Micro Motion Coriolis flowmeters are calibrated for density prior to shipment using air and water.

The sensor is operated with the flow tube full of air and the corresponding tube period is recorded as the K1 density calibration factor. The actual density of the air at the time of calibration, which has been determined by laboratory reference measurements, is recorded as the D1 density calibration factor. The tubes are then filled with water and the tube period is recorded as K2. The density of the water, again determined from traceable laboratory standards, is recorded as D2. These four factors, K1/D1 and K2/D2 characterize the response of a particular sensor to fluid density.

The relationship between the fluid density and the tube period squared is linear.  The illustration shows the linear relationship between the densities of the two calibration fluids and the tube periods.