In most level-measurement applications, we are concerned with knowing the volume of the liquid
contained within a vessel, and we infer this volume by using instruments to sense the height of the
fluid column. So long as the vessel’s cross-sectional area is constant throughout its height, liquid
height will be directly proportional to stored liquid volume. Pressure measured at the bottom of a
vessel can give us a proportional indication of liquid height if and only if the density of that liquid
is known and constant. This means liquid density is a critically important factor for volumetric
measurement when using hydrostatic pressure-sensing instruments. If liquid density is subject to
random change, the accuracy of any hydrostatic pressure-based level or volume instrument will be
correspondingly unreliable.