The dielectric constant (relative permittivity, εr) of the medium determines how much of the radar signal is reflected to the antenna. Media with εr > 3.6 reflect strongly and are easy targets; low-εr hydrocarbons (typically 1.4–4.0) reflect only a small fraction of the signal and require guided-wave radar or high-frequency (80 GHz) non-contact radar for reliable measurement common claim that “low dielectric constant liquids cannot be measured by radar” is therefore only partly true — what changes is which radar you specify.
If you are unfamiliar with the dielectric constant, please watch this video to help you understand the basics.
https://www.youtube.com/watch?v=4hpLxJYEBHA&t
What Is Dielectric Constant (εr)?
Dielectric constant, more precisely relative permittivity, describes how a material responds to an applied electric field: how much charge it stores per unit voltage compared with vacuum. For radar-level measurement, it matters for one reason — the size of the reflection at the medium surface is governed by the contrast in permittivity between the medium and the air (or vapour) above it. A large contrast produces a strong echo; a small contrast produces a weak one.
The two ends of the practical scale:
- High εr (polar, conductive): water and aqueous solutions, acids and bases. εr for water is about 80 at room temperature, so the surface behaves almost like a radar mirror.
- Low εr (non-polar): hydrocarbons, solvents, and liquefied gases. Benzene is around 2.3 and n-hexane around 1.9, so most of the pulse passes into the medium instead of returning.
Industry groups media into three bands, which is a useful shortcut when reading a specification sheet: non-polar (εr < 2.8), weakly polar (2.8 ≤ εr ≤ 3.6), and polar (εr > 3.6).
Dielectric Constant and Recommended Radar Type
The table below pairs typical εr values with the radar technology that measures each medium reliably. Values are relative permittivity at approximately 20 °C.
| Medium | εr (typical) | Recommended measurement method |
|---|---|---|
| Water | ≈ 80 | Non-contact radar — strong echo, easiest case |
| Acids / caustic solutions (conductive) | ≈ 40–80 | Non-contact radar with a corrosion-compatible antenna (PTFE / flange) |
| Methanol, ethanol, acetone | ≈ 20–33 | Non-contact radar, standard 80 GHz |
| Alcohols/solvents | ≈ 15–25 | Non-contact radar; guided wave where agitation or a narrow nozzle limits the beam |
| Acetic acid | ≈ 6.2 | Non-contact radar (80 GHz recommended) |
| Oils, edible and process oils | ≈ 2–5 | 80 GHz non-contact radar, or guided-wave radar where a stilling well is available |
| Hydrocarbons/gasoline/benzene | ≈ 1.8–2.3 | Guided-wave radar (best signal), or 80 GHz non-contact radar |
| Liquefied petroleum gas, No. 4 solvent | ≈ 1.4–1.8 | Guided-wave radar only |
| Powders and solids (dusty) | varies widely | 80 GHz non-contact radar; guided wave if build-up on the probe can be tolerated |
Common Liquids and Their Relative Permittivity
For reference work, the longer list below covers the solvents that generate most level-measurement enquiries. Values are relative permittivity at room temperature.
| Liquid | εr |
|---|---|
| Water (H2O) | 78.5 |
| Formic acid (HCOOH) | 58.5 |
| N,N-dimethylformamide (DMF) | 36.7 |
| Methanol (CH3OH) | 32.7 |
| Ethanol (C2H5OH) | 24.5 |
| Acetone (CH3COCH3) | 20.7 |
| n-Hexanol | 13.3 |
| Acetic acid | 6.15 |
| Benzene (C6H6) | 2.28 |
| Carbon tetrachloride (CCl4) | 2.24 |
| n-Hexane (C6H14) | 1.88 |
| No. 4 solvent | 1.78 |
How εr Affects Guided-Wave Radar
Guided-wave radar sends a low-energy pulse down a probe (steel cable or rod). At the probe/medium boundary, the change in dielectric constant reflects part of the pulse back; the remainder continues down the probe. Because the pulse is guided rather than radiated, the echo does not depend on a flat surface or a wide beam — which is why guided-wave copes with turbulence, inclined surfaces, and very low εr media that would defeat a non-contact unit.
Practical bands for guided-wave selection:
- Petroleum products and hydrocarbons in petrochemical service: 1.4 ≤ εr ≤ 4.0
- Alcohols, organic solvents and oil–water mixtures: 4.0 ≤ εr ≤ 10.0
- Conductive liquids such as aqueous solutions, dilute acids and bases: εr > 10.0
Guided-wave radar can also measure a gas/liquid, gas/solid, or liquid/liquid interface, which is the standard solution for oil–water boundary detection in separators and sumps.
