An existing storage tank has a process nozzle welded into the roof at an angle. A radar level sensor can be mounted mechanically without any problem, but its antenna axis is therefore not perpendicular to the liquid surface. The sensor still provides plausible values at a high filling level, while at lower levels the echo becomes weaker and the measured value occasionally jumps. Can this error simply be corrected through parameterization?
Only to a limited extent. A free-space radar level sensor transmits electromagnetic waves into the vessel along its antenna axis. With a calm liquid surface, a large portion of the reflection is directional. If the radar beam hits the surface at an angle, a significant portion of the energy is not reflected back towards the antenna.
With an angled installation, the position of the radar beam inside the vessel also changes increasingly with measuring distance. The beam may therefore hit the vessel wall, heating coils, ladders, pipes or other internal structures.
The most important correction for an angled tank nozzle should therefore, wherever possible, be mechanical: the antenna axis of the radar sensor should be aligned so that it points towards the intended measuring surface and, for liquids, is generally as perpendicular as possible to the liquid surface. Parameterization and false-echo suppression can support a good installation, but they cannot compensate for a severely incorrect beam direction.
How does radar level measurement work?
A free-space radar level sensor transmits electromagnetic waves towards the product surface.
The simplified signal path is:
Antenna → radar wave → product surface → reflection → antenna
From the transit time or phase information of the returning signal, the instrument determines the distance between a defined sensor reference point and the product surface.
If the vessel geometry is known, the level can be calculated from this.
In simplified form:
Level = vessel reference height - measured distance
However, this requires a sufficiently strong and clearly identifiable echo to return from the product surface to the antenna.
Why is the antenna axis crucial?
The radar energy is not transmitted arbitrarily throughout the entire vessel.
The antenna generates a defined radar beam.
Its centre axis is essentially determined by the mechanical orientation of the antenna.
With a correctly installed radar sensor on a liquid tank, this axis usually points:
as vertically downward as possible towards the liquid surface
.
If the sensor is tilted by a few degrees, the entire radar beam is tilted as well.
The electronics cannot simply restore the physical transmission direction of the antenna to vertical by entering a parameter.
How is radar reflected from a liquid surface?
In many applications, a calm liquid surface behaves partly like a directionally reflecting surface for radar.
In simplified form:
angle of incidence = angle of reflection
If the radar beam hits the liquid almost perpendicularly, a relevant proportion of the reflected energy is directed back towards the sensor.
With an oblique angle of incidence, the reflection direction changes.
Part of the energy is then reflected sideways away from the sensor.
The useful signal at the receiver can therefore become significantly weaker.
Especially over long measuring distances, a small mechanical misalignment can therefore be much more significant than it appears directly at the tank roof.
What happens with an angled mounting nozzle?
If a radar sensor is mounted directly on an angled process nozzle, its antenna axis normally follows the axis of that process connection.
Instead of:
Sensor ↓ liquid surface
the result may be, for example:
Sensor ↘ liquid surface
This creates two different problems.
First, the radar beam no longer strikes the product surface optimally.
Second, the beam shifts further sideways as the distance increases.
The radar beam can therefore enter areas that would not be detected at all with vertical installation.
How much effect can a few degrees of misalignment have?
The lateral displacement of the beam axis can be estimated geometrically.
In simplified form:
x = h × tan(α)
Where:
x= lateral displacement,h= measuring distance,α= deviation from vertical.
With only:
5°
of misalignment and a measuring distance of:
10 m
the lateral displacement is already approximately:
0.87 m
.
At:
20 m
it is approximately:
1.75 m
.
An angle that is barely noticeable directly at the process connection can therefore shift the radar beam by more than one metre in the lower part of the tank.
Why does the measurement sometimes still work at high filling levels?
At a high filling level, the distance between the antenna and the liquid surface is relatively small.
As a result, the lateral displacement caused by the installation angle also remains small initially.
As the level falls, the measuring distance increases.
