(645c) Accurately Calibrating Vibrating-Tube Densimeters for Operation Over Large Ranges of Temperature and Pressure

Accurate density data, particularly over large ranges in temperature and pressure aid greatly in the development of mathematical models (equations of state) used to predict the thermophysical properties of fluids. In turn, these models are used to facilitate the efficient design and operation of equipment and processes that have a wide range of applications. As a result of their commercial availability, vibrating-tube densimeters are increasingly being used to measure density, and they are being applied to ever more complex fluids. This type of instrument can provide valuable data, however it is important to fully understand the uncertainties associated with this measurement method.

The fact that the vibrating-tube method is not a direct measurement of density necessitates calibration of such instruments with well known fluids. The law of propagation of errors dictates that the overall measurement uncertainty can be no less than the uncertainty associated with the densities of the fluids used for calibration. Thus, the choice of calibration fluids can greatly contribute to the measurement validity and uncertainty particularly if measurements are to be made over large ranges of temperature and pressure, covering wide ranges of densities. Five criteria that should be considered when choosing a calibration fluid include:

1) Does a highly accurate equation of state exist for the fluid?

2) The density range of the fluid over the operating range of the instrument being calibrated;

3) The compressibility of the fluid;

4) The availability of high purity samples of the fluid;

5) Toxicity and other handling issues.

How each of these criteria can affect overall measurement uncertainty will be discussed, as well suggestions for choosing calibration fluids in order to obtain the most accurate results.