Radar vs Ultrasonic Level Measurement: How to Choose the Right Level Transmitter for Your Tank

Why the level transmitter choice decides more than the reading

A level transmitter is rarely bought for the reading alone. The reading feeds inventory reconciliation, pump interlocks, overfill protection, batch dosing and energy accounting. When it drifts, nothing announces the fault. A tank that reports 62 percent when it is actually at 71 percent will keep reporting confidently for months, and the error only surfaces when a manual dip, a shipment shortfall or an overflow forces someone to check.

That is why radar vs ultrasonic level measurement is a process question before it is a price question. The failure mode of an ultrasonic instrument in a steaming vessel is not a diagnostic alarm. It is a plausible wrong number. The failure mode of a radar instrument on a very low dielectric medium is the same. Choosing correctly means matching the physics of the sensing method to the physics inside the vessel.

Both instruments discussed here are non contact. Neither has a float, a cable, a displacer or a probe immersed in the medium, so neither suffers the coating, corrosion, fouling and mechanical wear that force contact instruments into a maintenance cycle. From there the two diverge sharply.

How each technology actually measures

Ultrasonic: time of flight through air

An ultrasonic level meter emits a focused acoustic pulse from a transducer mounted above the material and measures the elapsed time until the echo returns from the surface. Distance is derived from that flight time and the speed of sound, and the controller subtracts it from the tank height to give level.

The Sertel T-ULT-300-PR works exactly this way, delivering 0.3 percent ranging accuracy and 1 mm resolution across three measuring range options of 0 to 10 m, 0 to 15 m and 0 to 20 m. It handles liquids, pastes and small particle solids, and carries an IP65 rating with a process pressure rating below four atmospheres absolute.

The dependency to understand is the medium the pulse travels through. Sound needs air, and its speed changes with temperature, vapour composition and pressure. Heavy steam, dense dust clouds or a strong vertical temperature gradient in the vapour space will bend, absorb or scatter the pulse. That is the boundary of the technology, and it is also why the instrument is so effective everywhere that boundary is not crossed.

Radar: 80 GHz FMCW measurement

A radar level transmitter generates a continuously swept microwave signal and compares the frequency of the received echo with the frequency that is presently being transmitted. The resulting frequency difference is proportional to the distance. This constitutes Frequency Modulated Continuous Wave measurement, and since microwaves require no transmission medium, the quantity of vapour present is largely irrelevant.

The Sertel T-RL-300-80G works in the 76 to 81 GHz millimetre wave band and provides a 5 GHz ultra wide FM scanning bandwidth. Compared with conventional Ku band radar, the higher frequency yields a tighter beam and finer target resolution. It delivers plus or minus 1 mm accuracy across a maximum measuring range of 120 m, and it has a short blind zone ranging from 7 to 80 mm. It is rated with IP67 protection and ExibIIC T6 Gb explosion-proof classification.

Radar has its own limitations: the medium must be capable of reflecting microwaves. This requirement is expressed as dielectric constant, and the T-RL-300 needs a value of 2 or greater. Water, slurries, most chemicals, cement, and grain reflect well. Certain very light hydrocarbons and foamed plastics reflect poorly and may require a variant, a stilling well, or a different method entirely.

Side by side comparison

The table below sets out the practical differences that affect a specification decision, using published figures for the Sertel T-ULT-300-PR and T-RL-300-80G.

One nuance is worth flagging. The comparison is not a hierarchy. A 20 m water storage tank at ambient temperature does not become more accurate with a radar transmitter on top of it. It becomes more expensive.

Where an ultrasonic level meter is the better engineering answer

Ultrasonic measurement earns its place wherever the vapour space is quiet and the budget has to cover many measuring points rather than one.

