pH and EC calibration solution and probe maintenance

In your aquaponic system, the pH is a compromise between the fish, the nitrifying bacteria in the filter and the plants, and the target range is between 6 and 7, above the range aimed for in soilless growing. Nitrification releases hydrogen ions continuously, so the water gradually acidifies, at a rate that depends on its carbonate reserve. As you correct the pH in small doses, a discrepancy of a few tenths in the reading translates directly into a poorly dosed correction. Conductivity, for its part, stays lower than in hydroponics, since the nutrients come from the fish feed, so the 1.413 mS/cm calibration solution is the one you will use. The article on pH regulation in aquaponics details the possible corrections once the measurement is reliable.

The glass bulb of your pH probe must be kept clean and, for most models, permanently moist: once dry, it responds slowly and loses accuracy. The storage solution, based on potassium chloride, is there first and foremost for this purpose. After each reading, you rinse the tip with water, then pour about 2 cm of solution into the cap or into a small container, so that the probe stands upright and submerged in it. A probe should never be stored in pure water, and the solution in the cap should be renewed every 7 to 14 days. With some instrument manufacturers, such as Bluelab, poor probe maintenance can even void the warranty.

The same solution can also be used to refill liquid-electrolyte probes, those whose body has a filling hole: you top up the level regularly following your device's instructions, which ensures consistent readings. It is not suitable, however, for gel electrodes, which cannot be refilled. When the reading becomes slow or unstable despite a recent calibration, the probe is usually fouled: leave it to soak overnight in the cleaning solution, which dissolves the salts, debris and dirt accumulated during use, then rinse it and calibrate it again.

For a pH meter, you need at least the two buffers, pH 7 and pH 4, and the storage solution, which protects the probe between sessions; the cleaning solution completes the kit as soon as the probe becomes sluggish. For a conductivity meter, a single standard is enough, the one whose value is closest to your usual readings. A combined tester, which measures both pH and conductivity, therefore needs four bottles, and five with the cleaning solution. Each 300 ml bottle lasts a long time, since you only pour out a small amount for each check. It keeps best with the cap on, in a cool place and away from light, which preserves its value between uses.