Containers of liquid of known acidity are shown in picture. These liquids are used to calibrate many meters. Rather than using the pH 7 calibration solution, pure water can be used, although it is very hard to obtain water pure enough to use as such a reference. Instead, simple chemicals are mixed in known quantities to create a neutral formula. Rather than using pH 4 calibration solution, a solution of pH 1 could be used. This would be more accurate, however pH 1 is extremely acidic, making it awkward to handle and would very probably corrode the probes of most meters. Therefore the milder pH 4 solution is prefered.

A very small quantity of calibration fluid is required, merely enough to immerse the probe fully. Calibration fluid can be re-used, although this is not recommended because it may have been contaminated. The acidic pH 4 solution should not be used to lower the acidity of nutrient. Both formulations may react with nutrient - please rinse meter probes before use. You will need to calibrate your pH meter regularly to ensure maximum accuracy. Also, many CF (conductivity) meters require calibration.