• Water and wastewater: pump station wet wells, clarifier levels, balance tanks and effluent sumps, where the medium is water and the vessel is open to atmosphere
• Chemical day tanks and dosing tanks: the non contact transducer never touches the corrosive liquid, so there is nothing to attack, coat or replace
• Open pits, channels and bunds: no pressure, no vapour, no obstructions, and a 0.3 percent reading is comfortably better than the process needs
• Retrofits with limited budget: the T-ULT-300-PR accepts 2 wire loop powered wiring, so an existing 4 to 20 mA pair can carry the new instrument without a separate supply run
• Distributed measuring points: when a plant needs eleven level readings rather than one, the cost difference multiplies eleven times

The separate wall mounted transmitter is an underrated advantage. Display and keypad sit at eye level in an accessible location while the probe stays on the vessel, with up to 200 m of shielded RVVP cable between them. Nobody climbs a tank to read a value or change a set point.

Where radar is the only technology that works

Radar stops being an upgrade and becomes a requirement when the vessel breaks the assumptions ultrasonic measurement depends on.

• Steam and condensation: boiler feed tanks, condensate receivers and hot process vessels, where an acoustic pulse is absorbed before it returns
• Heavy dust: cement silos, fly ash hoppers, grain and powder storage, where the T-RL-300-80G-B variant adds universal air blowing to keep the antenna clear
• Agitated media: tanks with stirrers, where the echo library and 5 GHz sweep separate a moving surface from a rotating blade
• Tall vessels: anything beyond 20 m, where ultrasonic simply runs out of range and radar continues to 120 m
• Hazardous areas: ExibIIC T6 Gb classification allows deployment where an unrated instrument cannot be installed
• High temperature and pressure: the flange variants extend to plus 220 degrees C medium temperature and 2.0 MPa process pressure
• Strongly corrosive service: the anti corrosive flat lens variants use quartz isolated flange antennas rather than exposed metalwork

Narrow installations matter as well. The 3 degree beam angle keeps the microwave cone away from tank walls, internal ladders, heating coils and nozzles that would otherwise generate false echoes, which makes radar practical in vessels an acoustic instrument would struggle to survey cleanly.

Six questions that settle the decision

Rather than comparing datasheets line by line, answer these six questions about the specific vessel you are specifying.

• What is in the vapour space above the maximum level? If the answer includes steam, dense dust or heavy condensation for any part of the operating cycle, radar is the answer and the rest of the comparison is academic.
• How tall is the vessel? Beyond 20 m the decision is made for you. Below 10 m, ultrasonic is almost always sufficient unless another factor rules it out.
• What does the medium reflect? Water based and dense media reflect microwaves well. If the dielectric constant is below 2, radar needs a variant or a stilling well, and ultrasonic may be the more reliable choice.
• Is the area classified? An unrated instrument in a hazardous zone is not a cost saving, it is a compliance failure. Check the zone classification before comparing anything else.
• How much does a wrong reading cost? Inventory reconciliation on a bulk storage tank, custody transfer and overfill protection justify plus or minus 1 mm. A sump that only needs a pump start signal does not.
• How many points are you specifying? One critical vessel and forty ordinary ones are two different procurement decisions inside the same plant.

Installation details that ruin either instrument

More level loops are lost to installation than to instrument selection. Three details account for most of them.

Blind zone and dead band

Every non contact instrument has a minimum distance below which it cannot measure. The T-ULT-300-PR has a 0.35 m blind area below the transducer face. The T-RL-300 blind zone is far shorter at 7 to 80 mm. If the maximum expected level rises into that zone, the instrument does not report an error. It reports the last valid value or a nonsense one. Mount so that the highest process level stays clearly outside the dead band, and remember to account for foam, surge and filling turbulence, not just the static high level mark.

Beam path and false echoes

Mount the probe or antenna vertically with the acoustic or electromagnetic axis perpendicular to the surface. Keep it away from vessel inlets, agitators, ladders, heating coils and weld seams that produce reflections competing with the real surface echo. For radar, select the beam angle deliberately: 3 degrees for narrow or obstructed vessels, 6 degrees where a wider footprint helps average an uneven solids surface.

