"This weight of epsom salt should produce 85ppm -- which should increase conductivity by 113uS (which is more accurately measured than PPM)"
I don't think that's the case.
I am only guessing, but as I wrote earlier, my bet is that the ppm meter is just a conductivity meter, with the display scaled not in μS, but in whatevertheyareintendedtomean ppm units. So you won't get any better accuracy, just a better defined property of the solution.
You're absolutely right. This is a confusing topic because when I talk about PPMs, there are 1.
"PPMs I calculate from the fertilizer label (% of weight)".
That's accurate within the limitation of the label being merely a "minimum guarantee." (This topic could also be limited to the US. I've read that other countries may list fertilizer content as % of volume. I've never looked into that topic.).2.
"PPMs as reported by a meter."
I agree with you. They're all different. Even if they have the same scale they can be different. I have 4-5 pens and often measure a nutrient solution with all of them (and compare how they measure each fertilizer product added).
There are some fairly inexpensive PPM pens that display microsiemens. One is "Membrane Solutions" (ASIN: B07R58VQWJ on Amazon for $10 USD). I've been using that for over a year, and it's a real bargain. It displays the usual confabulated "PPMs." But, press the "shift" button, and it displays microsiemens. So, I can see how it's scaling conductivity into a "PPM" representation. It can be calibrated. I use 342ppm NaCl. Really awesome for $10. It seems to be stable, doesn't require recalibration too often.
I may be able to calibrate it to 1000ppm NaCl. I haven't tried yet. I have a 2nd pen on the way. I'm going to calibrate it to 1000ppm (if possible) and then see how the two pens compare from weaker to stronger solution. (I.e., I start with 150ppm water. I add potassium sulfate to get 60ppm (calculated).). After adding some various fertilizer, the solution might be 650-750. I'd like to see how the same meter calibrated to the low & high range reads low and high solutions. I'd like to graph that, and see how the two together might provide more accurate readings at the mid point.
I also have a HM Com-100 ($55 USD). It displays microsiemens too (as well as both .5 and .7 scales). It is calibrated to the same 342 reference solution. The $10 "Membrane Solutions" tracks that pretty well until the 550-650 range, then either the inexpensive meter under-reports. Or, the expensive one is over reporting. That's why I want to compare two of the inexpensive meters to see if one calibrated for 1000ppm (assuming it can be), becomes more accurate at higher levels. How the two together compare to the expensive meter across that 150 to 900 range I tend to measure within.
So, I'm thinking I should be able to mix 1g/L potassium sulfate, measure the conductivity, and create my own "scale" to match the calculated PPMs (415ppm K; 170ppm S).
I calculate those now, and then expect to see those PPMs using the meter. But, as you said, that's hocus-pocus. I think it would be more accurate to measure conductivity, and maintain my own "calculated-ppm to conductivity" scale. My goal is to stop reading PPMs with a meter (since that's nonsense). I think the best I can do is say "I know this product creates x
ppms when mixed 1g/L -- based upon it's 'guaranteed analysis.' 1g/L produces x
microsiemens." From that, I could measure the microsiemens of adding an amount of that product, and convert that to ppms (myself, not having a meter display ppms to me).