PH meters are very sensitive to factors such as the acidity of the air. This will gradually make them less accurate, so they should be calibrated regularly. How often they will need to be calibrated will depend mainly on the quality of the water. In hard water areas pH meters may need calibrating on a weekly basis. It is important that the pH meter is calibrated correctly if the readings are to be trusted. Making adjustments to the nutrient solution based on inaccurate readings can have a worse effect than simply leaving the nutrient solution to circulate.

Meters are calibrated using pH buffers. These are simply solutions of a known pH, against which the meter can be compared and corrected. They are usually available in pH values of 4, 7 and 10, as the ideal hydroponic pH level is 6.0, pH buffers 4 and 7 are used so that calibration can be made at both ends of the range.

HOW TO CALIBRATE A PH METER.

• Firstly make sure that the buffering solution is about the same temperature as the nutrient solution. Only use a small amount of the buffering solution and throw away afterwards to avoid contamination.

• Place the probe end into the pH buffer 7 and note the reading. Calibrate the meter, following the manufacturers instructions, until it shows the correct reading of pH 7.

• Repeat the above with the pH buffer 4.

• Now re-check against the pH buffer 7, as this may need calibrating again. Repeat the process until the reading is correct in both pH buffers.