As a result:
- the beam axis shifts further sideways,
- the illuminated area becomes larger,
- additional vessel internals may enter the beam,
- the reflected signal returning to the antenna may become weaker.
A typical fault pattern is therefore:
high level stable → low level unstable
This does not necessarily mean that the measuring range of the sensor is too small.
It may be a geometric installation problem.
Why are smooth liquid surfaces particularly critical?
A relatively smooth surface produces a more directional reflection.
This is generally advantageous with perpendicular antenna alignment.
With angled alignment, however, the same property can become problematic because the energy is preferentially reflected in another direction.
This is particularly relevant, for example, with:
- calm storage tanks,
- oils,
- water tanks without significant movement,
- uniformly calm process liquids.
Even the best signal processing cannot evaluate energy that no longer returns sufficiently to the antenna because of unfavourable reflection geometry.
Does a turbulent surface help with angled installation?
A moving or irregular liquid surface reflects radar energy in multiple directions.
As a result, even with slightly angled installation, part of the signal may still return to the antenna.
However, this does not mean that angled installation is therefore technically correct.
If the surface movement changes, the received signal strength may also change.
A measuring point might, for example, work while an agitator is running but suddenly exhibit significantly poorer echo quality when the agitator is stopped.
The installation should therefore not depend on turbulence randomly scattering enough radar energy back towards the antenna.
What influence does the mounting nozzle itself have?
The mounting nozzle forms the first section of the radar path.
With a metal nozzle, its walls can themselves reflect radar energy.
Particularly problematic are:
- long and narrow nozzles,
- rough internal surfaces,
- weld seams,
- protruding edges,
- deposits,
- strongly angled nozzle geometries.
Part of the radar energy may therefore already be reflected directly below the antenna.
This creates a strong near-range echo while at the same time less energy reaches the actual product surface.
Why must the nozzle not restrict the radar beam?
The antenna requires a sufficiently unobstructed transmission area.
With an unfavourable ratio of:
nozzle diameter to nozzle length
the radar beam can strike the inside wall of the nozzle.
This is particularly problematic if the antenna axis is additionally angled relative to the nozzle axis or the vessel.
When selecting the installation, the manufacturer-specified limits for:
- nozzle diameter,
- nozzle height,
- antenna position and
- process connection
should therefore be taken into account.
A very long existing tank socket should not automatically be used merely because its thread or flange mechanically fits the sensor.
How can an angled nozzle be corrected mechanically?
If the existing nozzle axis does not match the required radar axis, a mechanical solution should be considered first.
Depending on the installation, this may be achieved, for example, by:
- modifying or re-welding the mounting nozzle,
- using a suitable intermediate flange,
- using a suitably designed adapter construction,
- using another existing vertical process connection,
- using a manufacturer-approved alignment option.
The goal is not for the sensor housing to appear visually straight.
The decisive factor is:
The antenna axis must geometrically match the intended measuring path.
For liquids, this generally means alignment as perpendicular as possible to the average liquid surface.
Can you simply rotate the sensor housing?
Many process instruments have a rotatable housing so that the display, cable gland or operating interface can be positioned conveniently.
However, this normally does not change the transmission direction of the radar antenna.
Rotation around the sensor’s own axis:
changes the orientation of the housing
but not:
the tilt angle of the antenna axis
.
An angled process connection therefore cannot be corrected simply by rotating the electronics housing.
Which vessel internals become critical in case of misalignment?
An angled antenna axis can direct the radar beam towards fixed vessel internals.
Typical interfering reflectors include:
- ladders,
- agitator shafts,
- heating coils,
- bracing,
- pipework,
- filling pipes,
- vessel walls,
- welded structures.
Metallic internals can produce very strong radar reflections.
If such an echo is temporarily closer to the expected level echo than the actual useful signal, echo evaluation can become more difficult.
The mechanical beam alignment should therefore always be considered together with the complete vessel geometry.
