What Are Level Instruments?

Level instruments are devices used to measure, monitor, and control the level of liquids, solids, or slurries within a tank, vessel, silo, or open channel. Among continuous level transmitters, ultrasonic and radar are two of the most popular non-contact technologies.

Both ultrasonic and radar level transmitters are mounted at the top of the tank and measure the distance to the material surface without touching the process fluid. However, they use very different principles: ultrasonic uses sound waves, while radar uses electromagnetic waves. This fundamental difference gives each technology distinct strengths and weaknesses.

Choosing the wrong technology can lead to inaccurate readings, frequent maintenance, or complete failure. This article compares ultrasonic and radar level transmitters side by side to help you make an informed decision.

How Does Each Level Transmitter Work?

How an Ultrasonic Level Transmitter Works

An ultrasonic level transmitter emits high-frequency sound waves (typically 20–200 kHz) from a transducer mounted at the top of the tank. The sound waves travel through the air, reflect off the material surface, and return to the transducer. The instrument measures the time of flight (the time between sending and receiving the echo) and calculates the distance to the material surface using the formula:

Distance = (Speed of Sound × Time) / 2

The level is then calculated as: Level = Tank Height - Distance

The speed of sound varies with temperature, so most ultrasonic transmitters have a built-in temperature sensor to compensate. Some also compensate for humidity and pressure.

How a Radar Level Transmitter Works

A radar level transmitter emits high-frequency electromagnetic waves (typically 6–80 GHz) from an antenna mounted at the top of the tank. The waves travel at the speed of light, reflect off the material surface, and return to the antenna. The instrument measures the time of flight (or frequency difference in FMCW radar) and calculates the distance.

Distance = (Speed of Light × Time) / 2

Radar waves are unaffected by temperature, pressure, vapor, or dust (within limits). They travel at the speed of light in a vacuum and slow down only slightly in air or gas, so compensation is minimal.

Two Types of Radar Level Transmitters:

This article focuses on non-contact radar, as it is the direct competitor to ultrasonic.

Key Features of Each Technology

Advantages of Ultrasonic Level Transmitters

1. Low Cost

Ultrasonic transmitters are significantly less expensive than radar transmitters. For a typical water or wastewater application, an ultrasonic transmitter may cost 50–70% of a comparable radar unit. For large projects with many measurement points, this difference is substantial.

2. Works with Any Material

Ultrasonic does not depend on the dielectric constant (electrical property) of the material. It works equally well with water (εr ≈ 80), oil (εr ≈ 2–10), and even plastic pellets (εr ≈ 1.5). Radar struggles with low-dielectric materials.

3. Simple Installation and Configuration

Ultrasonic transmitters are generally easier to configure than radar. You enter the tank height, the blanking distance, and the output scaling. There is no false echo mapping or dielectric constant adjustment. For clean liquid applications, an ultrasonic transmitter can be installed and running in minutes.

4. Good for Short Ranges (Small Tanks)

Ultrasonic works well for small tanks (1–5 meters). The blind zone is the main limitation, but for tanks where the maximum level is well below the transducer, ultrasonic is perfectly adequate.

5. Suitable for Open Channels and Flumes

Ultrasonic transmitters are commonly used for open channel flow measurement (weirs and flumes). The transducer mounts above the channel and measures the water level, which is converted to flow rate using the Manning equation or a lookup table. Radar can also be used but is less common.

6. No Dielectric Constant Concerns

With ultrasonic, you never need to know the dielectric constant of your material. For applications with unknown or variable materials, this is a significant advantage.

Advantages of Radar Level Transmitters

1. Unaffected by Temperature, Pressure, and Vapor

Radar waves are not affected by temperature (except extremely high temperatures that change antenna properties). They are not absorbed by vapor or gas. For applications with high temperature (e.g., 200°C / 392°F), high pressure (e.g., 40 bar / 580 psi), or heavy vapor (e.g., steam, hydrocarbon vapors), radar is the only reliable choice.

2. Works Through Dust

In silos and hoppers with powders (cement, flour, plastic pellets, fly ash), dust absorbs and scatters ultrasonic waves. Ultrasonic transmitters often fail completely in dusty environments. Radar waves (especially low frequency, 6–26 GHz) penetrate dust with minimal attenuation.

