— A Practical Guide for Engineers, EPCs, and Project Teams
Radar level measurement has become the dominant technology for industrial level applications because it is non-contact (in its free-space form), unaffected by changes in density or temperature, and accurate across a wide range of services. But “radar” covers two distinct instrument families that operate on the same electromagnetic principle yet serve barely overlapping application ranges: free-space (non-contact) radar and guided wave radar (GWR) . Choosing between them is one of the most common—and most consequential—instrument selection decisions in process engineering, and the right answer is defined not by preference, but by the service conditions: dielectric constant, foam, vapour, turbulence, buildup, and vessel geometry.-34
This guide explains how each technology works, the key differences that drive selection, and the practical criteria for matching the right radar family to the application.
1. How Free-Space Radar Works
Free-space radar—also called non-contact radar—transmits electromagnetic waves through the vapour space of the vessel toward the product surface, and measures either the time of flight (pulsed radar) or the frequency shift (FMCW) of the reflected signal.-1
Key characteristics:
| Aspect | Detail |
|---|---|
| Frequency | Typically 6 GHz, 26 GHz, or 80 GHz (80 GHz is the current standard for most liquid and solids applications)-4 |
| Beam angle | 26 GHz: typically 10–15°; 80 GHz: 3–4° -4 |
| Measurement principle | Time-of-flight (pulsed) or FMCW (continuous frequency modulation) |
| Contact with process | None—the antenna sees the surface through the vapour space-34 |
| Typical accuracy | ±2–5 mm for high-frequency systems-1 |
The 80 GHz advantage: The narrow 3–4° beam of 80 GHz radar illuminates a spot only 0.5 m in diameter at 10 m distance, compared with 2.6 m for a 26 GHz sensor with a 15° beam. This focused measurement allows reliable measurement through small openings and around internal tank structures such as agitators, baffles, and heating coils that would otherwise produce false echoes.-4
2. How Guided Wave Radar Works
Guided wave radar (GWR) transmits a low-energy microwave pulse down a probe—a rigid rod, flexible cable, or coaxial waveguide—that extends into the medium. The pulse travels along the probe at the speed of light until it encounters the dielectric discontinuity at the product surface, where a portion of the signal reflects back to the transmitter. The time of flight determines the distance. This is the principle of Time Domain Reflectometry (TDR) .-1
Key characteristics:
| Aspect | Detail |
|---|---|
| Probe types | Rigid rod (short range), flexible cable (long range), coaxial tube (low dielectric/high pressure)-36 |
| Contact with process | Yes—the probe is immersed in the medium |
| Signal path | Guided along the probe; not affected by vapour space or vessel internals-34 |
| Dielectric capability | Media with ε ≥ 1.5 are measurable, including light oils and liquefied gas-36 |
| Typical accuracy | ±1–3 mm-36 |
Because the signal is guided, GWR is not affected by steam, foam, dust, agitators, or internal obstructions—the probe simply bypasses them. The trade-off is that the probe contacts the process and is therefore subject to corrosion, buildup, and mechanical wear.-34-36
3. Head-to-Head Comparison: The Selection Drivers
| Service Condition | Better Choice | Why |
|---|---|---|
| Low dielectric media (hydrocarbons, LPG, solvents, some organics, ε < 2–3) | Guided wave radar | The probe concentrates the signal; free-space radar struggles below roughly ε ≈ 1.5–2.0-34 |
| Heavy vapour, steam, condensation | Guided wave radar (or free-space with a stilling well) | Vapour attenuation and antenna condensation degrade free-space; the GWR probe is unaffected-34 |
| Foam on the surface | Guided wave radar (usually) | Free-space reflects off the foam top, producing a false level; GWR sees through most light foams-34 |
| Agitators, turbulence, vortexing | Guided wave radar (in a stilling well) or free-space with careful placement | GWR probes damp turbulence; free-space needs a calm spot or a stilling well-34 |
| Sticky, coating, or buildup services (asphalt, slurry) | Free-space radar (non-contact) | Buildup on a GWR probe attenuates the signal; a free-space antenna with air purge handles coating better-34 |
| Very large tanks, tall silos (> 20–30 m) | Free-space radar | GWR cables are limited in length and can sway in silos; free-space works to 100 m+-34 |
| Interface measurement (oil/water separation) | Guided wave radar | The probe detects the dielectric difference between the two liquids; free-space radar cannot measure an interface-34 |
| Sanitary, hygienic, high-purity | Free-space radar (flush antenna) | Non-contact avoids contamination risk and crevices; GWR probes require cleaning validation-4 |
The dielectric constant is the single most important electrical property affecting radar selection . A higher dielectric constant produces a stronger reflected signal. For liquids with εr > 3, 80 GHz radar provides reliable measurements even with internal obstructions. For εr < 2 (light hydrocarbons, some powders), careful installation and possibly GWR are required.-4
4. Installation and Application Considerations
4.1 Free-Space Radar
| Requirement | Practice |
|---|---|
| Antenna must “see” the surface | Nozzle height limits the beam; mount away from the fill stream and vessel walls-34 |
| Condensation and buildup | Manage with air purge, heating, or a stilling well-34 |
| Stilling well | Recovers 10–20 dB of return signal on weak reflectors; concentrates the microwave signal-27 |
| Verification | Echo curve stored as baseline; diagnostics show buildup and false echoes before the reading drifts-34 |
4.2 Guided Wave Radar
| Requirement | Practice |
|---|---|
| Probe clearance | Probe-to-wall spacing per manual; probe must not touch the tank bottom-34 |
| Probe selection | Rigid rod for small tanks; cable for tall tanks; coaxial for low dielectric/high pressure-34 |
| Buildup management | Sticky media require periodic cleaning; anti-coating algorithms help but do not eliminate maintenance-11 |
| Dead zone | The upper dead zone (where the pulse is still settling) sets the top measurement limit-34 |
A critical installation note: GWR is the ideal technology for chambers, bridles, and stilling tubes because the probe keeps the signal close to it. Even the 3° beam of 80 GHz non-contact radar is not focused enough to avoid false echoes from chamber walls, welds, and buildup in small-diameter chambers.-6
5. When Both Technologies Are Viable: The Decision Logic
For many applications, both radar families will “work”—the question is which will work better over the life of the plant.
