Radar vs Ultrasonic Level Measurement: Which to Choose?
Radar vs ultrasonic level measurement compared — how each works, performance in vapor/foam/dust, range, cost, and how to choose for your tank or vessel.
Quick Answer: Radar vs Ultrasonic Level Measurement
Ultrasonic uses sound waves that require an air medium. It is cost-effective, easy to commission, and works well in clean, open-air applications such as water tanks, sumps, and open channels. Radar uses microwave energy that travels through vapors, dust, and inert gases without attenuation. It is the right choice when process conditions make ultrasonic unreliable: high vapor pressure, foam, temperature gradients, vacuum, or pressure vessels.
Both technologies output a standard 4–20 mA analog signal (and increasingly HART or IO-Link) that feeds directly into your PLC analog input card. The choice is rarely about what your PLC can accept — it is about which sensor survives the process conditions and returns an accurate measurement.
| When to choose | Technology |
|---|---|
| Clean water, open-air, budget-constrained | Ultrasonic |
| Vapor, foam, pressure vessel, high temperature | Radar (free-space or guided wave) |
How Ultrasonic Level Measurement Works
Ultrasonic level transmitters operate on the time-of-flight (ToF) principle using sound waves, typically in the 20 kHz–200 kHz range. The sensor fires a short ultrasonic pulse from a piezoelectric transducer toward the liquid surface. The pulse travels through the air gap in the vessel, reflects off the liquid surface, and returns to the transducer. The electronics measure the round-trip travel time and calculate distance:
Distance = (speed of sound × time) / 2
The speed of sound in air is approximately 343 m/s at 20 °C, but it changes with temperature — roughly 0.6 m/s per degree Celsius. Quality ultrasonic transmitters include a temperature compensation circuit (typically an integrated temperature sensor) that corrects for this drift in real time.
The transmitter then converts distance to level using the programmed empty distance (span). The output is a linear 4–20 mA signal proportional to level, which your PLC reads through a standard analog input module and scales to engineering units (meters, percent fill, or volume).
What ultrasonic needs to work correctly:
- An air (or dry gas) gap between the sensor face and the liquid surface — the sound pulse must propagate through a medium
- A relatively calm, flat liquid surface to return a clean echo
- Minimal vapor or condensation in the headspace (both absorb or scatter sound energy)
- Operating temperature within the sensor's rated range (typically −40 °C to +80 °C for standard units)
- Sufficient signal-to-noise ratio — very short measurement distances can cause blanking zone issues
The blanking zone (also called the dead zone) is a distance directly below the transducer face — typically 0.1 m to 0.5 m — within which the electronics cannot distinguish the outgoing pulse from the returning echo. High-level alarms must account for this zone during installation.
How Radar Level Measurement Works
Radar level transmitters use microwave energy — typically in the 6 GHz, 26 GHz, or 80 GHz frequency bands — rather than sound. Because microwaves are electromagnetic waves, they do not require a medium to propagate and are not affected by the composition, density, or pressure of gases in the vessel headspace.
Modern industrial radar transmitters use FMCW (Frequency Modulated Continuous Wave) technology. Instead of a short pulse, the transmitter continuously varies (sweeps) its frequency over a defined range. The returned echo arrives at a slightly different frequency than the currently transmitted signal. The difference frequency between transmitted and received signals is directly proportional to the distance to the liquid surface. This approach provides superior resolution and noise immunity compared to older pulsed radar designs.
The transmitter converts measured distance to level and outputs a 4–20 mA signal with superimposed HART, or a digital fieldbus signal (PROFIBUS PA, Foundation Fieldbus, IO-Link), directly compatible with PLC analog input cards or fieldbus interface modules.
Key advantages of the microwave approach:
- Microwaves travel through steam, hydrocarbon vapors, CO₂, nitrogen, and other inert gases without significant attenuation
- Signal is unaffected by changes in temperature, pressure, or gas composition in the headspace
- 80 GHz (short-wave) radar has a very narrow beam angle — approximately 3–4° — enabling installation in tanks with internal agitators, heating coils, or structural bracing
- No moving parts, no acoustic transducer wear, suitable for abrasive or corrosive environments (lens or PTFE antenna options)
Empty spectrum mapping (also called false echo suppression or echo mapping) is a critical commissioning step for radar. With the tank empty, the transmitter records all echoes from vessel internals — nozzles, ladders, heating coils — and stores these as a reference map. During normal operation, echoes matching the stored map are suppressed, leaving only the true surface echo for measurement. This function is performed via the transmitter's local display or a handheld HART communicator, and the map is stored in non-volatile memory.
