Bulk solids do not behave like liquids in a silo. Instead of flowing down evenly as material is drawn from the bottom, cohesive or interlocking solids can arch across the vessel and hold themselves up, leaving an empty cavity underneath while the surface above barely moves. A level device sees that stationary top surface and reports a comfortably full silo, even though the material below has already discharged. The result is false inventory and, worse, a downstream process that starves because nothing is actually reaching the outlet. This page explains how bridging and ratholing form, how each level technology behaves under them, and how SCADA cross-checks level against outflow to catch a level that should be dropping but is not.
Level Bridging in one line: Level bridging is when bulk solids in a silo or hopper form a stable arch, or bridge, across the vessel that supports itself and leaves an empty cavity below, so material discharges from underneath while the top surface stays put. A level device measuring that surface reads a full or nearly full silo even though the usable inventory has already emptied out beneath the bridge, giving false inventory and starving the downstream process fed from the outlet. A rathole is a related failure where material flows only through a narrow central channel and leaves a stagnant annulus, producing similarly misleading level readings.
Bulk solids have cohesion and internal friction that liquids do not, and under the right conditions those forces let the material support itself rather than flow. A bridge, or arch, forms when particles interlock or stick together across the width of the silo, transferring their weight to the vessel walls instead of pressing down on the material below. Once an arch establishes itself above the outlet, material can keep discharging from beneath it, opening a void, while the arch and everything above it stays in place. The silo looks full from the top and is partly hollow inside.
Ratholing is the related pathology in which flow funnels through a narrow vertical channel directly above the outlet while the material in the surrounding annulus stays stagnant. The central channel empties down to the outlet, but the shoulders of material around it do not move, held up by their own cohesion and friction against the walls. Like a bridge, a rathole creates a mismatch between what is leaving the silo and what the bulk of the material is doing, and it can leave a large stagnant mass clinging to the walls while the center has voided out.
Both conditions are promoted by cohesive, damp, fine, or compressible materials, by long storage times that let material consolidate, and by vessel and outlet geometry that does not encourage mass flow. What they share operationally is a decoupling of the visible top surface from the actual state of the material below. Because the top of a bridge or the shoulders of a rathole can sit nearly still while material discharges from underneath, any measurement that infers total inventory from the top surface is being fed a misleading picture. The empty cavity is invisible from above, which is precisely what makes bridging so deceptive.
Different level technologies respond to a bridge in different ways, and understanding this helps interpret a suspicious reading. Non-contact radar and guided wave radar both locate the top surface of the material, so if a bridge holds that surface in place, they faithfully report the unchanged surface as the level, showing a full silo over a hollow cavity. They are not wrong about where the top surface is, they are simply measuring a surface that no longer represents the true inventory, because the material beneath it has gone. On a solids surface that is uneven or angled, radar readings can also be noisy, but the core issue under bridging is that the surface they track has stopped falling.
Weight-based measurement behaves differently and is often the tell. If a silo is instrumented with load cells or is weighed, the total mass reflects how much material is actually present regardless of where the surface sits. Under bridging, the weight drops as material discharges from beneath the arch even while a surface-tracking level device still reads full, so a divergence between a steady level and a falling weight is a strong signature of a bridge. Where weight measurement is available, it effectively sees through the deception that fools surface-based devices.
This contrast is the practical heart of detecting bridging. A single surface-tracking device, radar or guided wave radar, cannot by itself distinguish a genuinely full silo from a bridged one, because in both cases the surface it measures is high. Adding an independent view, whether a weight measurement, a point-level switch lower in the vessel that should have uncovered, or knowledge of how much material has been drawn from the outlet, is what breaks the ambiguity. The lesson is not that one technology is right and another wrong, but that surface level alone is insufficient to trust inventory in a solid prone to bridging.
The most robust way to catch bridging without adding hardware is to compare the reported level against what the discharge system is actually doing. If a feeder, screw, rotary valve, or belt beneath the silo has been running and removing material, the level should be falling at a rate consistent with that outflow. When the outlet has clearly been discharging but the level reading barely moves, the material is not reaching the outlet as it should, which is the classic signature of a bridge or rathole holding the surface up while a cavity opens below. A frozen level in the face of confirmed outflow is a red flag that the level device is being deceived.
This cross-check is exactly the kind of reasoning a cloud SCADA layer can automate, because it already has both signals in one place. When a platform like Merobix trends the silo level alongside feeder run time or measured outflow, it can compute the expected level drop from how long the discharge equipment has run and compare it to the actual level change, alarming when the level fails to fall as the outflow says it should. Instead of an operator discovering a starved downstream process and only then suspecting the silo, the system flags the frozen-but-should-be-dropping level as it happens, pointing at bridging as the cause.
The same history makes the problem legible over time and separates it from ordinary operation. A level that repeatedly stalls at a certain height whenever the silo is drawn down, then jumps suddenly when a bridge finally collapses, produces a recognizable sawtooth pattern in the trend that identifies a chronic bridging vessel rather than a one-off. Correlating those stalls with material type, moisture, or storage time helps target the fix, whether that is an aeration or vibration aid, a geometry change, or a discharge procedure. And because the platform holds both the inferred level and the outflow, it can maintain a truer inventory estimate based on material balance during a bridge, so the false full reading does not by itself corrupt the numbers people plan against or starve a downstream feed without warning.
Bridging, or arching, is when solids interlock across the width of the silo to form a stable arch that supports itself, leaving an empty cavity below as material discharges from underneath. Ratholing is when flow funnels through a narrow vertical channel above the outlet while the surrounding annulus of material stays stagnant against the walls. Both decouple the visible top surface from the true state of the material, and both can make a surface-tracking level device report far more inventory than is actually usable.
This is the classic symptom of a bridge. When solids arch across the vessel, material can keep discharging from beneath the arch while the arch and everything above it stays in place, so a level device measuring the top surface still reads full over a hollow cavity. The measurement is not malfunctioning, it is faithfully reporting a surface that no longer represents the real inventory, which is why a full reading with a starved downstream process points at bridging.
The most effective method is to compare the reported level against the discharge, since the level should fall at a rate consistent with how much the feeder or outlet has removed. When the outlet has clearly been running but the level barely moves, material is not reaching the outlet as it should, indicating a bridge or rathole. A monitoring platform can trend level alongside feeder run time or outflow, compute the expected drop, and alarm when the level fails to fall as the outflow says it should.
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