by Jorge Cervantes
Pure distilled water has no resistance and conducts no electrical current. However, when impurities are added to pure distilled water, it conducts electricity. Your water analysis will indicate the impurities or dissolved solids found in household tap water. These impurities conduct electricity. For instance, when you add fertilizer to pure water, the EC climbs.
TDS stands for Total Dissolved Solids. This is just one of the ways to arrive at EC, Electrical Conductivity. EC is the most consistent measure of gross nutrient solution strength.
Nutrient (salt) concentrations are measured by their 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: EC = Electrical Conductivity, CF = Conductivity Factor, PPM = Parts Per Million, TDS = Total Dissolved Solids and DS = Dissolved Solids. Most American growers use PPM, to measure overall fertilizer concentration. PPM is not as accurate, nor consistent 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. First let’s start with EC, the most accurate and consistent scale.
EC = Electrical Conductivity
CF = Conductivity Factor
PPM = Parts Per Million
TDS = Total Dissolved Solids
DS = Dissolved Solids
EC is measured in: (a) Milli-siemens per Centimeter (MS/CM) or (b) Micro-siemens per Centimeter (US/CM). One Micro-siemen/CM = 1000 Milli-siemens/CM.
PPM 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 means equates to a specific amount of nutrient solution". Consequently, the PPM reading is not precise at all, it is only an approximation, a ball park figure!
It gets worse! Nutrient tester manufacturers use different standards to convert from CF to the PPM reading.
1. Hanna 1 MS/CM = 500 PPM
2. Eutech 1 MS/CM = 640 PPM
3. New Zealand Hydro. 1 MS/CM = 700 PPM
PPM recommendations are inaccurate and confusing! To help you through this confusion, please see the following chart.
1 MS/CM = 10 CF e.g. 0.7 EC = 7 CF.
Conversion scale from PPM to CF and EC.
Every element in a multi-element solution has a different conductivity factor in these approximate measurements. Pure water will not conduct electrical current, but as 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 marijuana generally range between 500 and 2000 parts per million (PPM). If the solution concentration is to high, the internal osmotic systems can reverse and actually dehydrate the plant. For general purpose, 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 nutrient concentration. Adjust the concentration of the solution by adding fertilizer or diluting with more water.
Super Size Secret: Use an EC meter to check the overall strength of your nutrient solution. EC is the most consistent measure of overall solution strength.
Excerpted from Chapter Five, Indoor Marijuana Horticulture, The Indoor Bible
http://www.marijuanagrowing.com/dhtml/k ... p?faqid=14
