Every radar and capacitance level instrument depends on one property of the product, its relative permittivity. Water returns a strong echo while oils, LPG and plastic pellets return only a faint one, and knowing why helps you choose and set up the right sensor.
The strength of a radar echo and the size of a capacitance change both depend on the product. This guide explains the dielectric constant, typical values and practical rules for low reflection products.
What Is Dielectric Constant?
The dielectric constant, also called relative permittivity and written εr, tells how strongly a material stores electric field energy compared with a vacuum. A vacuum has εr = 1, dry air is almost the same, and water at room temperature is close to 80, as explained in what is capacitance.
In level work the value matters because microwaves reflect at any boundary where εr changes, and because a capacitor formed by a probe and tank wall changes its capacitance with the material between them. A bigger change gives a stronger, easier signal.
Polar liquids such as water, acids and alcohols have high values, while hydrocarbons, LPG, solvents and most plastics have low values near 1.5 to 3. That simple split decides whether a radar level transmitter sees a strong echo or a weak one.
Why the Dielectric Constant Controls the Radar Echo
A radar pulse travelling through vapour meets the product surface, and part of the energy bounces back while the rest enters the liquid. The larger the jump in dielectric constant at that surface, the larger the share that returns to the antenna or probe.
Process Instrumentation magazine explains that the dielectric constant of typical process media ranges from about 1.4 to 100, and that a higher value gives a stronger reflected signal. It compares oil at about 2.0 with water at about 80 as the classic weak and strong cases.
The same physics applies to the guided pulse in a guided wave radar level transmitter and to the free space beam of a non contact radar. Guided wave radar loses less energy on the way, so it handles lower values more easily, a trade off compared in guided wave radar vs non contact radar.
Because microwaves slow down inside a material by the square root of its dielectric constant, a guided wave radar pulse moves through oil at about two thirds of its speed in air. Instruments use this delay to measure interface layers below the top surface.
Reflected Power Formula for a Product Surface
For a wave striking a flat surface head on, the share of power reflected depends only on the dielectric constant of the two media. This is a simplified textbook model, but it shows clearly why low value products are hard to measure.
Reflected power % = Γ² × 100
Echo level in dB = 10 × log10(Γ²)
Example: vapour εr1 = 1.0, oil εr2 = 2.1
√2.1 = 1.449
Γ = (1.449 minus 1) ÷ (1.449 + 1) = 0.183
Reflected power = 3.36 %, about 14.73 dB below a perfect reflector
Repeat the same sum for water with εr2 = 80 and about 64 % of the power comes back. That is roughly nineteen times more echo energy than oil, before foam, turbulence or beam spreading reduce it further.
Dielectric Constant Echo Calculator
Second Worked Example: Oil Over Water Interface
Now set the upper medium to oil with εr1 = 2.1 and the lower medium to water with εr2 = 80. The calculator gives a reflected power of about 52 %, so the interface echo is far stronger than the oil surface echo above it.
This is why guided wave radar can measure both the top level and the interface in one tank, as described in interface level measurement. The weak first echo marks the oil surface and the strong second echo marks the water below.
Emerson states in its Rosemount interface technical note that the target is an upper product below 3 and a lower product above 20. The Rosemount 5300 needs a difference of more than 6, while the 3300 series needs a difference greater than 10.
Magnetrol gives a similar guideline, suggesting an upper layer between 1.4 and 5 and a lower layer above 15. It also warns that a thick emulsion or rag layer weakens the interface echo, while a displacer tracks the middle of the emulsion.
When an oil layer is thick, check the maximum upper layer thickness in the data sheet. Emerson lists about 19 m for the 5300 series with an upper product of 2 over a lower product of 20.
Typical Dielectric Constant Values for Common Products
| Product | Approximate εr | Radar Difficulty |
|---|---|---|
| Vacuum, dry air | 1.0 | Reference |
| LPG, propane and butane liquid | 1.6 to 1.9 | Very low, needs care |
| Plastic pellets in bulk | 1.5 to 2.5 | Low, plus sloping surface |
| Petrol, diesel, light crude | about 2 to 2.4 | Low |
| Hydraulic and lube oil | about 2.1 | Low |
| Liquid ammonia | about 16 | Medium |
| Ethanol | about 24 | Good |
| Methanol | about 33 | Good |
| Water and aqueous solutions | about 80 | Excellent |
These values are typical at room temperature and change with temperature, moisture and bulk density. Always confirm the dielectric constant with the supplier, a laboratory test or the instrument maker before final selection.
Process Instrumentation magazine notes that modern guided wave radar with advanced signal processing can detect a dielectric constant as low as 1.2. Free space radar usually needs more, so check the minimum value in each data sheet.
Effect on Guided Wave Radar Probe Choice
Inner rod inside an outer tube keeps the field concentrated.
Two parallel conductors share the field.
One conductor with the tank wall or nozzle as return.
Process Instrumentation magazine ranks the coaxial probe as most efficient and the single probe as least efficient but most tolerant of coating. Choose the coaxial type for clean low value liquids, and the single probe where buildup, as discussed in level measurement in high viscosity liquids, would block a coaxial tube.
Many modern transmitters also offer probe end projection, which uses the shift of the probe end echo to work out the level when the surface echo is too weak. Emerson highlights this feature for low value fluids on long probes.
