Measurement errors can be expensive.
Some manufacturers of density meters suggest calibration with air and water, but it is doubtful whether this meets the requirements of GLP, and it certainly does not give a calibration traceable to recognised national standards. It can place too much reliance on the behaviour of the measurement cell. Even the most modern dual frequency density meters can have their error halved if they are calibrated with a certified liquid and water. These standards are traceable to recognised national standards of mass temperature and pressure. To meet the differing demands of the wide variety of industries using standards, a range of liquids are available.