Are you saying that if the meter shows 1500 "ppm's" that it really is 1.5 EC?
No, that's not what I'm saying.
I'll try to break it down....
EC or "electrical conductivity", is very easy to measure. That's what you have. Easy to use, and easy to manufacture, and why companies sell them. Easy profit.
Let just take a couple of compounds, bease it will be easier to understand.
Say you have a two part mix, flora A and flora B. The company that made the mix knows the formula and tells you what scale that used to get the proper ppm's from it.
...but say you wanna change the mix up,
Flora part B has nothing but Iron (Fe), and Flora part A has nothing but Nitrates (NO3).
With this knowledge, flora part B will have a higher EC value, because iron is more conducting.
If you mix the directions up, they won't match the scale used. There are many scales used. Each premix follows one of these scales. Here's 3, but I'm sure there's many more..
Now, a EC/ppm/TDS meter only measures EC, then it that's that number and multiples that by what ever sCale it's programed for. Some meters can do all three scales.
...It's still just an EC meter.