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Old 12-03-2009, 02:21 AM
Luches Luches is offline
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PPM means parts per million and 1ppm = 1milligramm dissolved in 1 Liter. It's a unit that is actually used to determine elemental content in a nutrient solution in the first place.

For example 150 ppm of N (nitrogen).

TDS means total dissolved solids. Any of the commonly used instruments actually measure electric(al) conductivity and either display it millisimens/cm (EC) or converts it to what is supposed to be the solids of the elements in a solution in parts per million (ppm).

Obviously these instruments can only determine the total concentration and are not able to even make a difference between a nutrient solution and dissolved arsenic trioxide. Actually you can't even (directly) accurately measure the total ppm of your formula, because there are some other dissolved solids (molecules, ions) that aren't part of your actual formula. To put it in a simple way, they are provided by impurities or "contaminants" that are part of the chemical compounds which are used to manufacture nutrients.

Note that the final reading is always higher as the actual content in known nutrients. It depends on the composition, but the reading is always around 10-15% higher as it is supposed to be.

Watch out: with manufacturer's instruction, those 10-15% should actually be included. You do not have to add anything here.

About the ppm reading of your tap water and how to deal with it:
Basically you should add it, respectively put it on top of the recommended concentration. Why? Because if you subtract it from what you aim for, as a matter of facts - your actual nutrient concentration will be lower and not sufficient.

But a reading of 200 ppm is quite high and close to the limit to use without analyses. I'd rate tap water with 300 ppm as unsuitable to give you an idea.
But depending on the analyses, it's not so bad actually - because your tap water will most likely contain a lot of calcium, magnesium and iron in the best case scenario. And those elements are actually welcome. High content of chloride(s) on the other hand would not be really of any benefit but rather 'toxic'.

It's not a bad idea to have a look at the analyses of your tap water if available.

Conclusion: As long as you aim for the original concentration or lower, you should always put the ppm of the tap water on top. But in case you aim for a higher concentration, you actually should divide the tap water's ppm by 2. In your case you'd then only add 100 ppm to the ppm of the formula. Because seen from that end of the equation you will already have enough calcium, magnesium, iron or other trace elements as part of your nutrients. The content of macro elements will be "more" than sufficient as well in that case. Hence, no need to have all that "extra" from the tap water on top of it. Remark: I was using reverse logics here! The extra from the tap water will always be included, no matter how. You actually use 100 ppm less of your nutrient mix, when only considering half of the tap water's reading!

Last edited by Luches; 12-03-2009 at 02:49 AM.
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