Radar vs. Ultrasonic Level Transmitters: How to Choose

Every plant has at least one tank level reading that operators don't fully trust. It jumps when the mixer starts, freezes at the same number for hours, or drifts a few inches between morning and afternoon. Often the transmitter isn't broken. It's the wrong technology for that vessel, or it was installed in a way that makes it see the wrong thing.

Radar and ultrasonic are the two most common non-contact options for continuous level measurement. They look similar from the outside and both report a 4-20 mA signal, but they behave very differently once you put them on a real tank. Here is how HRE Construction looks at the choice when we install and commission level instruments in industrial facilities.

How Each One Measures Level

Both technologies work on time of flight. The transmitter sends a signal down toward the surface, waits for the echo, and converts the travel time into distance. Level is then the configured empty distance minus the measured distance.

Ultrasonic

An ultrasonic transmitter sends a pulse of sound, above the range of human hearing, through the air or vapor above the liquid. Because it relies on sound, its accuracy depends on the speed of sound in that space, and the speed of sound changes with temperature by about 0.17% per degree Celsius. Most units carry a temperature sensor to correct for this.

Non-Contact Radar

A radar transmitter sends microwave energy instead of sound. Microwaves don't need air to travel, so temperature, pressure, and gas composition in the vapor space have very little effect on the reading. What matters for radar is how well the surface reflects microwaves, which depends on the material's dielectric constant. Common non-contact radar frequencies are in the 6, 26, and 80 GHz bands, with 80 GHz now the standard choice for most new liquid applications.

There is also guided wave radar, which sends the pulse down a rod or cable that sits in the process. It is a contact device, and we cover where it fits further down.

Where Ultrasonic Works Well

Ultrasonic has a reputation as the "cheap" option, but in the right application it is a solid, low-maintenance choice. It tends to perform well on:

• Open or vented atmospheric tanks holding water or water-based liquids
• Sumps, lift stations, and wet wells
• Open channel flow measurement through flumes and weirs
• Clean, calm surfaces with no heavy vapor or foam

The transducer has no wetted parts, there is nothing to plug or coat in the process, and setup is usually simple. For a straightforward water tank, it's often all you need.

Where Ultrasonic Struggles

Most ultrasonic problems we see in the field come from asking the technology to do something sound can't do reliably.

Temperature gradients. The built-in temperature probe measures the air at the transducer, not the full column the sound travels through. If the vapor space is stratified, with hot air near the liquid and cooler air at the top, the correction will be off. As a rough example, if the probe reads 20 °C cooler than the air the sound actually travels through, a reading 15 feet down can be off by about half a foot.

Vapor and gas composition. Speed of sound is different in gases other than air. Tanks with solvent vapors, or digesters with a CO2-rich atmosphere, can cause large errors or a complete loss of echo.

Vacuum and pressure. Sound needs a medium to travel through. Ultrasonic will not work in a vacuum, and pressurized vessels are generally outside its range.

Foam, steam, and dust. Dry foam absorbs ultrasonic energy, and heavy steam or dust scatters it. The result is a weak or lost echo.

Blanking distance. Right after it fires, the transducer is still "ringing," so it can't see anything very close to its face. This blanking distance is typically around 12 to 18 inches, longer on long-range sensors. If the tank fills into that zone, the reading becomes unreliable right when it matters most.

Beam spread. Ultrasonic beams are often around 5 to 10 degrees wide. In a narrow tank or one with ladders, heating coils, or agitators, part of the beam will hit those objects and return false echoes.

Where Radar Earns the Higher Price

Radar costs more, though the gap has narrowed. In return, it handles conditions that would take an ultrasonic unit out of service:

• Vacuum, pressure, and high-temperature vessels
• Solvent and hydrocarbon vapors, and stratified temperatures in the vapor space
• Dusty silos and bulk solids
• Tall vessels and long ranges

Why 80 GHz Matters

Beam angle depends on antenna size and frequency. A 26 GHz radar with a roughly 3-inch antenna has a beam of about 10 degrees. An 80 GHz unit with the same size antenna can focus to about 3 degrees. At any distance, the 3-degree beam is about one-fourth as wide, so it can be aimed past coils, baffles, and agitators, and it fits into smaller nozzles. Higher frequency units also typically measure closer to the antenna and have more signal headroom for difficult surfaces.

Radar Still Has Limits

• Low dielectric materials. Water, with a dielectric constant around 80, reflects strongly. Oils, solvents, and many hydrocarbons sit roughly between 1.9 and 4 and return much weaker echoes. Non-contact radar typically needs a dielectric constant of at least about 1.4 to 1.9, depending on the instrument. Check the manufacturer's data for your exact material.
• Foam. Radar is not a cure-all for foam. Wet foam can reflect the signal, but dry foam can absorb radar energy much like it absorbs sound.
• Condensation and buildup on the antenna. Coating on the antenna creates strong close-range echoes and weakens the true surface echo.
• Heavy steam. Dense steam can attenuate the signal in some services.

