Quote:
Originally Posted by Shadeflower
Why do we know that the msu has a ppm of 264?
Yep because someone decided to stick a ppm meter in the bucket and check... That's all it is.
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Nope.
First, do you even know what Total Dissolved Solids is, and do you know how it is accurately determined? (Your earlier response says “no”.)
TDS is determined by evaporating 100% of the water from a known mass of clear solution (i.e., no suspended solids) and weighing the solids that were left behind.
In the case of MSURO, it takes 0.74 g of dry fertilizer powder to produce a liter of 100 ppm N solution. Assuming we used pure water, evaporating it would give us the 0.74g back, and that would mean that the
true TDS of that solution is 740 ppm.
Since there are 3.785 liters in a gallon, that same amount of powder in a gallon would give 740/3.785=195 ppm TDS (and 26 ppm N), but since we were originally referring to a full gram in that gallon, that would increase the true TDS to 1000/3.785=264 ppm (and about 36 ppm N).
TDS meters DO NOT “measure” dissolved solids, they
infer it from the electrical conductivity, and that’s where the problem is.
The EC of a solution relies on the travel of ions between electrodes. The magnitude of that is determined by the charge of the ion, its physical size, and the quantity in the solution. (For the purposes of this, I am ignoring the effect of temperature on the solution viscosity, but that also affects it.). The more charge that travels per unit time, the greater the conductivity.
- Smaller ions travel through the solution faster than large ones, and that means they contribute to greater conductivity.
- More highly charged ions carry more free electrons, so they have higher conductivity than less-highly charged ones.
- The greater the concentration, the greater the number of free electrons in the solution, so the higher the EC.
TDS meters are cheap EC meters with a built-in conversion factor, but in the majority of the cases, that conversion is based upon a
sodium chloride solution of known (weighed) concentration. That would be great if you only wanted to measure sodium chloride solutions, but fertilizers have entirely different chemistries, so they contain many kinds of ions of all sizes, charges and concentrations, so the EC versus dissolved solids content is different for every fertilizer formula, and does not fit the NaCl “standard”.
It is certainly possible to create a calibration curve so that you’d know the “corrected conversion factor”, but it would have to be done for literally every different solution you made up.
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