Silos for grain, animal feed, cement or plastic pellets are at the heart of many production and logistics processes. Accurate fill level data is the basis for purchasing, production planning and financial reporting. In practice, however, few operators know exactly how much material their silos contain. Some sites have no continuous measurement at all and rely on high and low level switches, book inventory or manual checks from the silo roof. Others have a level sensor installed but do not fully trust its readings: the value jumps after filling, drifts during discharge and rarely matches what deliveries and consumption suggest. In both cases, companies compensate with higher safety stock, earlier orders and silos that are never filled to capacity.
How Silo Volume Is Calculated
Many silos consist of a cylindrical section with a conical hopper at the bottom. For a fully filled hopper and a material surface within the cylindrical section, the volume can be calculated using the following geometric formulas:
- Cylindrical section: V = π × R² × H, where R is the silo radius and H is the fill height within the cylinder, measured at the silo wall.
- Conical hopper: V = (π × h / 3) × (R² + R × r + r²), where R is the silo radius at the top of the hopper, r is the radius at the outlet, and h is the hopper height.
- Material cone on top: V = (π × R² × c) / 3, where the cone height c = R × tan(angle of repose). This assumes a symmetrical conical material surface.
During discharge, an inverted crater may form in the material surface. If approximated as a symmetrical cone, its volume can be subtracted from the calculated fill volume. The total material volume is determined by combining the relevant sections. However, these formulas rely on simplified assumptions about the material surface, which may not reflect its actual shape.
Why Silo Levels Are Difficult to Measure
The surface of bulk material in a silo is rarely flat. During filling, the material forms a cone beneath the inlet, with a slope influenced by its angle of repose. During discharge, a crater forms above the outlet while material remains banked against the walls. Cohesive materials can also form bridges or ratholes.
A measurement taken at a single point cannot capture this shape. A simple example illustrates the effect. In a silo with a diameter of 10 metres and a material with an angle of repose of 30 degrees, the cone rises almost three metres from the wall to the centre. If the level measured at the centre were assumed to represent a flat surface across the full diameter, the volume estimate would be roughly 150 cubic metres higher than the actual volume. For cement, this can correspond to around 200 tonnes. The wider the silo, the more important the shape of the surface becomes.
Silo Level Measurement Methods Compared
A range of technologies is used to monitor silo levels today:
- Point level switches only signal whether a silo is nearly full or nearly empty.
- Book inventory and manual checks: book inventory accumulates errors between corrections, and manual checks require staff to work on silo roofs.
- Ultrasonic and radar sensors measure continuously, but only at one point. Radar is robust in dusty environments and well suited to tall, narrow silos with free-flowing material.
- Load cells determine weight independently of surface shape, but can be costly to retrofit.
Most of these methods reduce a three-dimensional surface to one or a small number of measurements. When the surface is close to flat, that can work well. When it is not, the resulting volume depends increasingly on assumptions.
How 3D LiDAR Calculates Silo Volume
LiDAR sensors take a different approach. A sensor such as Blickfeld’s QbVolume emits laser pulses across a field of view of 90 by 50 degrees and measures the time of flight of each pulse. The result is a detailed 3D point cloud of the entire material surface.
The software compares this surface with the geometry of the silo and calculates the volume directly. Cones, craters and uneven deposits can be taken into account rather than approximated from a single measuring point. Where areas are temporarily hidden, for example directly below the sensor, the software interpolates the surface or extrapolates it using defined cone zones. For large bunkers, data from several sensors can be combined.
The system delivers accurate material volumes and the total material stock in each silo. Continuous monitoring provides detailed insights into stock level changes over time and helps optimize supply chain decisions. These results are available via a dashboard and can be transferred to SCADA or ERP systems through a standard interface. To convert volume into tonnes, the bulk density of the material is required.
Practical Requirements for LiDAR-Based Silo Monitoring
The sensors are mounted on the silo roof or on existing structures, typically at a height of 3 to 20 metres, without any contact with the material. A simulation of the sensor layout before installation helps to minimize blind spots. An unstable mounting point can also affect measurement quality, so mounting and data integration are best confirmed during a site assessment. During pneumatic filling, heavy dust can temporarily affect the measurement, which, depending on the application, can be addressed by filtering, averaging or measuring between fill cycles.
For combustible dusts such as grain, feed or certain plastics, QbVolume is available in an ATEX-rated housing. Both the housing and the installation must match the hazardous-area zone classification at the mounting location.
Where 3D LiDAR Makes the Biggest Difference
3D LiDAR is particularly beneficial for wide silos, bunkers and flat storage, where even a modest cone represents a large volume. It is also well suited to silos with several inlets or outlets, to cohesive materials such as animal feed that tend to form bridges and craters, and to sites with many silos, where one consistent measurement method simplifies maintenance and reporting. For narrow silos with free-flowing material, a radar sensor is often sufficient.
Conclusion
3D LiDAR turns silo level measurement from a single-point reading into a measurement of the entire material surface. Operators gain reliable volume data for planning, purchasing and reporting, without manual checks. At the same time, they can see the shape of the material surface instead of only its height. Changes such as a deepening crater or material persistently banked against one wall can indicate potential flow problems such as ratholing or bridging before they affect the process. More details on Blickfeld’s solution for bulk solids measurement are available on our application page.
