In all my years of research, these are the best tools I've acquired in developement of grow techniques.
#1 Brix Meter; "The hand-held Brix meter was originally designed for farmers to know nutrient levels of their produce and when to harvest crops. This precision instrument (refractometer) has a drop or two of the sap (juice) of the produce placed on the glass and held up to the light to read the Brix scale of zero to 32. The higher the Brix reading, the higher the nutrient levels. Total dissolved solids (or Brix) are measured with the Brix meter. The number on its scale indicates the level of balance of nutrient uptake and complexing of sugars or proteins in the photosynthesis factory – the leaf. If Brix is low, even after several hours of sunshine, some element(s) are missing in the photosynthesis factory. Ions, if present, have not been “complexed” into sugars or proteins."