Calibration is performed on any instrument that you use to ensure that it is within a speicifed range. I check the calibration on my pipettes before using them by sucking up a known amount of water and weighing it. There is an acceptable range and if it is within that it is ok to use. If not we have to have a wee look at it. I also run a calibration on my instruments before using them . That involves running about 5 solutions that I know what their concentration is. They should give a calibration curve or straight line. This lets the instrument know that at this count or value it is that specific concentration. This allows us to quantify an unknown solution.