The need for interface measurement arises whenever immiscible liquids–those incapable of mixing—reside within the same vessel.
The lighter material rises to the top and the heavier material settles at the bottom. In oil production, for example, water or steam is used to extract oil
from a well. Well fluids then route to production separators where they settle into their primary constituent parts as a water-hydrocarbon interface.
Water may also be used as a transport medium or a cleaning agent and forms an interface with an allied material which is later extracted.
Interfaces are most commonly found in the diverse separation processes that are essential to every industry. Separation recovers additives, catalysts
or solvents, extracts impurities, and routes media into different processing channels. Though our emphasis is on liquid/liquid interface, interfaces also form between liquid and solids, liquid and foam, or liquid and a gas—such as gases (other than air) that are used in tank blanketing.
Immiscible liquids meet along an interface layer where they undergo some amount of emulsification. This emulsion layer (also called a rag layer)
may form a narrow and precise boundary; but more frequently it is a broader gradient of mixed liquids—or liquids mixed with particles that form
a slurry. Generally, the thicker the emulsion layer, the greater will be the measurement challenge.
Knowing the position of a process interface is necessary for maintaining product quality and operations efficiency. The interface is measured and controlled by precision level switches and transmitters. Though at least 20 different types of liquid level measurement devices are in service today, only a very few are suitable for accurate and reliable interface measurement.