away any excess fertilizer salt buildup in the growing medium.If the EC level of a solution is too high, increase the amount of runoff you create with each flush. Instead of a 10-20% runoff, flush so 20-30% of the solution runs off. To raise the EC, add more fertilizer to the solution, or change the nutrient solution.
A DS measurement indicates how many ppm of dissolved solids exist in a solution. A reading of 1800 ppm means there are 1800 parts of nutrient in one million parts solution, or 1800/1,000,000.
An EC meter measures the overall volume or strength of elements in water or solution. A digital LCD screen displays the reading or the EC of the electrical current flowing between the two electrodes. Pure rainwater has an EC close to 0. Check the pH and EC of rainwater to find out if it is acidic (rain acid) before using it.
Distilled bottled water from the grocery store often registers a small amount of electrical resistance, because it is not perfectly pure. Pure water with no resistance is very difficult to obtain and not necessary for a hydroponic nutrient solution.
Electrical conductivity measurement is temperature-sensitive and must be factored into the EC reading to retain accuracy. High-quality meters have automatic and manual temperature adjustments. Calibrating an EC meter is similar to calibrating a pH meter. Simply follow the manufacturer's directions. For an accurate reading make sure your nutrient solution and stock solution are the same temperature.
Inexpensive meters last for about a year, and expensive meters can last for many years. However, the life of most EC meters, reguardless of cost, is contingent upon regular maintenance. The probes must be kept moist and clean at all times. This is the most important part of keeping the meter in good repair.
Read instructions on care and maintenance. Watch for corrosion buildup on the probes of your meter, corrosion will give a false reading.
To check the EC of the nutrient solution, collect samples from both the reservoir and the growing medium. Save time and effort, collect EC and pH samples simutaneously.
Collect samples with a syringe or baster at least two inches deep int othe rockwool and coco. Collect a seperate sample from the resivoir. Place each sample in a clean jar. Use calibrated EC meters to measure the samples. Under normal conditions, the EC in the slab should be a little higher than the nutrient solution in the resevior. If the EC of the solution drawn from the growing medium id substantially higher than the one from the resevoir, there is a salt build up on the substrate. Correct the imbalance by flushing substrate through with dilluted nutrient solution. and replace with new solution. Regularly check the EC of your water, slab, and runoff.