For most people, a liquid colorimetric pH test kit works well and costs from $5 to $ 10. A type of pH indicating paper is also available, but is not as accurate as the liquid test. Serious pH measurement is accomplished using electronic meters that can resolve the pH to the nearest 0.1 of a pH unit. They need calibration against a solution of a known value, which is known as a buffered pH reference, or simply "pH buffer." The pH buffer solutions come in three major values: pH 4, pH 7, and pH 10. The pH 7 buffer is the most important for Hydroponic work, as we want to be just below 7 for most crops. The glass electrode tips of the electronic meters are very fragile. They should be stored in a slightly acid storage solution to prevent the microscopic pores from clogging with dissolved minerals.