What influence does the medium have?
The strength of the reflected radar signal depends, among other things, on the electromagnetic properties of the medium.
Media with good reflection characteristics generally produce a stronger echo than products with very poor reflection characteristics.
With good antenna alignment, modern radar measurement can also operate reliably with more difficult media.
However, geometrically unfavourable installation reduces the available signal reserve.
For this reason, especially with weakly reflecting media, it should not be assumed that an angled antenna can be compensated for by additional parameterization.
What applies in the case of foam, vapour and condensation?
In addition to the installation geometry, process conditions can influence the radar signal.
These include, for example:
- foam on the liquid surface,
- strong turbulence,
- condensation on the antenna,
- deposits,
- vapour or changing process conditions.
Unfavourable antenna alignment and difficult process conditions can reinforce each other.
A measuring point with sufficient signal reserve has considerably more tolerance for such changes than an installation that already receives only a weak echo under ideal conditions.
How does the echo profile help with diagnostics?
The echo profile is one of the most important diagnostic tools in modern radar level measurement.
It shows the received reflections over the measuring distance.
This makes it possible to determine, for example:
- where the actual product echo is located,
- whether strong nozzle echoes are present in the near range,
- whether a vessel internal produces a fixed false echo,
- how the echo conditions change as the level falls,
- whether the signal reserve is sufficient.
With an angled sensor installation, it is particularly useful to compare the echo profile at several filling levels.
If the useful signal is strong at a high filling level but decreases unusually sharply as the level falls, the geometric alignment should be checked.
Can false-echo suppression correct the misalignment?
Automatic or learned false-echo suppression can take fixed reflections from known vessel internals into account during signal evaluation.
It can, for example, help suppress a constant echo from a:
- nozzle,
- ladder,
- pipe or
- brace
.
However, it does not change the physical direction of the radar beam.
If the actual level echo becomes too weak because the antenna is angled, false-echo suppression cannot restore the lost signal energy.
False-echo suppression is therefore a signal-processing tool, not a mechanical antenna alignment method.
Which parameters should be checked after correcting the installation?
After a mechanical change to the measuring point, the parameterization should also be checked.
Typical points include:
- sensor reference point,
- vessel height,
- zero point,
- full point,
- measuring range,
- near range,
- false-echo suppression,
- response behaviour,
- fault behaviour in the event of echo loss.
If the sensor has been mounted higher or lower by means of an adapter, the geometric reference distance may also change.
This difference must be taken into account in the configuration.
After a major mechanical modification, an existing false-echo profile should not be reused without checking, because the echo conditions may have changed.
Which diagnostics and alarms are useful?
For an important level measuring point, the displayed level value should not be the only information considered.
Depending on the measuring instrument, additional diagnostic information can be used.
Useful messages include, for example:
- echo loss,
- insufficient measurement quality,
- device fault,
- sensor or antenna problems,
- implausible changes in measured value.
A measured value may appear plausible for a long period even though the available signal reserve has already deteriorated.
A diagnostic message can then enable maintenance before the actual process value fails.
Practical example: 5° misalignment over a 10 m measuring distance
A liquid tank has a maximum measuring distance of:
10 m
The existing flange nozzle is approximately:
5°
off vertical.
If the radar level sensor is installed without correction, the lateral displacement of the antenna axis at the tank bottom is approximately:
x = 10 m × tan(5°)
which is approximately:
0.87 m
.
With an almost full tank, however, the measuring distance may be only:
1 m
The lateral displacement is then only approximately:
0.09 m
.
The measuring point therefore operates without problems at a high filling level.
At a low level, however, the beam increasingly moves towards a vessel wall or an internal pipe.
In the echo profile, the fixed false echo increases while the product echo becomes weaker.
A mechanical correction realigns the antenna axis perpendicular to the liquid surface.
After checking the vessel parameters and false-echo suppression again, the measurement then remains stable over the entire level range.