Side-by-Side Comparison

When Neither Is the Right Answer

Sometimes the best choice is a different technology entirely:

• Guided wave radar works well for low dielectric liquids, foam, narrow nozzles, and liquid-liquid interfaces such as oil over water, because the pulse travels along a probe instead of through open space.
• Hydrostatic or differential pressure transmitters read level from the bottom of the tank and are a reliable choice for many closed vessels, as long as density is known and stable.
• Point level switches belong on any tank where an overfill would cause a spill, damage equipment, or create a safety hazard. A continuous transmitter should not be the only line of defense. For petroleum storage, API 2350 lays out overfill prevention expectations.

Installation Mistakes That Make Either One Fail

A good transmitter in a bad spot will still give bad readings. These are the issues we see most often:

1. Mounting over the fill stream. Incoming product splashes through the beam and creates false echoes. Mount away from the inlet.
2. Mounting too close to the tank wall. The beam picks up the wall and welds. Follow the manufacturer's spacing for that beam angle.
3. Long or narrow nozzles. A recessed antenna can bounce energy off the nozzle walls before it ever reaches the tank, creating strong near-range echoes.
4. Skipping false echo mapping. Most modern transmitters can record fixed echoes from ladders, coils, and nozzles while the tank is empty and ignore them later. If this step is skipped, the unit may lock onto an obstruction and report a "phantom" level that never moves.
5. Wrong reference points. Empty distance measured from the flange instead of the instrument's reference point, or a full point set inside the blanking zone, will throw off every reading.
6. Leaving lost-echo behavior at default. Decide whether the transmitter should hold its last value or drive the loop to a failure current when it loses the echo, and make sure the control system responds the right way. Holding the last value on a tank that is still filling can hide a developing overfill.
7. Ignoring the radio approval. In the U.S., radar level devices are approved either as level probing radar (LPR), which can be used in open air, or tank level probing radar (TLPR), which is meant for closed tanks. Check the nameplate before mounting a radar over an open sump or a plastic tank.
8. Wrong area classification or enclosure rating. In classified locations, the transmitter must carry the right hazardous area approval. In washdown or corrosive areas, the housing and conduit entries need to match the environment, the same issues we cover in our post on corrosion in industrial electrical systems.

Commissioning and Upkeep

Level instruments are easiest to get right during startup and planned outages. A short checklist:

• Save the echo curve at commissioning as a baseline. Comparing it later is the fastest way to spot buildup or a new obstruction.
• Verify the reading against a manual measurement, such as a tape gauge or sight glass, at a few points across the range.
• Use HART diagnostics to trend echo strength. A slowly weakening echo often points to antenna coating before it becomes a lost signal.
• Clean the antenna or transducer face on a schedule set by your process, not just when the reading fails.
• Confirm the loop end to end so the control system scales the 4-20 mA signal the same way the transmitter does.

If you're adding or replacing level instruments, a planned shutdown is the best time to do it. Our post on holiday shutdown electrical maintenance covers how to plan that window. For readings that slowly wander over months, see why your transmitters keep drifting out of calibration. And if you're also working through temperature measurement, our guide to RTD vs. thermocouple selection follows the same application-first approach.

Connected smart transmitters also add network touchpoints on the plant floor. If your instruments report back through networked controllers, our post on PLC cybersecurity and the 2026 CISA alerts is worth a read.

How HRE Can Help

HRE Construction installs, wires, and commissions process instrumentation for industrial facilities. For level measurement, we can:

• Review the vessel, process conditions, and existing readings to recommend the right technology
• Install transmitters, conduit, and wiring to the correct area classification and enclosure ratings
• Set up empty and full points, false echo mapping, and lost-echo behavior during commissioning
• Complete loop checks from the transmitter to the control system
• Troubleshoot readings that jump, freeze, or drift on existing installations

Learn more about our instrumentation services.

Frequently Asked Questions

Is radar always better than ultrasonic?

No. Radar handles vacuum, pressure, vapors, and temperature swings that ultrasonic can't, but for a clean water tank, sump, or open channel, ultrasonic is often reliable and costs less. The right choice depends on the vessel and the process.

Can an ultrasonic level transmitter work in a closed tank?

Yes, as long as the tank is near atmospheric pressure and the vapor space is mostly air with stable temperatures. Solvent vapors, CO2-rich atmospheres, vacuum, and heavy foam or steam are where it runs into trouble.

Why does my radar level transmitter show a level that never changes?

It is often locked onto a fixed echo from a ladder, coil, nozzle, or buildup on the antenna instead of the liquid surface. Reviewing the echo curve and redoing false echo mapping with the tank empty usually reveals the cause.

What dielectric constant does radar need?

Non-contact radar typically needs a dielectric constant of at least about 1.4 to 1.9, depending on the instrument and frequency. Water reflects very strongly, while oils and solvents return weaker echoes. For very low dielectric materials, guided wave radar is often the better fit.

Get the Right Level Reading the First Time

A level transmitter that fits the application saves hours of troubleshooting and helps prevent spills and overfills. If you have tanks with readings your operators don't trust, or a new project that needs level instruments specified and installed, contact HRE Construction to request a quote.