A small sensor, part of a production logging tool, which determines the type of fluid in its vicinity as it moves up and down a production well. Typically there are four or more sensors, or probes, held on arms to measure the four quadrants of the well cross-section. The probes may be electrical, to distinguish hydrocarbon from water; optical, mainly to distinguish gas from liquid, but also oil from water; or dielectric, mainly to distinguish water from hydrocarbon, but also, with less resolution, oil from gas. They can detect bubbles that are larger than about 1 mm diameter. Their response is essentially digital, indicating either one fluid or the other, so that the percentage of time that they see a fluid is a direct measure of its holdup. The rate of change between the two fluids is known as the bubble count.

The results can be averaged to give the mean holdup and bubble count, or converted into an image, showing the holdup or bubble count at different locations across the well at different depths. The image is particularly useful in highly deviated or horizontal wells where different flow regimes may be found in different quadrants.