The error was therefore not caused by the maximum measuring range of the radar level transmitter, but by just a few degrees of incorrect antenna alignment.
Systematically diagnosing unstable radar readings
- Check the mounting position of the sensor.
- Check the antenna axis relative to the liquid surface.
- Determine the angle of the mounting nozzle.
- Check nozzle diameter and nozzle length.
- Check the antenna and nozzle for deposits.
- Consider the radar path over the entire vessel height.
- Take ladders, pipes and other internals into account.
- Compare the echo profile at high and low filling levels.
- Identify false echoes in the near range.
- Assess the product echo and signal reserve.
- Check the vessel reference height as well as zero and full points.
- Check the plausibility of the existing false-echo suppression.
- Correct the mechanical installation before changing only the signal processing.
- Then check the measuring point again over several filling levels.
Systematically planning a radar measuring point
- Determine the medium and process conditions.
- Determine the maximum measuring distance.
- Select a suitable radar frequency and antenna version.
- Take the beam angle of the selected sensor into account.
- Check the actual axis of the mounting nozzle.
- For liquids, provide antenna alignment that is as perpendicular as possible.
- Dimension nozzle diameter and length according to the manufacturer’s specifications.
- Ensure an unobstructed radar path over the entire measuring distance.
- Keep the vessel wall and internals outside the relevant beam area.
- Avoid installation directly above the inlet wherever possible.
- Take the agitator, foam and possible surface movement into account.
- Plan a mechanical alignment option at an early stage for difficult tank geometries.
- Parameterize the measuring range and vessel geometry correctly.
- Check the echo profile during commissioning.
- Integrate diagnostic and alarm functions into the PLC or control system.
Common mistakes
- Simply allowing the sensor to follow the nozzle axis: A mechanically compatible process connection does not automatically mean the radar axis is correct.
- Trying to correct an angled nozzle solely through parameterization: Software does not change the physical direction of the beam.
- Testing only with a full tank: At a short measuring distance, an angular deviation is much less noticeable.
- Underestimating small angles: A 5° misalignment already causes approximately 0.87 m of lateral displacement over a distance of 10 m.
- Rotating only the sensor housing: This normally does not change the tilt angle of the antenna axis.
- Using false-echo suppression as a substitute for poor installation: A useful signal that is too weak cannot be restored this way.
- Ignoring nozzle diameter: A nozzle that is too narrow can influence the radar beam itself.
- Failing to consider nozzle length: Long metal nozzles can produce strong near-range reflections.
- Looking only at the tank roof: The alignment relative to the product surface and the entire measuring path is what matters.
- Considering vessel internals only directly below the sensor: With an angled radar beam, more distant internals may enter the measuring beam.
- Considering good readings during turbulence as proof of correct installation: Scattered reflections can temporarily mask unfavourable geometry.
- Reusing old false-echo settings without checking after mechanical modification: The reflection geometry may have changed fundamentally.
SITRANS LR510 and LR530 for liquids
Siemens SITRANS LR510
The Siemens SITRANS LR510 is an 80 GHz radar level transmitter with a lens antenna and threaded connection for liquids and slurries.
The small process connection is particularly suitable for measuring points with limited installation space.
Depending on the process connection size, the antenna provides a correspondingly focused radar beam.
Functions of the LR500 series include:
- graphical echo-profile display,
- automatic false-echo suppression,
- near-range suppression,
- Process Intelligence signal processing,
- HART communication,
- diagnostic functions.
These functions simplify commissioning and diagnostics in difficult tank geometries.
Even with a high-performance 80 GHz radar sensor, however, correct mechanical antenna alignment remains the basis for stable measurement.
Siemens SITRANS LR530
For flange connections and more demanding process applications, the Siemens SITRANS LR530, for example, is available.
It uses an encapsulated PTFE antenna and is designed for non-contact continuous level measurement of liquids and slurries.
Different beam angles are available depending on antenna or flange size.
