Radar measurements are challenged by the dynamic motion of objects as well as undesirable conditions like foam. The greatest hazard is configuring the radar’s detection thresholds under ideal conditions without mitigating the real-world dynamics. Taken to the extreme, the radar is blind to the measured process.
This engineering brief demonstrates radar dynamics in a Tank Level Indicator (TLI) application using a radar “looking” into a 55-gallon metal drum as shown in Figure 1. While it is a crude setup, it’s approachable engineering that allows us to explore water in motion and the challenges of foam.
Knowledge is the first step toward designing a robust control system that will continue running or fail gracefully.
Key Takeaways
- Radar is sensitive to the angle of the reflective surface relative to the radar beam.
- This angle includes the motion of water with large signal strength changes in response to “small waves” in the drum.
- Verify radar operation in the anticipated environment. Select a radar for power, frequency, and sensitivity. Configure the radar thresholds based on worst case observations.
- Turbulent water and foam are hazards to the TLI and may result in delayed, absent, or even misleading measurement responses.
- Foam degrades the return signal in terms of strength and the spread of the returned pulse.
This article is part of the DigiKey Field Guide for Industrial Automation
Location: Understand It → Sensors
Difficulty: 
Student — difficulty levels explained
Author: Aaron Dahlen | MSEE | Senior Applications Engineer, DigiKey
Last update: 01 Apr 2026
Figure 1: Radar setup to measure the liquid level in a 55 gallon drum.
Parts used in the TLI Experiment
The following components were used in the demonstration:
- 55-gallon drum
- Banner K50RPB-4030-LDQ radar or equivalent
- Support to center the radar sensor to the center of the drum. MDF (1/4 inch) was used for this demonstration. Wooden feet were added to prevent the wood from falling into the tank.
- The Banner 2170-PRO-KIT-ND is required to configure the radar and to observe the real-time data.
- PC to host the Banner Measurement Sensor Software
- 5-gallon bucket
- Water and soap
Tech Tip: The Banner K50 is 60 GHz radar. Lower frequency 24 GHz radar provides improved long-distance detection while high frequency 122 GHz devices may detect smaller challenging objects with a lower dielectric constant.
From an education standpoint, the K50’s programmable multicolor display provides an easy way for students to visualize the radar’s operating state.
Setup
The setup is shown in Figure 1. The sensor sits atop the drum. The radar is then connected to the PC’s USB port using the Banner Pro kit.
Figure 2 presents a screen capture of the empty drum using Banner’s Measurement Sensor Software. The display shows a strong single peak indicating a depth of 0.83 meters. A distinct advantage is Banner’s near real-time graphical display of signal strength and distance. This allows us to see the dynamic interaction between the radar and liquid. For example, we can see when the return signal falls below the user-set detection threshold.
Figure 2: Screen capture showing a stable high SNR radar response for an empty drum.
Initial Radar Readings
The strength of the radar return is dependent on the material type (dielectric constant), size, and angle of reflection. In theory the 55 gallon metal drum will provide a reflective radar surface. Yet, it’s far from perfect as the drum’s bottom was slightly domed. Consequently, small movement of the radar would cause a large change in signal strength but not distance.
Add the Water
Figure 3 shows the change in signal strength when approximately 4 gallon of water were added to the drum. The measured distance changed from 0.83 to 0.72 meters (less distance as the water surface is closer to the top of the tank). The signal strength is significantly reduced from 215.5 to 59.3. Note that this measurement a few minutes later after the water motion had settled down. Also note that the measured signal strength is higher than the user-set threshold of 30 (orange line).
Figure 3: Screen capture showing the lowered, but clearly discernible, return signal strength from still water.
Make Surface Waves
The Banner software displays radar data in near real-time. This allows us to see how the system responds to water in motion. We can see the impact of sloshing water such as filling or tank movement (think maritime applications).
Dynamic Signal Strength with Water in Motion
The radar’s return signal strength is highly dependent on the angle of the reflecting surface. Now, instead of the dome of the drum’s bottom plate, we see the reflection on the water’s surface. In Figure 3 we stated that the water in the drum was no longer moving. In practice, the return signal strength varies with water movement in the tank. The signal strength would oscillate between approximately 100 and 15 in response to a 2-inch wave. Note that the measured distance did not appreciably change.
This has important implications for system-level design and configuration settings.
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The orange threshold line must be set low enough to prevent radar lockout. Remember that the radar is effectively blind if the signal strength falls below this line.
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There is also a possibility that the radar’s update time will become excessive. For example, the radar may not respond in a timely manner when water is violently moving such as when the tank is being filled.
Radar Response to Foam
I once worked on a ship that featured a vacuum-type sanitary system. While the system worked well, it was susceptible to foaming. In fact the tank level indicator detection would provide erratic or misleading indications. Our solution was to inject an anti-foam solution and then run the recirculating pumps to clear the tank.
We can verify the radar’s “foam performance” by adding liquid soap and then inject air via a bubbler. In this example, the return signal strength dropped in response to the foam. As expected, foam lowers the signal strength while simultaneously spreading the signal pulse, which was so prominent in Figures 2 and 3. There is value in running the experiment again with a higher power 122 GHz radar.
Bottom Line
Be sure to verify the radar against the conditions you expect in your industrial environment. Match the radar’s power, frequency, and design. Then set the radar’s detection threshold to worst-case condition.
Radar selection is not just about distance; it’s about the entire system including target type, target material, and the dynamic motion of the material relative to the radar’s axis.
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About This Author
Aaron Dahlen, LCDR USCG (Ret.), is a Senior Applications Engineer at DigiKey in Thief River Falls. His background in electronics and industrial automation was shaped by a 27-year military career as both technician and engineer, followed by over a decade of teaching.
Dahlen holds an MSEE from Minnesota State University, Mankato. He has taught in an ABET-accredited electrical engineering program, served as coordinator of an electronic engineering technology program, and instructed military technicians in component-level repair.
Today, he has returned to his home in northern Minnesota, completing a decades-long journey that began with a search for capacitors. Read his story here.