Many new and even experienced growers get confused with the EC vs ppm debate. To confuse matters further EC can also be termed CF, or measured in a number of different units. EC, or electrical conductivity, is a measure of the concentration of ions in the nutrient, or its strength. It is measured with an EC meter which passes a small electrical current between two electrodes while in the solution and gives a reading. Typical EC levels for most hydroponic crops run between 0.6 and 3.5 with lettuces and other greens being at the lower end of the scale and fruiting plant such as tomatoes at the higher end of the scale. CF or conductivity factor is just EC multiplied by 10 and is more often used in countries such as Australia and New Zealand. In the US and Canada, ppm is still being used by many growers, although it is not as accurate as EC. ppm (parts per million) meters are actually just EC meters which then use an approximate conversion factor to convert the EC readings into ppm. Since this conversion factor actually varies with the composition of each different solution it's only an approximation and EC is a much more accurate way of measuring and monitoring a nutrient solution. Many meters these days have both EC and ppm (often temperature and pH as well), so these are worth looking at if there is still confusion over the EC vs ppm measurement of nutrient solutions.
The simple rule for small systems is that if the EC is increasing over time, then the plants are taking out more water than nutrients, so lowering the strength of the EC is a good idea. If the EC is decreasing over time, then the plants are taking out more nutrient ions and less water, so increasing the strength of the EC would then be the best option in media systems. Whether the EC increases or decreases over time depends on how much water the plants need under the current growing conditions, also their stage of development and rate of growth.
A fruiting plant can lower the EC rapidly as it draws up large amounts of potassium and other ions. Small seedlings may not change the EC much at all. pH also changes over time and should be adjusted from time to time. pH can change as plants take certain ions out of solution faster than others and also with each addition of fresh water if the pH is on the high side.
relevant info pulled from
http://www.growingedge.com/magazine/back_issues/view_article.php3?AID=170144