How εr Affects Non-Contact Radar
Non-contact radar radiates from an antenna and relies on the reflection from the product surface. The higher the dielectric constant, the stronger the reflection — and, all else being equal, the longer the range you can achieve. The further the target, the more signal is lost to beam spread and attenuation, so a low-εr medium has to be closer to the antenna for the same reliability.
Modern 80 GHz (millimetre-wave) radar largely solves the historical limitation: a much larger dynamic signal range and a narrower, more concentrated beam mean low-εr media can be measured more stably than with older 26 GHz instruments. Our 80 GHz radar level sensor range is specified for exactly this duty — for example, the KLD804 radar level sensor reaches ±2 mm accuracy with a minimum blind zone of 0.05 m, and is explicitly designed to hold measurement on low dielectric constant media, adhesion, and condensation.
Measuring Low-εr Media (εr < 2.5): 80 GHz vs Guided Wave
Below roughly εr = 2.5, the reflection is weak enough that instrument choice stops being a preference and becomes a constraint. The two viable options have different boundaries:
| Situation | Choose | Why |
|---|---|---|
| εr 1.8–2.5, clean liquid, large nozzle, no build-up on a probe | 80 GHz non-contact radar | Millimetre-wave dynamic range recovers the weak echo; nothing contacts the product |
| εr 1.4–1.8, or a stilling well/bypass chamber is available | Guided-wave radar | Pulse is guided along the probe, so path loss is minimal and the weak reflection is still detectable |
| Turbulent surface, foam or a strongly inclined liquid level | Guided-wave radar | The echo does not depend on a flat, horizontal surface |
| Aggressive media, hygienic service, or build-up that would foul a probe | 80 GHz non-contact radar | No wetted probe to clean or corrode |
| Liquid/liquid interface (oil–water) | Guided-wave radar | Resolves the interface reflection, not just the top surface |
| Tall silo with dusty solids, very long range | 80 GHz non-contact radar | Narrow beam and high dynamic range, unaffected by dust and condensation |
Where a tank has neither a nozzle suitable for 80 GHz non-contact radar nor a stilling well for a probe, the practical alternative is to change the measurement principle altogether rather than accept an unreliable radar reading — a capacitive or hydrostatic instrument may be the correct engineering answer.
Interface Measurement: What εr Requires
Both guided-wave and non-contact radar can measure an interface by following the portion of the pulse that is not reflected at the upper surface down to the lower product. The requirements are stricter than for simple level:
- Guided-wave radar: the upper medium is typically required to be at least 10–20 cm thick so the two echoes can be resolved; the upper medium εr should be below about 3 and the lower above about 20, with a difference greater than 10.
- Non-contact radar: the dielectric constants of both products must be known and stable, and the difference between them must be greater than about 6.
- In both cases, the measurement degrades if the upper layer is emulsified, if its thickness varies rapidly, or if foam sits at either boundary.
Choosing the Right Radar for Your Liquid
A short selection summary: water and aqueous solutions are the easy case — standard non-contact radar handles them. Solvents and light oils sit in the middle band and generally want 80 GHz. Hydrocarbons and liquefied gases are the hard case and usually want guided wave. If your tank is remote and cabling is the constraint rather than the medium, a wireless radar scheme changes the installation economics — see our 433 M wireless radar technical solution, and for the head-to-head comparison between the two technologies read 80 GHz radar vs guided-wave radar. The full catalogue sits under radar level sensors, with guided-wave models in a dedicated category.
FAQ
Q: What is the minimum dielectric constant a non-contact radar can measure?
A: Manufacturers typically specify a minimum of about εr 1.5–2 for non-contact radar. Below that, guided-wave radar is the recommended technology, because the pulse is guided along a probe rather than radiated through air.
Q: What dielectric constant difference is needed for oil–water interface measurement?
A: Guided-wave radar typically needs the upper medium below about εr 3 and the lower above about 20, with a difference greater than 10; non-contact radar requires a difference greater than about 6. Both media must have known, stable dielectric constants.
Q: Does foam affect radar level measurement on low-εr liquids?
A: Yes. Foam adds a second, diffuse reflecting layer and attenuates the signal before it reaches the real surface. On low-εr media, the margin is already thin, so guided-wave radar — whose echo does not depend on a flat surface — is normally the safer choice.
Q: What is the dielectric constant of water?
A: Water has a high relative permittivity of about 80 at room temperature, which gives a very strong radar echo and makes it the easiest common medium to measure.
References
- Vega, Radar level measurement — technology notes on non-contact and guided-wave radar.
- Endress+Hauser, Radar level measurement — dielectric constant requirements and interface measurement.
- Emerson, Level measurement instrumentation — application guidance.
- CRC Handbook of Chemistry and Physics — source of the relative permittivity values tabulated above.
- Kacise, KLD804 80 GHz radar level sensor datasheet.
