Hey man I found this on PPM hopefully it's useful. That shit is hard to understand contains a lot of math.
Method A:
To determine the pounds (or ounces) required if you know the ppm of chemical per gallon of water needed follow the steps below.
The following example is a fertilizer application, but the principle applies to any chemical solution.
Example: You want to fertigate at 150 ppm of nitrogen using a 20-10-20 soluble fertilizer. You have your injector set at a 1:100 ratio. How much fertilizer should be put in the solution tank?
Ounces of fertilizer =
150 ppm x 100
75 x 20
Answer = 10 ounces of fertilizer per gallon of water.
The 150 = the recommended ppm for the crop; the 100 = the injection ratio; the 20 = the % of active ingredient; the 75 is a constant conversion factor used for finding ppm (1 ounce of any 100 percent soluble fertilizer or chemical in 100 gallons of water always equals 75 ppm). The tank size or gallons is one gallon here. If you have a tank size of more than one gallon, multiply your answer by the number of gallons.
Method B:
To calculate what injection ratios would provide other ppms of the same chemical mixture, follow the steps below.
If mixing the fertilizer for various ppms, you will need to choose one ppm as your base for a 1:100 injection ratio and then calculate the other ratio settings for the other ppms.
Example: If you have various fertilizers such as 20-20-20, 10-52-17, etc., choose nitrogen (the second number) as the chemical to determine your ppm.
Formula: Recommended ppm x injection ratio/75 (constant) x % fertilizer. Choose 20-20-20 and solve for 50 ppm, 100 ppm and 200 ppm. Select 100 ppm as your base. You could choose 50 or 200, but choosing 100 simplifies calculations. Also, choose 1:100 as your injection ratio. You could choose any ratio setting to find the ounces per gallon needed, but choosing 1:100 makes it much easier to calculate your settings.
100 ppm x 100
75 x 20
= 6.67 oz per gallon
Answer = 6.67 oz per gallon to obtain 100 ppm at a 1:100 ratio setting.
If you are using an 8 gallon container, multiple 6.67 oz. by 8 = 53.3 oz or 3.33 lbs. per 8 gallons, and so on for other gallon containers.
