PPM meters and EC meters are useful tools but nothing to live by. The readings they give are fairly accurate but not exact. Most meters are calibrated using a sodium solution, using the electrical conductivity of sodium as a baseline. The meters cant tell that the elements floating around in your solution aren't all sodium, so it just takes the general EC reading and applies it to the sodium baseline. Anything that conducts more (or less) electricity than sodium will still be measured as if it were sodium so the readings are less than accurate. Best way to get an accurate level is to the mathematical calculations. For example:
Lets say you have some nutes that are 3-5-4 with an additional 7% of other nutrients (add up all percentages of micronutrients and other macronutrients listed on the side of the bottle)
If you add all percentages (7+3+5+4) you end up with 19%, meaning this solution starts off at 190,000 ppm (19% of 1,000,000 is 190,000). You then take this number and divide by a given ratio. 1/2tsp of nutrients to 1 gallon of water is roughly a 1:1500 ratio. So, to figure out what ppm's you get from 1/2 tsp you simply divide 190,000 by 1500, coming to 126.66. So, for this fictional product, you get about 125 ppm from every 2.5ml (1/2tsp) added to a gallon of water. From this, you can figure out exactly how many ppms you will have at any feeding level.
To recap:
Add all percentages. Figure out total ppms in undiluted form by multiplying your percentage by 1 million. Take this number and divide by 1500 to get your actual ppms at 2.5ml per gallon.
Once you have this true number, you can double check with your ppm meter and use that as a baseline to monitor change. So, if you know that your solution is at 1000ppm based on mathematical calculations, but your meter says 1200, you know your meter is off by about 20%, so in the future, you can just take off 20% from your reading to find your actual level.
NOTE:
To satisy government labeling criteria, the numbers on the front represent MINIMUM quantities. So, something at 3-4-5 must have a minimum of 3-4-5, but could have more. Most companies get pretty close but to be safe, assume about 10% overage. To account for this, simply take the total percentage (in the example it was 19%) and add an additional 10% of that number. So, in the example, you take 10% of 19 and get 1.9. You then add that 1.9 to your total percentage, bringing it to 20.9%.
IOW, dont add 10% to the total percentage (not 19% + 10% = 29%), you take 10% of the total percentage and add it back to the total (19 x 10% = 1.9%) + 19% = 20.9% total.
You then take this updated number and finish the calculations:
20.9% x 1 million = 209000ppm undiluted.
209000 / 1500 = 139.33 = (140 ppm) per 1/2tsp per gallon.
Lets say you have some nutes that are 3-5-4 with an additional 7% of other nutrients (add up all percentages of micronutrients and other macronutrients listed on the side of the bottle)
If you add all percentages (7+3+5+4) you end up with 19%, meaning this solution starts off at 190,000 ppm (19% of 1,000,000 is 190,000). You then take this number and divide by a given ratio. 1/2tsp of nutrients to 1 gallon of water is roughly a 1:1500 ratio. So, to figure out what ppm's you get from 1/2 tsp you simply divide 190,000 by 1500, coming to 126.66. So, for this fictional product, you get about 125 ppm from every 2.5ml (1/2tsp) added to a gallon of water. From this, you can figure out exactly how many ppms you will have at any feeding level.
To recap:
Add all percentages. Figure out total ppms in undiluted form by multiplying your percentage by 1 million. Take this number and divide by 1500 to get your actual ppms at 2.5ml per gallon.
Once you have this true number, you can double check with your ppm meter and use that as a baseline to monitor change. So, if you know that your solution is at 1000ppm based on mathematical calculations, but your meter says 1200, you know your meter is off by about 20%, so in the future, you can just take off 20% from your reading to find your actual level.
NOTE:
To satisy government labeling criteria, the numbers on the front represent MINIMUM quantities. So, something at 3-4-5 must have a minimum of 3-4-5, but could have more. Most companies get pretty close but to be safe, assume about 10% overage. To account for this, simply take the total percentage (in the example it was 19%) and add an additional 10% of that number. So, in the example, you take 10% of 19 and get 1.9. You then add that 1.9 to your total percentage, bringing it to 20.9%.
IOW, dont add 10% to the total percentage (not 19% + 10% = 29%), you take 10% of the total percentage and add it back to the total (19 x 10% = 1.9%) + 19% = 20.9% total.
You then take this updated number and finish the calculations:
20.9% x 1 million = 209000ppm undiluted.
209000 / 1500 = 139.33 = (140 ppm) per 1/2tsp per gallon.