A small beam angle makes it easier to keep vessel internals and the vessel wall outside the relevant radar path.
However, a narrow radar beam does not mean that any angular deviation of the process nozzle is irrelevant.
Over long measuring distances, even a narrow but incorrectly aligned beam axis can shift significantly sideways.
Further radar, ultrasonic, hydrostatic and other measuring systems can be found under level measurement technology at ICS Schneider.
Conclusion
A radar level sensor must not only mechanically fit the existing tank connection.
The decisive factor is where its antenna axis actually points.
With a largely horizontal liquid surface, alignment that is as perpendicular as possible usually provides the most favourable reflection geometry. If the sensor is tilted by an angled mounting nozzle, part of the radar signal is reflected sideways away from the sensor.
At the same time, the beam axis shifts increasingly with measuring distance. Even a few degrees of misalignment can result in lateral displacement of several decimetres or metres in the lower part of a tall tank.
As a result, the vessel wall, pipes, ladders or other internals can suddenly enter the radar path.
The fact that the measuring point works at a high filling level is therefore not sufficient proof of correct installation.
False-echo suppression, near-range suppression and modern signal processing help with diagnostics and with suppressing unavoidable reflections. However, they cannot fully compensate for a fundamentally incorrect antenna direction.
For reliable radar level measurement, the following therefore applies: do not consider only the process connection, but check the actual antenna axis over the entire measuring distance. With an angled nozzle, the geometry should wherever possible be corrected mechanically so that the radar beam reaches the intended product surface as perpendicularly as possible and without interfering internals.
FAQ: Radar level sensor in an angled nozzle
Can a radar level sensor be installed at an angle?
A small deviation may still work depending on the vessel, medium, measuring distance and sensor. For liquids, however, alignment that is as perpendicular as possible to the product surface is normally significantly more favourable and reproducible.
Why is angled installation problematic with liquids?
A calm liquid surface reflects a large proportion of the radar energy directionally. With oblique incidence, less energy is therefore reflected directly back towards the antenna.
Can parameterization correct an angled nozzle?
Not physically. Parameters can influence echo evaluation, measuring range and false-echo detection, but they do not change the actual direction of the radar beam.
How much effect does an installation angle have?
The lateral displacement can be approximated using x = h × tan(α). With a 5° misalignment and a measuring distance of 10 m, it is approximately 0.87 m.
Why does an angled radar sensor often work better when the tank is full?
At a high filling level, the measuring distance is short and therefore the lateral displacement is also small. As the level falls, the displacement continues to increase.
Can the alignment be corrected by rotating the housing?
Normally not. A rotatable electronics housing primarily changes the position of the display and cable connections, but not the tilt angle of the antenna axis.
What must be considered with a long mounting nozzle?
Nozzle diameter and length must match the antenna. With unfavourable geometry, the nozzle walls themselves can reflect radar energy and generate strong near-range echoes.
Can false-echo suppression compensate for incorrect alignment?
It can take fixed unwanted reflections into account during echo evaluation. However, it cannot restore a severely weakened level echo if insufficient radar energy returns to the antenna because of the geometry.
Why should the echo profile be checked?
The echo profile shows useful and interfering reflections over the measuring distance. This makes it possible to identify whether the product echo becomes weaker as the level falls or whether vessel internals become increasingly relevant.
Which internals can interfere with radar?
Typical examples include ladders, pipes, heating coils, agitator shafts, bracing, vessel walls and inlet pipes.
Is an 80 GHz radar insensitive to angled installation?
No. Although the narrower beam of modern 80 GHz instruments often reduces the influence of surrounding internals, its centre axis must still be sensibly aligned with the product surface.
Which radar level sensor is suitable for liquids?
Specific examples are the Siemens SITRANS LR510 with threaded connection and the SITRANS LR530 with encapsulated flange antenna. Selection depends, among other things, on measuring distance, process connection, pressure, temperature, medium and vessel geometry.