A device used to measure the average density of the fluids contained in a fixed length of the well bore located between sensitive membrane-type pressure sensors, irrespective of the fluid distribution. The measurement recorded as a function of depth is called specific gravity. The recorded curve would represent a specific gravity profile of fluids in the borehole for the conditions under which the survey was run. In some cases it may be affected by hole deviation, a friction component, and a kinetic component.