ABSTRACT
This chapter describes the differential pressure between two levels in the liquid is determined by the use of two “bubbler tubes” vertically separated by a distance. A hydrometer is an instrument that can be used to determine the density or the specific gravity of liquids used in flow measurements. The hydrometer floats with the stem partially immersed in a liquid of which the density or specific gravity is to be determined. Reference standard hydrometers can be calibrated by relating their flotation levels to liquid bath densities independently determined by hydrostatic weighing of solid object density standards. Working hydrometers are tested or verified by floating the working hydrometer together with a reference standard hydrometer in a suitable liquid in a comparator. The density of a liquid can be inferred from the differential pressure measured between two probes immersed at different heights in the liquid.
