Interface level measurement is very important topic in industrial level measurements. In industries like oil & gas, chemicals, and petrochemicals, it is very common to have tanks or vessels filled with two or more liquids.
For example, oil and water often exist together in a tank. These liquids don’t mix well and form separate layers because they have different densities (or specific gravity).
In simple terms, the lighter liquid floats on top of the heavier one.
Knowing where these two liquids meet called the interface level is very important.
Let us understand how engineers measure it using different types of level measuring techniques.
Imagine a bottle of salad dressing with oil and vinegar. The oil stays on top, and the vinegar stays at the bottom. The boundary between them is called the interface.
In industrial tanks, it is the same.
For example:
Crude oil floats on water
Diesel floats on green diesel
Soap may float on black liquor
Sometimes the layers are clearly separate. Other times, there is a mixing layer in between, called a rag layer or emulsion layer — A cloudy zone where the two liquids partially mix.
Please see image below
Why Interface level measurement?
Knowing the exact position of the interface helps operators:
Prevent water from entering oil-processing systems (which could damage equipment).
Avoid losing valuable oil mixed with water.
Ensure clean separation of products (like removing methanol from water or soap from black liquor).
Reduce waste and extra processing costs.
Even a small mistake in separation can cause:
Product contamination
Increased energy usage
Higher operating costs
The Role of Density (Specific Gravity) in Interface level measurement
Most Interface level measurement techniques depends on the difference in density between two liquids. The bigger the difference, the easier it is to detect the interface. But if the liquids have similar densities, it becomes harder to measure the exact boundary.
Sometimes we also need to know:
The overall level in the tank (top of the upper liquid)
The thickness of the upper layer
Whether the rag layer is too thick or manageable
Technologies Used for Interface Level Measurement
Let us go through the most common measurement methods. Each has its own advantages and limitations.
Floats and Displacers – The Oldest but Simplest Tools
How It Works:
A float rides on the surface of the top liquid. If there are two layers (like oil over water), it only measures the oil level.
A displacer, on the other hand, is submerged and designed to settle at the interface between two liquids. It balances based on the specific gravity (density) of the liquid it is sitting in.
Advantages
– Very affordable and easy to understand
– Good for basic interface detection
– Can be used in non-hazardous, low-pressure tanks
Limitations
– Only works if you know the densities of the liquids and they don’t change.
– If the liquids stick to the float or displacer, readings become inaccurate.
– Turbulence in the tank causes float instability.
– Rag layers can confuse the reading.
Where It is Used?
– Simple storage tanks
– Oil-water separation vessels
– Industrial water tanks
Capacitance Sensors – Best for Conductive vs. Non-Conductive Liquids
How It Works?
A probe inside the tank acts like part of a capacitor.
One liquid (e.g., water) is conductive, and the other (e.g., oil) is non-conductive.
The probe detects the change in capacitance at the interface.
Advantages
– Works even if a rag layer is present.
– Good for narrow interface zones.
– Can be used in small or narrow tanks.
Limitations
– If conductivity of the fluid changes, accuracy drops.
– Can only measure one parameter at a time: either interface or thickness of conductive fluid — not both.
– Not suitable for changing product types.
Where It is Used?
– Chemical mixing tanks
– Oil/water separator units
– Food & beverage and pharma industries (when materials are stable)
Guided Wave Radar (GWR) – Smart & Accurate Technology
How It Works:
A radar signal travels down a rod or cable inside the tank.
The signal bounces back at two points:
Top surface of the upper liquid (like oil)
Interface between oil and water (due to difference in dielectric constant)
Advantages:
– A radar signal travels down a rod or cable inside the tank.
– The signal bounces back at two points:
– Top surface of the upper liquid (like oil)
– Interface between oil and water (due to difference in dielectric constant)
Limitations:
– Measures both top level and interface.
– No effect from density, pressure, or vapor.
– Works in pressurized or high-temp environments.
– Can be installed in long vertical tanks (up to 30 ft).
Where It is Used?
– Needs minimum 10 dielectric difference (dk) between the two liquids.
– Does not work well with thick rag layers.
– Must be configured properly for accurate multi-layer detection.
Multi-Parameter Sensors – Hybrid of Radar and Capacitance
How It Works:
Combines guided wave radar + capacitance in a single probe.
If radar signal is blocked by an emulsion, the device automatically switches to capacitance mode.
Advantages:
– Handles rag layers + gives top and interface levels.
– Switches automatically based on conditions.
– Tracks multiple tank echoes and ignores false readings from tank structures.
Limitations:
– Higher cost
– Requires skilled configuration
– May not be needed in simple two-liquid systems
Where It is Used?
– Advanced oil-water-gas separator tanks
– Tanks with variable or unpredictable fluids
– Refining and high-precision chemical processing
Gamma (Radiometric) Level Sensors – Powerful and Non-Invasive
How It Works:
A gamma source emits radiation across the tank.
A detector on the opposite side measures how much radiation gets through.
Denser materials (like water) block more radiation, lighter ones (like oil) block less.
Multiple detectors allow mapping of multiple interfaces or layers.
Advantages:
– Works without touching the liquid (non-invasive).
– Can penetrate metal tanks, foam, vapor, and sludge.
– No moving parts so no maintenance
– Great for very large or pressurized tanks
Limitations:
– Expensive
– Requires nuclear safety permits and regulations
– Needs trained personnel for installation and compliance
Where It is Used?
– Heavy oil & gas applications
– Refineries and multiphase separators
– Harsh and high-pressure chemical reactors
Ultrasonic Sensors – Best for Solids in Liquids
How It Works:
The sensor sends an ultrasonic sound wave into the liquid.
When it hits a layer of solid material (like sludge or salt), the signal reflects back.
The time taken tells how deep the solid layer is.
Advantages:
– Useful in solid/liquid interfaces
– Works well in wastewater, mining, and chemical sedimentation tanks
– Non-invasive if mounted externally
Limitations:
– Does not work with liquid-liquid interfaces (e.g., oil/water)
– Not reliable in bubbly or foamy liquids
– May need calibration if solids settle unevenly
Where It is Used?
– Sludge monitoring in wastewater treatment
– Brine/salt separation tanks
– Mining thickener applications
Sight Glass, DP Cells, and Bubble Tubes – Basic Tools
How It Works:
Sight glass: A transparent tube shows the liquid levels directly. This is very rate method used in Interface level measurements.
DP (Differential Pressure): Measures pressure difference between top and bottom to calculate level.
Bubble tubes: Blows air from bottom of tank, back pressure gives level information.
Advantages:
– Very simple and low cost
– Good for visual checks or non-critical applications
Limitations:
– Manual reading (sight glass)
– Can not show multiple layers or interfaces clearly
– DP depends on stable fluid density
– Bubble tubes may get clogged or affected by coating fluids
Where It is Used?
– Small tanks
– Backup readings
– Educational labs or training facilities
Points to be considered while Choosing the Right Sensor
Not every sensor fits every situation. Here is what you should think about before selecting:
1) What two liquids are you measuring?
2) Are they conductive or non-conductive?
3) Do they form a rag/emulsion layer?
4) Is the interface clearly defined or changing?
5) Do you need just the interface, or interface + overall level?
6) Is the tank pressurized, hot, or dirty?
7) What is your budget?
There is no single sensor that solves all interface level challenges. Often, two sensors are used together.
For example:
Capacitance to measure the interface
Ultrasonic to measure total level
Whenever in doubt, it is always best to:
Talk to a sensor expert or supplier
Share all process details (liquid types, temperature, tank size, etc.)
Ask for real application experience
Here we learned what is Interface level measurement and different technologies used for the same.
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