Cabling and interference

Use shielded twisted pair between transmitter and probe, keep the run away from high voltage cabling, and earth the screen at one end only, at the transmitter. On the T-ULT-300-PR the recommended cable is RVVP 4 by 0.5 with a maximum run of 200 m. On the T-RL-300, AWG18 or 0.75 square millimetre shielded cable with an M20 by 1.5 entry. Earthing the screen at both ends creates a current loop through the shield and injects exactly the noise the shield was fitted to exclude.

Getting the data out: 4 to 20 mA, Modbus and HART

A level reading is only useful where it lands. Both instruments support the analogue and digital paths that plants actually run.

4 to 20 mA analogue

Still the workhorse. Simple, immune to voltage drop across long runs, and understood by every PLC input card in service. On the ultrasonic unit the loop can also power the instrument in the 2 wire configuration, with red carrying signal positive and black signal negative. On radar, both 2 wire and 4 wire configurations are supported.

RS-485 Modbus RTU

Digital, multidrop and far richer than a single analogue value. One twisted pair can carry many instruments back to a gateway, and the reading arrives without the digital to analogue and analogue to digital conversion losses of a current loop. Both the T-ULT-300-PR and the T-RL-300 support Modbus over RS-485.

HART

Optional on the T-RL-300. HART overlays digital configuration and diagnostics on the existing 4 to 20 mA pair, which lets a plant keep its analogue infrastructure while gaining remote configuration and device status. In a brownfield site with an installed analogue base, that is often the least disruptive upgrade path available.

Whichever path is chosen, the display side matters too. A Sertel T-DPM-300 panel meter and controller takes the 4 to 20 mA signal directly, scales it to engineering units through PC configuration software, and drives relay outputs from a set point, so a level loop can trigger a pump or an alarm without any PLC involvement at all.

How Sertel builds both instruments

Sertel Electronics supplies both technologies, so the recommendation is not steered by what is on the shelf. The specification is matched to the vessel.

T-ULT-300-PR Ultrasonic Level Meter

• Three measuring ranges: 0 to 10 m, 0 to 15 m and 0 to 20 m, with 0.3 percent ranging accuracy and 1 mm resolution
• 0.35 m blind area, non contact operation on liquids, pastes and small particle solids
• 4 to 20 mA analogue or RS-485 Modbus RTU output, 24 V DC or 220 V AC supply
• IP65 enclosure, minus 20 to plus 60 degrees C ambient, process pressure below 4 atmospheres absolute
• Two probe sizes with G2, G1 and a half or M70 by 2 threaded fixing, wall or panel mounted transmitter at 183.5 by 125 by 41 mm
• Up to 200 m between transmitter and probe using shielded RVVP 4 by 0.5 cable

T-RL-300-80G Radar Level Transmitter

• 76 to 81 GHz FMCW with 5 GHz ultra wide scanning bandwidth, plus or minus 1 mm accuracy
• Up to 120 m measuring range, 7 to 80 mm blind zone, dielectric constant of 2 or greater
• 3 degree or 6 degree beam angle depending on antenna variant, six antenna configurations available
• IP67 protection, ExibIIC T6 Gb explosion proof, minus 40 to plus 85 degrees C standard and up to plus 220 degrees C on the high temperature flange variant
• 4 to 20 mA or RS-485 with Modbus standard and HART optional, 18 to 28 V DC or 220 V AC supply
• 128 by 64 dot matrix backlit LCD, 4 button keypad and PC configuration software
• Aluminium alloy or stainless steel body, threaded or flanged connection, DN80 minimum flange

Antenna variant selection at a glance

Specifying with confidence

Radar and ultrasonic level transmitters are not competitors so much as answers to different vessels. Ultrasonic measurement delivers dependable non contact level on open, ambient, moderately sized vessels at a cost that scales across many measuring points. Radar measurement delivers millimetre accuracy through steam, dust, pressure, heat and hazardous classification where an acoustic pulse cannot survive the journey.

Work through the six questions for each vessel rather than for the plant as a whole. The specification usually writes itself, and the capital lands where the process risk actually sits.