Radar Level Measurement: Differences Between Guided Wave Radar (GWR) and Non-Contact Radar (FMCW)
Both radar technologies emit microwave signals that reflect off the material surface and, by measuring the time of flight, calculate the level. The key difference is how the signal travels: guided by a metallic probe (GWR) or in the free space of the tank (FMCW).
1. Guided wave radar (GWR) principle
Guided wave radar sends a pulse along a probe (cable, rod or coaxial) that descends to the material. The signal travels through the probe and reflects when it finds the surface or interface. The return time is converted into level.
- Advantages: Not affected by foam, vapors, dust or density changes that attenuate the free signal; suitable for materials with low dielectric constant; excellent for interfaces (liquid-liquid); ideal in tight tanks or with obstructions.
- Limitations: The probe must remain free inside the tank (risk of damage by agitators or turbulent flow); shorter range than FMCW; requires that the level contacts or slides along the probe.
- Applications: Small and medium tanks, solids, interfaces, areas with vapors, liquids with low dielectric constant, applications where the probe fits the geometry.
2. FMCW (non-contact) radar principle
FMCW (Frequency Modulated Continuous Wave) radar emits a signal from an antenna mounted at the top of the tank, without contact with the material. It modulates the frequency continuously and calculates distance by the frequency difference between the emitted and reflected signal.
- Advantages: No contact with the process (ideal for corrosive or hygienic); long range (up to 60 m or more); no probe inside the tank; easy to remove without emptying; tolerates dust, vapors and moderate pressure.
- Limitations: Can be affected by dense foam, condensation on the antenna, deposits in the nozzle, and by very low dielectric constant materials that reflect little signal; requires a free opening angle without obstacles.
- Applications: Large tanks and silos, bulk liquids, solids, reactors, pressurized vessels, processes where contact with the media is not desired.
3. When to choose each technology
| Condition | GWR (guided) | FMCW (free) |
|---|---|---|
| Dense foam or vapors | Excellent | May attenuate |
| Liquid-liquid interfaces | Excellent | Difficult |
| Solids / tall silos | Medium range | Long range |
| Sanitary non-contact processes | Requires probe | Ideal |
| Low dielectric constant | Excellent | Weak reflection |
| Corrosives in contact | Probe material | Non-contact |
4. Installation considerations
For FMCW radar, the antenna must have a clear view of the surface and be separated from the wall and from nozzles that could reflect false signals. For GWR, the probe must be kept as vertical as possible and free of obstacles, avoiding damage by the agitator or transverse flow; in agitated tanks a stilling well is recommended.
Both require calibrating the tank parameters (height, dead zone, dielectric constant of the media) and defining the real measurement range. Good aiming or correct probe/antenna selection substantially improves reliability.
Conclusion
Guided wave radar (GWR) is the most robust option for interfaces, vapors, foam and low dielectric constant materials, while FMCW radar delivers excellent non-contact performance, long range and easy maintenance. Correct selection depends on tank geometry, process conditions and the material measured.
MARPATECH SAC represents in Peru, Argentina and Colombia leading radar level instrumentation manufacturers, including SOR and Berthold, advising on the selection and installation of the right solution for each tank and silo.
Frequently asked questions
Yes, GWR is especially suitable for measuring liquid-liquid interfaces because the wave travels along the probe and detects both the top surface and the interface with the second liquid, provided the densities differ sufficiently.
Thick foam attenuates and scatters the signal that travels through free space, producing weak or erratic reflections. Guided radar, traveling along the probe, crosses the foam better and detects the true liquid surface.
It is the ability of a material to reflect microwaves. A high value (water >80) reflects well; a low value (hydrocarbons or some dry solids <2) reflects little. The lower the dielectric constant, the harder radar measurement is, and GWR usually performs better in those cases.