Comparison Table: Radar vs Ultrasonic Level
| Parameter | Ultrasonic | Radar (Free-Space) |
|---|---|---|
| Measurement principle | Sound time-of-flight | Microwave FMCW / pulsed ToF |
| Medium requirement | Requires air/gas medium | Propagates through any gas or vapor |
| Vapor/steam tolerance | Poor — absorbs/scatters sound | Excellent — microwaves unaffected |
| Foam tolerance | Poor — foam absorbs sound | Moderate — depends on foam density; 80 GHz often acceptable |
| Dust/powder (solids) | Moderate for light dust | Good — preferred for bulk solids |
| Temperature range | Typically −40 °C to +80 °C (standard) | Typically −40 °C to +200 °C (process versions higher) |
| Pressure range | Atmospheric only (standard) | Rated to full vessel pressure (flanged versions) |
| Measurement range | Typically 0.1 m to 15 m | Typically 0.1 m to 30 m+ |
| Accuracy | ±0.25% to ±0.5% of range (typical) | ±1 mm to ±3 mm (typical for FMCW) |
| Output | 4–20 mA, HART, IO-Link | 4–20 mA, HART, PROFIBUS PA, FF, IO-Link |
| Installation | Top-mounted, open flange or BSP thread | Top-mounted, flanged or threaded process connection |
| Relative cost | Low | Medium to high |
| Commissioning effort | Low — set span, temperature compensation active | Moderate — false echo mapping recommended |
Where Ultrasonic Level Measurement Struggles
Understanding the failure modes of ultrasonic is the fastest way to know when radar is mandatory.
Vapor and steam. Sound energy is attenuated (absorbed and scattered) by vapor molecules and water droplets in the headspace. In hot water tanks, boilers, evaporators, or any vessel where the liquid temperature is near its boiling point, the headspace fills with steam or high-humidity vapor. The returning echo becomes too weak for reliable detection. Condensation on the transducer face compounds the problem by dampening the piezoelectric element.
Foam layers. Foam presents a soft, diffuse acoustic reflector. Rather than returning a sharp echo from a well-defined surface, the sound energy scatters into the foam matrix. The transmitter either loses the echo entirely or locks on to the foam top rather than the true liquid level underneath — which can be a significant measurement error. This is a common failure mode in fermentation tanks, wastewater aeration basins, and chemical mixing vessels.
Vacuum and pressurized vessels. Standard ultrasonic sensors are designed for atmospheric open-air applications. A vacuum vessel has a different gas density in the headspace, which changes the speed of sound in an unpredictable way (temperature compensation algorithms assume atmospheric air). Pressurized vessels require sealed sensors rated for the process pressure, which significantly increases the cost and often exceeds the capability of standard ultrasonic products.
Temperature gradients. In tall vessels with significant temperature stratification — hot product at the bottom, cooler air at the top — the speed of sound varies through the air column in a non-uniform way that a single temperature sensor cannot fully compensate for. This introduces a systematic measurement error that worsens as the vessel height increases.
Turbulent or splashing surfaces. Agitated reactors, mixer tanks with open impellers, or filling applications with heavy product splash create a chaotic surface that returns multiple partial echoes. The transmitter's echo processing must work harder to identify the true surface echo, and in severe cases the measurement becomes unstable or unreliable.
Where Radar Wins
Radar's immunity to gas and vapor conditions makes it the default choice for a wide range of industrial applications where ultrasonic is unreliable:
- Hydrocarbon storage tanks — crude oil, naphtha, condensate, LNG. The vapor space contains hydrocarbon gases that would attenuate ultrasonic signals. Radar is the regulatory-preferred technology for custody transfer measurement in the oil and gas industry.
- Chemical process vessels — reactors, distillation columns, and evaporators with aggressive vapor environments.
- Boiler feedwater tanks and hot wells — near-boiling water produces steam that immediately defeats ultrasonic.
- Pressurized storage — ammonia bullets, propane/butane spheres, CO₂ tanks. Radar transmitters with flanged process connections are rated for full vessel pressure.
- Bulk solids silos — cement, grain, fly ash, plastic pellets. Dust does not significantly attenuate microwave energy, and the narrow 80 GHz beam navigates internal silo geometry more precisely than the wider beams of older 6 GHz units.
- Wastewater primary clarifiers with foam — 80 GHz radar with appropriate false echo mapping can track the sludge/scum blanket interface where ultrasonic loses signal.
From the PLC integration standpoint, radar transmitters configured for HART overlay on the 4–20 mA loop allow the control system to read secondary variables (signal strength, echo confidence, device temperature) through a HART multiplexer or a HART-capable analog input card — useful for predictive maintenance and continuous loop health monitoring.
