Who makes a hydrometer that's sensitive enough, though, GG? I've tried measuring SG of just a sugar water solution and the hydrometer I have (float) just isn't sensitive enough. I was thinking a refractometer might be necessary to make anything resembling an accurate measurement, but again, am not sure if it will measure the low amounts I use. So, I remain stuck in Volumetric Measure Land.
Here's the body of a post (that i'm quoting verbatim) that proved useful to me in figuring out elemental %'s from applicable, water soluble organic compounds. Unfortunately, i don't remember the forum or the poster's name. Thank you for the breakdown... whoever and wherever you are, kind sir.
ppm=parts per million.
Its refered to any compound dissolved in a liquid or gaseous medium, and its used when solucion concentration is low.
When its a solution over water, its very easy calculate ppm, because 1l of pure water weights 1Kg=1000gr=1000000mg. So, if you dissolve 1 mg of any compum¡nd in 1l of water, the concentration is exactly 1ppm (1mg in one million mg of water).
The problem on how calculate the ppm dissolved of a given molecule comes because often you add a compound wich split in two charged (ion) molecules.
For example, you add MgSO4 (magnesium sulphate), wich splits on a Mg+ cation and a SO4- anion. But you want to know just how many ppm of Mg you are adding, for example. Say you add 1 gr of MgSO4 in 1l of water. How many ppm of mg you are adding?
In order to calculate it, you need to know the atomic weight of all elements wich forms the molecule. In this case: Mg=24,3 ; S=32,06 ; O=16 (you can find this data on a element's periodic table, or in wikipedia).
So the atomic weight of the mg sulphate molecula is 24,3+32,06+ (4*16)=120,36.
And the weight percentage of Mg in it is 24,3/120,36=0.20 (20%).
So, of the 10gr of Mg sulphate added, the 20% correspond to Mg+ (electron's weight is negligible), so you added 1gr*0.2=0.2gr=200mg of Mg in 1l (one million mg of water), thus you have a solution with 200 ppm of Mg.
Most Excel spreadsheets have calculated the percentage of each ion of many compounds, so calculate ppms very easily.
Dont confuse the ppm you add of each element, with the measured ppm, it isnt the same.
TDS (total dissolved solids) meters are EC (Electroconductivity) meter wich applies a conversion factor to give an aproximation of the total ppm dissolved.
There is different conversion factors between EC (mS/cm) and TDS (ppm): 0.5, 0.7, 0.64... and depends of the nutes used what gives the best aproximation.
Nutes manufacturers rarely give the best conversion factor for their products. I only know of AN because they changed their recomendation to 0,5.
This is due each gr of salt dissolved have a different EC. Im working in doing my own nutes, and the first step is check the conductivity/ph of each salt in deionized water, in order to do a good aproximation between EC to TDS.
Aditionally, EC is measured for the whole nutrient mix, not for a single element. I know of some meters for individual elements/compounds, but they are pricey, so ive never used it.