A float is a buoyant device that rides on the surface of the upper liquid. In a two-layer system, it measures the total liquid surface level (the top of the upper layer), not the interface itself. A displacer is different: it is a heavy, fully submerged rod or cylinder that does not float. Instead, it uses Archimedes' principle: the buoyant force on the displacer changes as it crosses from one liquid into another. When configured for the known densities of both liquids, the displacer transmitter can calculate the interface position from the changing buoyant force it experiences.

The key requirement is that the densities of both liquids must be known and stable. For a DP transmitter or displacer, the calculation relies on stable specific gravity values. If the oil density changes with temperature or composition, the displacer reading becomes unreliable without density compensation.

Typical applications: Simple oil-water separator vessels, storage tanks, industrial water treatment.

A capacitance probe is a rod or cable inserted vertically into the tank. It acts as one plate of a capacitor; the tank wall or a reference electrode is the other plate. The dielectric constant (permittivity) of the liquid between them determines the capacitance. Since oil is non-conductive (low dielectric constant around 2) and water is highly conductive (high dielectric constant around 80), the capacitance changes sharply at the oil-water interface. The electronics measure this change continuously and convert it to an interface position signal.

Capacitance sensors are commonly wired with 4-20 mA outputs to DCS or PLC systems. Some smart versions also communicate via HART for remote configuration and diagnostics.

Typical applications: Chemical mixing tanks, oil-water separators, food and beverage, pharmaceutical processing.

Guided Wave Radar is one of the most widely used technologies for interface measurement in modern process plants. A microwave signal (typically 1-2 GHz) travels down a probe (rod or cable) immersed in the liquid. When the radar signal encounters a change in dielectric constant, such as at the oil-gas surface or the oil-water interface: part of the signal reflects back to the instrument. By measuring the time-of-flight of each reflection, the GWR transmitter calculates both the total level (surface of upper liquid) and the interface level simultaneously from a single probe.

GWR works because oil and water have very different dielectric constants (oil dk 2, water dk 80). The signal strongly reflects at the oil-water interface because of this contrast. The minimum required difference in dielectric constant between the two liquids is typically dk 10, which the oil-water pair far exceeds. Related to how radar signals behave in process vessels, the same principles govern DP transmitter level measurement where hydrostatic head rather than electromagnetic waves is used.

Typical applications: Oil-water separators, crude oil storage, refinery desalters, condensate drums, fuel storage vessels.

Multi-parameter sensors combine guided wave radar and capacitance measurement in a single probe. When the radar signal is blocked or attenuated by a thick emulsion or rag layer (which has intermediate dielectric properties that weaken the interface echo), the device automatically switches to capacitance mode to detect the conductive water layer beneath. This hybrid approach handles the most challenging interface applications where neither technology alone is reliable.

A multi-parameter sensor probe. The outer capacitance electrode (blue sheath) and the inner radar waveguide work together to provide accurate interface detection even when a thick rag layer is present.

Typical applications: Advanced oil-water-gas three-phase separators, tanks with variable emulsion layers, refining and high-precision chemical processing.

Gamma level gauges use a radioactive source (typically Caesium-137 or Cobalt-60) mounted on one side of the vessel and a scintillation detector on the opposite side. The gamma radiation passes through the vessel wall and the process liquid. Denser liquids (water, sludge) absorb more radiation, so less reaches the detector. Lighter liquids (oil, gas) absorb less. By measuring the radiation intensity at different heights using multiple detectors or a continuous detector array, the system can map the complete level profile inside the vessel without any process contact.

A gamma gauge installation showing the radiation source holder on the left and the detector array on the right of the process vessel. The gamma beam passes through the vessel wall and liquid without any process connection.

Typical applications: Heavy crude oil separators, coke drums, high-pressure chemical reactors, multiphase separators where no process nozzles are available.

Ultrasonic sensors transmit sound pulses (typically 40 kHz to 200 kHz) into the liquid. When the pulse hits a layer of solid material such as settled sludge, salt cake or sediment, the acoustic impedance difference causes the pulse to reflect back. The time-of-flight gives the distance to the solid-liquid interface. Ultrasonic is particularly effective for sedimentation and thickener applications where a solid layer settles at the bottom of a liquid-filled tank.

Note that standard ultrasonic sensors are not suitable for liquid-liquid interface measurement (oil-water boundary) because the acoustic impedance difference between oil and water is too small to produce a reliable reflection. The reflection coefficient depends on density and sound speed differences, and oil-water pairs have insufficient contrast for most practical ultrasonic detectors at industrial frequencies.

Typical applications: Sludge monitoring in wastewater treatment, brine and salt separation tanks, mining thickeners, clarifiers.

A differential pressure (DP) transmitter connected with tapping points at the top and bottom of a two-liquid zone can estimate the interface level if both liquid densities are known and stable. The total hydrostatic head measured by the DP cell equals the sum of contributions from each liquid layer. If total level is known from a separate measurement, the interface position can be calculated mathematically. This approach is widely used in existing plants as a low-cost add-on because most vessels already have DP transmitters for other purposes.

Sight glasses provide direct visual confirmation of the interface position for local operators. Bubble tubes blow a small air or gas flow from the bottom of the tank, and the back-pressure provides a level indication. Both sight glass and bubble tube methods are best suited for non-hazardous, low-pressure backup readings or training applications, not for precise continuous control.

Typical applications: Small tanks, backup readings, educational demonstrations, cost-sensitive applications where precision is not critical.