Guided Wave Radar: A Brief Note
Guided wave radar (GWR), also called time-domain reflectometry (TDR) in process instrumentation, is a third variant worth understanding. Instead of transmitting a free-space beam, a GWR transmitter launches the microwave pulse down a probe — a single rod, twin rod, or flexible cable — immersed in the vessel. The signal is guided along the probe and reflects at the gas/liquid interface.
GWR is effective for:
- Low dielectric constant liquids (dielectric constant < 2, such as light hydrocarbons) where free-space radar gets a weak reflection from the liquid surface
- Interface measurement between two liquid layers (e.g., oil over water) using the dielectric contrast between the two liquids
- Narrow or obstructed vessels where there is insufficient clearance for a free-space radar beam
- Agitated surfaces — the probe physically guides the signal and is less susceptible to surface turbulence than free-space radar
The trade-off is that the probe contacts the process fluid, introducing mechanical and chemical compatibility considerations, and the probe can be damaged by high viscosity, crystallizing, or polymerizing products.
For a broader overview of the sensor families that feed signals into PLC systems, see Types of Industrial Sensors.
How to Choose: A Controls Engineer's Decision Framework
Making the right sensor selection starts with characterizing the process conditions, not the sensor specifications.
Step 1: Characterize the headspace. Ask: Is the vapor pressure of the liquid significant at operating temperature? Is there steam, solvent vapor, or hydrocarbon gas in the headspace? If yes, radar is the correct technology. If the headspace is dry air at atmospheric pressure, ultrasonic is viable.
Step 2: Characterize the surface. Ask: Is there foam, froth, or turbulent splash? Foam disqualifies standard ultrasonic. Agitation requires evaluation of radar beam angle and echo processing capability.
Step 3: Check the process conditions. Is the vessel rated for pressure above atmospheric? Is the operating temperature above 80 °C? Does the process fluid contact the sensor (eliminating non-contact options)? Pressurized, high-temperature, or highly corrosive service points toward flanged radar with chemical-resistant antenna materials.
Step 4: Evaluate the measurement range. For ranges beyond 15 m, radar is typically required. For short ranges (under 5 m) in benign conditions, ultrasonic offers the best value.
Step 5: Consider the PLC integration. Both technologies output 4–20 mA and are compatible with standard PLC analog input modules. See Level Measurement Types for a comparison of all level technologies including differential pressure, float, and magnetostrictive types. If your project also involves pressure measurement on the same vessel, Pressure Transmitter Explained covers the 4–20 mA loop wiring and PLC scaling for pressure signals using the same principles.
Step 6: Budget realism. Ultrasonic transmitters for standard applications are significantly less expensive than free-space radar. However, the cost of a measurement failure — process upset, product loss, environmental incident — typically exceeds the price difference between technologies. Specify radar where conditions warrant it, not to save capital cost.
For a practical example of level measurement in a real control system, the Water Treatment PLC Programming Guide shows how level signals from both ultrasonic and pressure-based transmitters are scaled and used in ladder logic for pump control and tank management.
Frequently Asked Questions
What is the difference between radar and ultrasonic level measurement? Ultrasonic level transmitters measure distance using sound waves (time-of-flight in an air medium). Radar level transmitters use microwave energy (electromagnetic waves), which propagates through vapors, gases, and varying pressures without attenuation. Both convert measured distance to a 4–20 mA output signal for PLC input.
Is radar better than ultrasonic level measurement? Radar is more capable under difficult process conditions — vapor, foam, pressure, high temperature — but not inherently "better" for all applications. In a clean water sump at atmospheric pressure, a well-commissioned ultrasonic transmitter delivers reliable measurement at lower cost. The correct technology depends on the process conditions.
Why does ultrasonic level measurement struggle with vapor? Sound waves are mechanical pressure waves that require a medium to propagate. Vapor — whether steam, hydrocarbon gas, or solvent — attenuates the sound pulse through absorption and scattering. The returning echo weakens to the point where the transmitter cannot reliably detect it, causing lost signal or erratic readings.
Which is more expensive, radar or ultrasonic level measurement? Ultrasonic transmitters for standard water/wastewater applications are generally the lower-cost option. Free-space radar transmitters, particularly 80 GHz units with flanged process connections and HART communication, carry a higher purchase price. Guided wave radar falls between the two depending on probe length and material. Total installed cost — including installation, commissioning time, and maintenance — should be compared, not just the transmitter list price.
Can both radar and ultrasonic transmitters connect to the same PLC analog input card? Yes. Both output a standard 4–20 mA signal over a two-wire loop that is compatible with any PLC analog input module. HART communication (superimposed on the 4–20 mA loop) is supported by both technologies and requires a HART-capable input card or external HART modem to access digital diagnostic data.
