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Ultrasonic vs radar level measurement is often reduced to a comparison of range and price. Adding differential pressure makes another question important: which process change can produce a false change in level? Ultrasound depends on an acoustic path, radar depends on a usable reflected electromagnetic signal, and a pressure-based calculation depends on liquid density and the pressure reference.
Three instruments can disagree without any of them being broken
Consider a tank whose level is steady while its contents or operating conditions change. A pressure transmitter may correctly detect a density-related pressure change. An ultrasonic instrument may receive an altered echo through a changing gas space. A radar may track a different reflection as foam develops. Treat disagreement as a diagnostic clue before deciding which instrument to replace.
Specify what "level" means for the process: liquid below foam, the top of an aerated layer, a volume-conversion input or a pump-control value. Instruments responding to different physical features cannot be compared fairly until the required measurement surface and reference are agreed.
Ultrasonic vs radar level: examine the path above the liquid
An above-surface ultrasonic instrument calculates distance from sound travel time. Temperature compensation addresses the temperature effect within its intended application, but it does not automatically correct every change in gas composition. Review the vapour space, sensor temperature, condensation and any obstruction between the transducer and surface. An otherwise adequate range specification does not prove this path will remain usable.
Free-space radar uses electromagnetic waves and does not depend on sound velocity. That removes one acoustic dependency, not every installation problem. Antenna contamination, weak reflection, foam and unwanted echoes still require assessment. Likewise, a radar measurement principle does not establish the pressure or vacuum rating of its housing, seal or process connection.
For a liquid tank, METRAVON MV-RD14 offers a defined 80 GHz lens-antenna route to assess. Its use must be checked against the actual medium and mounting arrangement. The ultrasonic selection guide is useful when evaluating whether the existing acoustic path can be retained.
Differential pressure converts pressure into level
For a simple open tank containing a uniform liquid, hydrostatic pressure follows p = rho x g x h. Here rho is density, g is gravitational acceleration and h is the liquid height above the pressure reference. With the reference arrangement unchanged, calculating height from pressure requires an assumed or measured density.
For illustration, take a real liquid height of 2.00 m and a calibration density of 800 kg/m3. If the actual density becomes 900 kg/m3 while height stays constant, the ideal indicated height becomes 2.00 x 900 / 800 = 2.25 m. The 0.25 m difference comes from the density assumption, not necessarily from transmitter error. This is a simplified calculation, not a METRAVON performance specification.
In a closed tank, vapour pressure must also be accounted for. A correctly arranged differential-pressure measurement can cancel the common vapour-pressure component by applying the appropriate pressure reference to the other side. Reference legs, remote seals and their temperature behaviour introduce installation-specific effects. Have the selected arrangement and calibration reviewed; do not treat an open-tank calculation as a complete closed-tank design.
| Change | What to check | Useful evidence |
|---|---|---|
| Density | Pressure-to-level conversion and any compensation used. | Liquid identity, density basis and temperature at the comparison. |
| Vapour | Acoustic propagation for ultrasound; pressure reference for closed-tank DP. | Operating state and instrument trend before and after the change. |
| Foam | Which surface supplies the usable echo and which surface the process needs. | Observed foam condition and an independent level reference where safe. |
| Zero | The physical reference used by each instrument and the control system. | Elevation drawing, calibration endpoints and displayed engineering units. |
Test the likely cause, not just the displayed value
Log the raw measurement where the equipment makes it available. For radar or ultrasound, retain distance or echo information together with calculated level. For differential pressure, retain the pressure reading and conversion settings. When only the final percentage is recorded, a scaling change can be mistaken for a sensor problem.
Compare readings at matched times and reference elevations. If one instrument is measured from the tank roof and another from a lower pressure connection, reconcile those datums first. Use an independent reference with known limitations; an old instrument is not a reference merely because it has been installed longer.
Then repeat the comparison under the conditions linked to the complaint: a different product batch, a change in temperature, filling or a recurring foam condition. Keep safe operating limits and approved access procedures intact. Do not alter a pressure connection or open a pressurised system as an informal diagnostic step.
Selection questions
Q1. Is radar always more accurate than ultrasound or DP?
No. Compare the specified instrument performance under the intended installation, together with the errors introduced by the process and conversion. A well-suited pressure arrangement can outperform a poorly mounted radar. A catalogue accuracy value cannot settle that installed comparison on its own.
Q2. When is density variation a reason to reconsider pressure measurement?
When the resulting level error exceeds what the process can tolerate and reliable compensation is unavailable or impractical. Estimate the effect over the expected density range first. A surface-ranging method may avoid that conversion dependency, but its own reflection and installation conditions still need qualification.
Q3. Can temperature compensation make ultrasound suitable for any vapour?
No. Check what the selected instrument actually compensates and the gas conditions for which it is specified. A temperature sensor does not by itself identify a changing gas mixture. Use the supplier's application limits or a representative test to resolve an uncertain acoustic path.
Q4. Does radar need liquid-density compensation?
Distance-based liquid level does not use the hydrostatic density conversion. However, converting that level into mass still requires density, and liquid properties can affect reflection suitability. Distinguish the distance measurement from the inventory calculation performed later in the system.
Q5. Which option is cheaper to install?
Price the complete feasible installation. Existing nozzles, pressure connections, access, seals, cabling and commissioning can change the cost order. Do not compare a bare transmitter with another offer that includes mounting work and site testing.
Q6. What information will make a replacement review useful?
Send the existing arrangement, the symptom trend, medium changes, operating conditions and reference used to judge the error. For a radar alternative, include the tank and nozzle drawing. Ask METRAVON to review the liquid-radar application against that evidence rather than requesting a generic technology substitution.
Summary
Choose a measurement method whose physical dependencies remain controlled in the process. Separate acoustic-path problems, echo-selection problems and pressure-to-level conversion errors during diagnosis. The useful comparison is the expected installed result, including its reference and operating conditions, rather than an unconditional ranking of three technologies.
