IP65 pH meter with replaceable electrode, pH and temperature

To take a measurement, immerse the electrode in the sample, stir gently to speed up the response, wait for the stability symbol, then hold the reading on the screen. It is best to draw the water into a clean cup rather than measure directly in the tank, which avoids dropping the meter in and lets you note the value in peace. The meter stores two sets of buffers in its memory, 4.01, 7.01 and 10.01 or 4.01, 6.86 and 9.18, depending on the solutions you use. Clean the electrode regularly by soaking it in a cleaning solution, then calibrate again, because cleaning slightly alters the probe's response.

A single reading says little, whereas a series taken at the same time of day shows the trend. Note your values in a notebook: a steady drop from one week to the next signals nitrification consuming the alkalinity, and gives you time to correct before reaching the threshold where the bacteria slow down. The FAO manual recommends taking this reading every week, before the fish are fed, and it is worth doing more often during start-up. To raise the pH towards 7, the usual practice is to alternate calcium hydroxide and potassium hydroxide, which supply calcium and potassium to the plants at the same time. Conversely, a pH that stays too high locks out iron, and this is often what explains young leaves turning yellow, as the article on iron deficiency in aquaponics explains.

The same device is used in hydroponics, where the pH is set below the aquaponics range since no fish or biofilter imposes a compromise. The feeding charts for coir fertilisers, for example, aim for a pH of 5.8 to 6.5. Check the solution after each addition of fertiliser and after each correction, because a mineral fertiliser and a corrector both alter the pH, then let the solution circulate for a few minutes before measuring again. At the edge of a garden pond, the pH55 tracks how the water changes through the seasons, and its temperature compensation makes summer and winter readings comparable.

The pH55 does not measure conductivity, which is read with a separate conductivity tester; together, the two devices cover the two basic measurements of a nutrient solution. The tester comes with its Mi56P electrode already fitted, its protective cap, four LR44 batteries and two 20 ml sachets of pH 4.01 and pH 7.01 buffer solutions, enough to carry out the first calibration straight out of the box. Pour a few drops of storage solution into the cap before first use, then store your device upright, away from frost and direct sunlight, so that the electrode stays responsive from one season to the next.