The real deal on EC, TDS and ppm. First you need to know there is the true ppm based upon the actual mass (weight) of the salts actually in your liquid concentrate fertilizer. When you read a label of guaranteed analysis and it says something like NPK 10:5:10 that means by mass (weight) of the actual salts present. this is easily converted to true ppm by multiplying each number by 100. ie a guaranteed analysis of 10 is 1000 ppm. When that manufacturers states that at a solution at so much per gallon or liter will produce a mix with say Nitrogen 200 ppm, Phosphorus 100 ppm and Potassium 100 ppm it is NOT the same reading that you see when you use a meter. They are a basing the ppm levels on the actual amount of each salt on a gram per liter of water basis.
A meter provides a reading based on electrical conductivity and then uses a preprogrammed conversion factor to provide a reading of TDS or ppm based on electrical conductivity of the combined slats in the solution. This is not a true ppm measurement. If you actually put say 100 ppm by weight pr volume of six different common fertilizer ions in six separate solution jars you would get six different EC readings. Only one of those readings would possibly read 100 ppm. That is because the meters TDS or PPM output readings are based upon the electrical conductivity of just one specific ion at 100 ppm concentration by weight for a specific volume of solution. Each ion conducts electrical energy which different levels of efficiency. This means each will give a different reading even though in reality all the solution samples really contain 100 ppm by actual mass concentration for the volume of solution. So really all an EC meter give you is a best guess approximation of what is there based upon the electrical conductivity of all the ions combined even though they all conduct electricity at different efficiencies. Simply put it gives you no idea of how much of any specific fertilizer ions are in the solution or even an accurate estimate of how much is there in entirety.
For am example: you can have a liquid fertilizer concentrate that has say 3800 grams per liter of salts in it. That means specifically and truly 3800 ppm.
However you can use an EC meter and get a reading of 3.0 or a CF of 30. The meter will if it is a 0.70 conversion meter may say that the TDS or ppm is 2100. In reality the ppm is 3800, but an EC meter is not an ion specific meter so its simply provides a math based best guess estimate based upon electrical conductivity of all the combined salts.
So when you mix up your nutrients with the hope of giving them say a TDS of 750 ppm and you add the fertilizer based on the ppm of the fertilizer that is bad based upon actual concentrations by nmass you will not get a reading of 750 ppm on your EC meter when you put it in the solution. You will get a lower number. How much lower the number will be is based upon the ratios of the salts actually in the fertilizer.
What all of this really means is an EC meter is just a tool that makes things easier to get repeated same results without having to measure with a measuring cup and it gives a good estimate of the
rough approximation of what is till in your reservoir after is use for a while.. But it does not in any way supply you with an accurate number as to just how much fertilizer is really in your solution.
In reality the way to mix your fertilizer is to mix it by the directions for the ppm you desire and then take an EC reading. If you mixed in enough fertilizer to get a 750 ppm reading and you get a reading on your EC meter that says 550 do not add more fertilizer. Just make a note in you log book, or on the fertilizer zero bottle that true 750 ppm is equal to an EC meter reading of 550 ppm. Next time you top of your reservoir orr mix a new reservoir of fertilizer you do not m need to measure by weight or volume but merely add more fertilizer until you get the EC meter reading of 550 ppm. you will then if it sis afresh batch it is truly 750 ppm concentration. If it is a topped of reservoir you will know it is close as you can estimate to the true 750 ppm.
As far as how much nutrients to use at different stages of growth just use something like the General Hydroponics nutrient calculator but use about half to 2 two thirds of what they recommend as they are pretty heavy handed with the amounts they tend to recommend.
http://www.genhydro.com/calculator/index.html
