DIY recertification obviously. What's the lowest cost for the hobbyist ? A high-end, stable, double-wall PH-meter or finding a cheap way to build single-wall PH

DIY recertification obviously. What's the lowest cost for the hobbyist ? A high-end, stable, double-wall PH-meter or finding a cheap way to build single-wall PH meters ? https://www.instructables.com/cheap-DIY-electronic-pH-meter/ https://www.instructables.com/cheap-DIY-electronic-pH-meter/ You explore the solution-space, and suddenly, out of nowhere it changes your perspective on another aspect, and you realize you may not need a spectrometer at all to measure ion concentartion in aeroponic nutrient solutions. http://jupiter.plymouth.edu/~jsduncan/courses../2011_Fall/InstrumentalAnalysis/Labs/9-IonSelectiveElectrodes.pdf http://jupiter.plymouth.edu/~jsduncan/courses../2011_Fall/In...