Regular calibration of EC meters is essential. If EC meters are not reading correctly than the grower is misinformed as to the strength of the nutrient solution which could have deleterious effects on the plants in the system.
This standard is 2.76mS/cm and meters that are calibrated in milliSeimens should be calibrated to this level. For meters that read in parts per million (ppm) things are a little more complicated. All EC meters measure EC in mS/cm. The meters that have a read-out in ppm have electronically converted mS to ppm. The problem is that different manufacturers convert mS to ppm in different ways.
1 mS = 500 ppm meters : 2.76 mS = 1380 ppm
1mS = 666 ppm : 2.76 mS = 1838 ppm
1 mS = 700 ppm : 2.76 mS = 1932 ppm
Some meters like the brilliant Bluelab meters do not need calibrating, but it is still good to check that the EC is still reading correctly. With these meters an incorrect reading is usually due to low batteries or the EC probe needing cleaning.
Pour as much EC standard as is necessary to cover the electrodes. Discard the standard after calibration. Do not return it to the bottle it came from.