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The nutrient profiles in my calculator are copied from various academic publications. Those profiles typically come with less discussion of how they were designed than I might hope, but I assume it's from review of trials of the form mentioned in the other reply to your comment. For more profiles, I've never used anything from
https://cdnmedia.eurofins.com/corporate-eurofins/media/12142...
but I suspect from the author's bio that it's trustworthy.
For probes, see
https://github.com/hydromisc/hydromisc/blob/master/doc/senso...
EC probes should last basically forever if correctly excited (zero net DC). pH probes are consumables, but my $10 probe is still happy after a year continuously immersed. As process fluids go, hydroponic solution is pretty mild, neutral-ish pH and decent ionic content.
I bought the potassium nitrate from MBFerts in combination with other fertilizer salts, so if I'm on a list it's probably "cannabis growers" and not "mad bombers". If you'd rather not use it, then a quick experiment in the calculator shows you can still hit a decent profile with just calcium and magnesium nitrates.
Outside of light, nitrogen (N), phosphorus (P), and potassium (K), or NPK for short
Finding the right combination of these elements is what hydroponics farmers do everyday, pushing the limits of their crop for maximum quality/yield. We are starting to see a big shift in hydroponic fertilizers from liquid to powder since nobody wants to pay for shipping water.
I highly recommend https://github.com/kizniche/Mycodo for anyone doing anything in hydroponics.