Abstract: A differential level hydrometer reduces the possibility for user error and provides an accurate, predictable, easy to use, reliable instrument for measuring the density (specific gravity) of a liquid. The differential level hydrometer works by automatically measuring the level of a liquid within a vessel and floating, in the liquid, a calibrated float having a precisely known volume and density. The instrument automatically measures the level of the liquid within the vessel while the float is floating in the liquid, and automatically calculates the density (specific gravity) of the liquid based on the two measured levels and the volume and density of the float. Temperature and meniscus correction may be used to improve repeatability and accuracy.