The measurement of density specific gravity

Instruments are available for the continuous measurement of the density and relative density (specific gravity) of both liquids and gases. When specific gravity is a sufficiently sensitive function of composition for a particular process stream, sensors employed to monitor density can also be employed as composition analysers. 

Certain instruments which have been designed to measure liquid level (Section 6.5) can be modified easily to record variations in liquid density. Such instruments are those that measure the pressure P exerted by a given depth of liquid z in a vessel, i.e. P = pgz. If z is fixed then  

Typical are the pressure transducer (Section 6.3.2) and the DP cell (Section 6.3.4). In these cases, of course, the level must be maintained constant. 

These are generally in the form of a U-tube attached by flexible connectors to the inlet and outlet lines of the instrument (Fig. 6.34). The instrument has a substantial volume (ID = 0.025 m) and, where flows are small (recommended minimum velocity = 1 m/s), it is probable that the whole process fluid stream, rather than simply a sample, will be sent through the instrument. 

Sensing by Continuous Measurement Discrete Measurement Suitable for Comments 

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