An in situ record of the capability of the fluid passing through a sensor to store electrical charge. Since water has a high dielectric constant, and hence capacitance, it can be distinguished from oil or gas. The capacitance, or fluid capacitance log, can therefore identify water and be scaled in terms of water holdup. However, the relation between capacitance and holdup depends strongly on whether the water is the continuous phase, complicating quantitative evaluation.

The log was introduced in the 1960s with the so-called holdup meter. It was mainly used in three-phase flow, or when fluid-density measurements were insufficiently sensitive to water at low holdup, or with heavy oils. Since the late 1980s, other holdup measurements have been preferred.