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SEPTEMBER 2024 I PCB007 MAGAZINE 83 clean again. e probe can also be used for the processes themselves, but the ranges will be different, and the probes need to be com- patible with the chemistries involved. I give more details on these probes in my book on automation and advanced procedures 2 . Because of the popularity of aquaculture and hydroponics (including fish aquariums), there are many available conductivity controllers (Figure 4) and schematics to help build your own. Units e units of conductivity are siemens per cm (S/cm). Derived units are µS/cm (one- millionth of a S/cm) and mS/cm (one-thou- sandth of a S/cm). S/cm is the same as the older unit mho/cm. Certain high-purity water Figure 4: Amp-hour totalizers can be purchased, or a simple voltage-to-frequency circuit can be designed and built for less than $20 and attached to a purchased 50 or 100 milli-ohm power shunt.