JBL ProFlora CO2 pH Sensor Set pH probe, BNC gel electrode

Once connected to its controller, the probe is calibrated for the first time on the day you unpack it, even before it is installed in the tank. The bottles and test tubes in the kit are enough for this first calibration, which is done on two points separated by a rinse with distilled water. The order matters: always pH 7 first, then pH 4, never the reverse, because the first point sets the probe's zero and the second the slope of its measurement curve. Then throw away the leftover solution in the test tubes, and repeat the procedure every 30 days, 60 at most, since the probe ages continuously and each calibration compensates for this drift.

Allow about 24 months. Your electrode ages chemically from the day it is made, and this ageing shows itself before it becomes a problem: the reading takes longer and longer to stabilise in the buffer solution, and then calibration no longer reaches a correct value. At that point, replace it, because a worn probe controlling a gas injection puts the fish at risk. Note the commissioning date on the casing or in a notebook: it is the only way to know where you stand 2 years later, as an electrode carries no counter.

A controller that displays the probe voltage lets you track this wear in figures. A new probe delivers roughly 0 mV in pH 7 buffer, and its voltage changes by about 59 mV per pH unit, which is around 177 mV in pH 4 buffer. As it ages, two things change: the voltage at pH 7 drifts away from zero, for example towards 28 mV, and the difference per pH unit decreases, for example to 46 mV. The JBL control computer accepts a probe as long as the offset at pH 7 stays below 115 mV and the slope exceeds 35 mV per unit; beyond that, it rejects the probe at the end of calibration.

Fix your probe with the two suction cups supplied, vertical or slightly tilted, tip pointing down, in a well-circulated area that is dimly lit. The orientation matters: if placed horizontally, it ends up trapping an air bubble against its membrane, and the displayed value becomes unstable. A shaded position slows the growth of algae that would settle on the glass and skew the measurement. If a deposit still forms on it despite this, clean the tip with a soft cloth, dabbing gently and never rubbing, then recalibrate straight away. When out of the water, always put the storage cap back on, and store a probe left unused for several weeks in a suitable storage solution rather than in clean water.

In aquaponics, continuous monitoring is particularly useful, because the pH of a system where nitrification is active falls slowly, week after week, as the alkalinity is consumed. A probe installed in the sump or in the fish tank shows this trend well before a weekly test flags it, and it tells you when the buffer needs to be topped up. Place it away from the water inlet of the grow beds and from any addition of pH adjuster, so that it measures the system water and not that of an input that has not yet mixed in. The article on pH control in aquaponics details the possible corrections.