| If the primary concern is… | Choose… |
|---|---|
| Process conditions (foam, steam, low dielectric, interface) | Guided wave radar |
| Maintenance access (sticky, coating, sanitary) | Free-space radar |
| Vessel size (very tall, large diameter) | Free-space radar |
| Vessel geometry (narrow chambers, bypass, complex internals) | Guided wave radar |
| Cost | Free-space radar is generally lower for simple liquid level; GWR is justified where its performance advantages are required-1 |
A practical rule from industry experience: For media with a dielectric constant greater than 4, evaluate 80 GHz high-frequency radar first. For light hydrocarbons and light oils with a dielectric constant below 3, guided wave radar usually provides more stable echoes. This is a starting point—not a hard rule—and must be verified against the full set of process conditions.-11
6. Why Choose Anhui Tiankang for Radar Level Measurement?
Anhui Tiankang (Group) Co., Ltd. has nearly five decades of experience in industrial instrumentation. We offer both 80 GHz non-contact radar and guided wave radar (TWR series) —so our engineers can recommend the right technology for each application without bias toward one family.
Guided wave radar (TWR series) :
| Model | Probe Type | Range | Best For |
|---|---|---|---|
| TWR-B | Rigid rod | 0–6 m | Small vessels, reactors, viscous liquids-36 |
| TWR-C | Flexible cable | 0–35 m | Large storage tanks, tall towers, silos-36 |
| TWR-T | Coaxial tube | 0–10 m | Low dielectric media (light oils, LPG), high-pressure sealed vessels-36 |
Key specifications:
Accuracy: ±1–3 mm-36
Dielectric capability: ε ≥ 1.5 (light oil and LPG measurable)-36
Temperature: up to 300°C (high-temperature versions)-36
Pressure: PN16/PN40 standard; higher on request-36
Output: 4–20 mA + HART standard; RS485 Modbus and relay alarms optional-36
Explosion protection: Ex d IIC T6 -36
Materials: PTFE coating or Hastelloy for corrosive media-36
80 GHz non-contact radar (TKLD series) :
Narrow 3–4° beam for complex vessels and internal obstructions
Non-contact measurement—no probe to maintain or clean
Suitable for corrosive, hygienic, and high-purity applications
Core advantages:
Both technologies from one supplier—unbiased technology selection
Complete certifications: CCC Ex, ATEX, IECEx, SIL, CCS marine
CNAS-accredited laboratory: full performance testing
Long-term supplier to CNPC, Sinopec, CNOOC, and international EPC projects
One-stop supply: from radar level instruments to cables to Ex accessories
7. Conclusion
Radar and guided wave radar are not competitors—they are complementary technologies with distinct application domains.
Key takeaways:
| Aspect | Free-Space Radar | Guided Wave Radar |
|---|---|---|
| Contact | Non-contact | Contact (probe immersed) |
| Signal path | Through vapour space | Guided along probe |
| Dielectric capability | εr > 1.5–2 (80 GHz improves the margin) | ε ≥ 1.5 |
| Foam/vapour | Degraded | Unaffected |
| Buildup | Handles coating well | Susceptible—requires cleaning |
| Tank size | Very large (100 m+) | Limited by probe length |
| Interface measurement | Not possible | Yes |
| Best for | Coating services, tall tanks, sanitary applications | Low dielectric, foam/steam, interface, chambers |
The most important rule: Select by the service, not by the technology name. Start with the dielectric constant, then evaluate foam, vapour, buildup, vessel geometry, and interface requirements. For most liquid and solids applications, 80 GHz non-contact radar is the current standard—but where the service is foamy, vaporous, low-dielectric, or requires interface measurement, guided wave radar is the correct choice.-4
Remember: A radar that is installed correctly but selected for the wrong service will produce unstable readings and require costly rework. The selection decision is made at the desk—not in the field.
Contact Us
For radar level measurement selection advice, technical documentation, or project quotations, please contact:
Yin Shuangjie
International Sales Manager
📧 Email: [email protected]
📱 WhatsApp / Zalo: +86 17856068126
🌐 Website: http://www.tiankang-global.com/
Anhui Tiankang – Your partner for reliable radar level measurement solutions.