3. Works with Foam

Foam absorbs sound waves. Ultrasonic transmitters typically cannot measure through foam. Radar waves are reflected by the liquid surface beneath the foam (low frequency radar) or can be configured to ignore foam (guided wave radar).

4. Unaffected by Condensation on Antenna

Ultrasonic transducers are sensitive to condensation. Water droplets on the transducer face disrupt the sound beam. Radar antennas (especially PTFE-faced or drop antennas) are much less affected. Condensation does not significantly attenuate radar waves.

5. Very Small Blanking Zone (Dead Zone)

Radar has a much smaller blind zone than ultrasonic. The minimum distance from the antenna to the maximum high level can be as little as 50–200 mm (2–8 inches). Ultrasonic typically requires 300–500 mm (12–20 inches). This allows radar to be used in very short tanks or when the tank must be filled nearly to the top.

6. High Accuracy

Radar transmitters are more accurate than ultrasonic, especially over long ranges. For inventory management, custody transfer, and precise batching, radar is preferred. Accuracy of ±1–5 mm is common for radar; ultrasonic is typically ±0.2–1% of range (e.g., ±10–50 mm at 5 meters).

7. Long Range

Radar can measure levels up to 40 meters (130 feet) for liquids and up to 120 meters (400 feet) for solids with high-gain antennas. Ultrasonic is typically limited to 15–20 meters (50–65 feet) and performance degrades at longer ranges.

8. Works in Vacuum

Ultrasonic requires a medium (air or gas) to transmit sound. In a vacuum, there is no sound transmission, so ultrasonic fails. Radar works in vacuum because electromagnetic waves do not require a medium.

9. High Temperature and Pressure Capability

Radar transmitters with process seals can operate at temperatures up to 450°C (842°F) and pressures up to 400 bar (5,800 psi). Ultrasonic is typically limited to 80–120°C (176–248°F) and 2–4 bar (30–60 psi).

Factors to Consider When Choosing

Use this decision matrix to select the right technology for your application.

Application-Specific Recommendations

Common Misconceptions

Misconception #1: "Radar is always better."

False. Radar is better for challenging conditions (dust, foam, high temperature, high pressure). For simple clean liquid applications (water tank, wastewater lift station, ambient conditions), ultrasonic is often the better choice because it costs less and works perfectly well.

Misconception #2: "Ultrasonic is obsolete."

False. Ultrasonic remains the most popular level measurement technology for water and wastewater applications. Millions of ultrasonic transmitters are in service worldwide. They are not obsolete—they are appropriate for specific applications.

Misconception #3: "Radar works with any material."

Not exactly. Radar requires a minimum dielectric constant (εr) to get a strong enough reflection. For very low dielectric materials (εr < 1.6, such as LPG, pentane, some plastic pellets), standard non-contact radar may not work. Guided wave radar or ultrasonic are better choices.

Misconception #4: "Ultrasonic is not accurate."

Ultrasonic is accurate enough for most level applications. ±0.25–0.5% of range is typical. For a 5-meter tank, that is ±12.5–25 mm (±0.5–1 inch). For overflow prevention, pump control, and inventory management, this is perfectly adequate. Radar offers higher accuracy but at higher cost.

Summary Comparison Table

Cost Comparison Example

When to Use Guided Wave Radar Instead of Non-Contact

In some cases, guided wave radar (GWR) is a better choice than either ultrasonic or non-contact radar:

Conclusion

Choosing between ultrasonic and radar level transmitters comes down to your specific application conditions. Use ultrasonic for clean liquids, ambient temperatures, atmospheric pressure, and when budget is a primary concern. Use radar for dusty environments, foam, high temperature, high pressure, vacuum, condensation, or when you need high accuracy or very long range. Both technologies have been proven over decades of industrial use. The "best" choice is not about which technology is superior—it is about which technology fits your specific application.

Tianjin ZINACA Intelligent Equipment Co., Ltd. , located in Tianjin, China, is a high-tech company specializing in instrumentation sales, engineering design, and management consulting. ZINACA offers both ultrasonic and radar level transmitters, as well as guided wave radar for challenging applications. Our engineering team can help you compare these technologies based on your material, tank conditions, temperature, pressure, accuracy requirements, and budget. We do not push one technology over another—we recommend the right solution for your specific need