Gel electrode pH probe, 2 m cable, BNC connector

Once connected to its controller, the probe is calibrated for the first time on the day it is unpacked, before being installed in the tank. Pour approximately 10 ml of pH 7.0 buffer into a test tube, submerge the tip, wait for the reading to stabilise, then confirm. Rinse with distilled water and repeat with the pH 4.0 buffer, then confirm. The order matters: always pH 7 first, then pH 4, never the other way round, because the first point sets the zero of the probe and the second establishes the slope of its calibration curve. Discard the remaining buffer solution afterwards, and repeat the procedure every 30 days, 60 at most, as the probe ages continuously and each calibration compensates for this drift.

Allow around 24 months. Your electrode ages chemically from the moment it is manufactured, and that ageing shows itself before it becomes a problem: the reading takes longer and longer to stabilise in the buffer solution, then calibration can no longer reach a correct value. At that point, replace it, a worn probe controlling gas injection puts your fish at risk. Note the date you put it into service on the unit or in a logbook: it is the only way to know where you stand 2 years later, as an electrode carries no usage counter. See also the article on pH regulation.

A controller that displays the probe voltage lets you track this wear in figures. A new probe delivers approximately 0 mV in pH 7 buffer, and its voltage varies by approximately 59 mV per pH unit, giving roughly 177 mV in pH 4 buffer. As it ages, two things change: the voltage at pH 7 moves away from zero, for example to 28 mV, and the variation per pH unit decreases, for example to 46 mV. The JBL regulation controller accepts a probe up to 115 mV offset at pH 7 and down to 35 mV per unit, and rejects it beyond those limits at the end of calibration.

Fix your probe using the two suction cups provided, positioned vertically or at a slight angle, tip pointing downwards, in a well-circulated and low-light area. Orientation matters: lying horizontal, the probe will eventually trap an air bubble against its membrane, causing the displayed value to become unstable. A shaded position slows the growth of algae that would otherwise build up on the glass and distort the reading. If a deposit forms despite this, clean the tip with a soft cloth, dabbing gently, never rubbing, then recalibrate immediately afterwards. When out of the water, always replace the protective storage cap, and store a probe that will not be used for several weeks in an appropriate storage solution rather than in plain water.

In aquaponics, continuous measurement really comes into its own, because the pH of a system where nitrification is active falls slowly, week by week, as alkalinity is consumed. A probe installed in the sump or in the fish tank reveals this trend well before a weekly test would flag it, and indicates the moment when the buffer needs to be replenished. Position it away from the water inlet of the grow beds and from any corrective additions, so that it measures the system water rather than an input that has not yet mixed in.