Capacitance Level Sensors and Dielectric Constant
A capacitance level probe and the tank wall form a capacitor whose value rises as product with a higher dielectric constant replaces vapour. For a non conductive product, the capacitance change is roughly proportional to (εr minus 1), so a low value oil gives only a small signal.
For this reason capacitance instruments are calibrated on the real product, as shown in capacitance level transmitter calibration. A change in product grade, water content or temperature changes the dielectric constant and shifts the reading.
Point level devices behave the same way, so a capacitance level switch needs its sensitivity set for the product in use. Conductive liquids act differently because the probe insulation, not the product value, then decides the capacitance.
5 Vital Rules for Low Dielectric Constant Products
False echoes matter more with weak products because a nozzle edge can return more energy than the surface itself. The mapping procedure is described in radar level transmitter false echo handling, and agitator effects in level measurement in agitated vessels.
On LPG and light hydrocarbon spheres, specify the minimum dielectric constant at the highest operating temperature. Values fall as temperature rises, so the hot case is the worst case for echo strength.
Vapour Space, Temperature and Foam Effects
Control Global, in a discussion moderated by Béla Lipták, notes that radar needs a product dielectric constant above about 1.4. It also explains that hot, high pressure steam in the vapour space raises the vapour value, slows the pulse and makes the level look lower than it is.
The same source describes a reference target fixed on the probe in the vapour space so the transmitter can correct this delay, which matters on boiler drum level service. It adds that for most media the dielectric constant falls as temperature rises.
Foam and coating also reflect or absorb microwaves, and Lipták notes the error grows with higher foam value, extent and density. Practical fixes are covered in why foam affects level sensors.
- Strong, stable echo at long range
- Less sensitive to beam spreading and foam
- Simple probe and antenna choice
- Clear interface signal under a low value layer
- Weak echo easily masked by obstacles
- Needs coaxial probe, still pipe or larger antenna
- Sensitive to temperature and grade changes
- Capacitance signal becomes very small
Commissioning and Troubleshooting Checklist
- Enter the correct product dielectric constant or media group in the setup
- Check the echo curve for a clear surface peak above the threshold
- Run a false echo map with the tank empty or at low level
- Confirm probe or antenna is clean and free of condensate
- Verify vapour space compensation on steam and high pressure service
- Compare the reading with a manual dip or sight glass
- Record the echo amplitude as a baseline for future diagnostics
If the reading jumps or freezes, look first at the echo curve rather than the output. A weak or drifting peak points to a product or vapour issue, while a fixed peak points to an obstacle, and the overall choice of technology is discussed in how to select the right level sensor.
For displacement based alternatives that ignore the value completely, see the displacer level transmitter. A radar level selection guide can also help when both radar types look possible.
Emerson Guided Wave Radar Interface Technical Note
Radar and Product Permittivity Video
Dielectric Constant FAQ
It is the relative permittivity of the product compared with a vacuum. It decides how much microwave energy reflects from the surface and how much the reading of a capacitance probe changes.
Water has a value near 80, while oils and LPG are close to 2. A low dielectric constant gives a weaker signal and calls for more careful instrument selection.
Oils have a value close to that of the vapour above them, so the boundary is only a small step. Most of the microwave energy enters the oil instead of bouncing back.
In the simple reflection model only about 3 percent of the power returns from oil. Water returns more than 60 percent under the same conditions.
Control Global quotes about 1.4 as a common practical minimum for radar level work. Guided wave radar with advanced signal processing can work down to about 1.2.
Each instrument model has its own limit stated clearly in the data sheet. Check it against the lowest value you expect at the hottest operating condition of the vessel.
The pulse reflects weakly from the low value top layer and strongly from the high value lower layer. The transmitter measures the time to each echo.
The speed through the top layer depends on its permittivity, so that value must be entered correctly. Emerson suggests below 3 for the top layer and above 20 below.
Yes, for most liquids the value falls as temperature rises. A product that measures well when cold may therefore give a noticeably weaker echo when hot.
Steam in the vapour space works the other way, and its value rises steadily with pressure. That slows the pulse and needs proper vapour compensation on high pressure boiler service.
The capacitance between probe and wall rises with the product value. A low value product gives only a small change, so the instrument must be calibrated on the real product.
Grade changes, water content and process temperature can all shift the reading. Conductive liquids behave differently, because the probe insulation then sets the measured capacitance.
Start with published tables and the media list of the instrument maker, then confirm with the product supplier. A laboratory measurement at process temperature gives the best figure.
Some radar transmitters can also estimate the value from the echo amplitude during setup. Record that figure in the instrument datasheet so maintenance teams can use it later.
Related Articles
- Guided Wave Radar Level Transmitter
- Capacitance Level Measurement
- Interface Level Measurement
- Radar Level Transmitter False Echo Handling
- Why Foam Affects Level Sensors
External References
- Guided Wave Radar Interface Measurement Technical Note, Emerson
- Radar Gauge Considerations, Control Global
- Relative Permittivity, Wikipedia
What We Learn Today
- The dielectric constant of a product decides how much microwave energy reflects from its surface, from about 3 percent for oil to over 60 percent for water.
- Guided wave radar with a coaxial probe, 80 GHz radar or a still pipe are the usual answers for low value liquids such as LPG and oils.
- Interface measurement needs a low value upper layer and a much higher lower layer, and the value must be correct for the pulse speed.