Oil-water interface measurement is not one task. Depending on the vessel and control objective, an instrument may need to locate a clean boundary, track an emulsion zone, indicate local hydrocarbon-water concentration, or profile several phases. This guide compares the main technologies and the conditions that influence selection.
Start by defining what the control system actually needs. Boundary position, total liquid level, emulsion thickness, local concentration and a full vessel profile are different measurements. A technology that is strong at one may not provide the others.
Start with the measurement objective
Five questions are often grouped together
A boundary measurement answers where one liquid changes to another. Total level establishes the liquid surface. Emulsion measurement follows the thickness or movement of a transition zone. Local concentration indicates the phase composition at a sensing point. Full profiling shows several layers through a vertical range. The application should state which outputs are required before a technology is shortlisted.
RF energy absorption and the Agar ID-200
The ID-200H family uses RF energy absorption at an intrusive probe to indicate local hydrocarbon-water concentration. Oil and water absorb RF energy differently, allowing a continuous 4-20 mA signal with HART for monitoring and control. This is useful where a changing emulsion makes a simple clean-boundary switch inadequate, but the reading is local to the sensing point. One probe does not create a full vertical profile or independently provide total liquid level.
What the ID-200 measures
The published ID-201H and ID-202H brochure describes measurement from 0 to 100% hydrocarbon or water by volume, two-wire loop power and pressure options up to 3000 psi. Temperature range, materials, connection and probe configuration depend on the selected model. Final suitability must be confirmed against the vessel, process conditions and wetted-material requirements.
Guided-wave radar
Guided-wave radar sends a microwave pulse along a probe and interprets reflections from dielectric changes. It can provide total level and interface measurements in one installation when the upper and lower liquids create a distinct reflection. Thick emulsions, weak dielectric contrast, deposits and internal obstructions can reduce confidence, so probe style and process conditions matter.
Capacitance
Capacitance instruments respond to changes in dielectric properties around a probe. They can be effective for level or interface duties where the two phases have a stable and meaningful dielectric difference. Coating, changing product composition, conductive build-up and a broad emulsion layer can change the response, making application data and calibration important.
Differential pressure
Differential pressure infers level or interface from hydrostatic head. It is familiar and can be economical when phase densities are known and stable. Its accuracy is more vulnerable when density changes with crude blend, temperature, pressure, entrained gas or emulsion composition. Remote seals and impulse arrangements also add installation and maintenance considerations.
Radiometric multiphase profiling
Radiometric systems measure attenuation through the vessel wall and can be configured for non-intrusive multiphase level or interface profiling. They can suit severe vessels where process contact is undesirable and several layers must be resolved. The trade-offs are project complexity, specialist engineering, source licensing and site radiation procedures.
Where ultrasonic measurement fits
Ultrasonic instruments are widely used for non-contact total-level measurement, especially in open or simple vessels. For liquid-liquid interfaces inside a closed refinery vessel, signal path, gas space, foam, vapour, temperature and attenuation through changing media can limit the application. An ultrasonic proposal should therefore be checked against the exact vessel arrangement rather than treated as a universal interface solution.
Selection questions for a real application
Is the required output total level, a clean boundary, emulsion thickness, local concentration or a full multiphase profile?
How much can density, dielectric constant, conductivity and product composition change during normal operation?
Will coating, solids, wax, salt, rag layers, foam, vapour or entrained gas affect the sensing path or probe?
What insertion, isolation, nozzle, pressure, temperature, hazardous-area and maintenance constraints apply?
No technology wins every application. A sound choice begins with the control objective and process envelope, then checks the claimed measurement against published limits, installation details and the behaviour of the actual fluids. For an ID-200 application, PMA should confirm probe position, model, materials, connection, temperature and pressure before final selection.
