Silo Weighing Retrofit: Adding Load Sensing to Existing Silos Without Downtime

A silo weighing retrofit lets you measure the actual mass of material inside a legged silo without cutting the roof, emptying the vessel, or stopping production. Instead of inferring level from a top-mounted radar or guessing from delivery records, you read the load carried by the support legs and convert it to stored mass. For plants running cement, fly ash, lime, mineral powder, animal feed, grain, or plastic pellets, this is usually the fastest path to trustworthy inventory data on equipment that is already in service.

This guide walks through the practical sequence a field crew follows: structural review first, then sensor installation, wiring and communications, on-site calibration, and post-commissioning verification. It also covers the cases where a retrofit should wait until a structural concern is resolved.

How patch-mount weighing works on a legged silo

The patch-mount silo weighing system uses external strain-gauge sensors bonded to the silo support legs. As material accumulates, the load path through each leg increases, and the steel undergoes microstrain. The sensor measures that strain change, and an algorithm with filtering converts the combined signal into stored mass. Nothing is installed inside the silo, so there is no contact with the product and no need to open the vessel.

Because the measurement comes from the existing structure, the approach fits silos already standing on legs. Typical coverage runs from roughly 5 t up to 3000 t, with accuracy of typically ±0.5–3% of full scale after on-site calibration. The achievable figure depends heavily on how symmetric the structure is and how well the calibration is performed, which is exactly why the steps below matter.

Step 1: Structural review before anything is bonded

The retrofit starts with an engineering walk-down, not with a sensor. The goal is to confirm the silo is a sound candidate and to decide how many legs to instrument.

Check the following:

• Leg count and configuration. Systems typically use 2 to 12 legs. The number you instrument depends on the silo’s leg arrangement and is settled during a structural review, not assumed in advance.
• Symmetry. Evenly spaced legs sharing load predictably give better accuracy. Asymmetric layouts, added bracing, or one leg carrying piping or a walkway change how load distributes and must be accounted for.
• Foundation condition. Settled, cracked, or uneven foundations distort the load path and corrupt the strain signal. The foundation has to be sound before instrumentation is meaningful.
• Leg surface and integrity. Each instrumented leg needs a clean, sound section of steel for the sensor to read. Heavy corrosion, prior repairs, or deformation at the intended location are disqualifying for that spot.
• Load path clarity. Confirm the legs actually carry the silo’s weight, rather than the vessel resting partly on a skirt or external frame that bypasses them.

If the walk-down surfaces a structural concern — visible foundation movement, corroded or modified legs, or an unclear load path — do not proceed with bonding. The right move is to flag it for structural assessment and resolve it first. A weighing system bonded to a compromised structure produces unreliable numbers and can mask a real safety issue.

Step 2: Sensor installation on the legs

Once the silo passes review, the crew prepares each selected leg and bonds the strain-gauge sensor to the cleaned steel surface. The work is external and does not require entering the silo or interrupting filling and discharge.

Plan around these points:

• Per-leg time. Installation runs typically 2–4 hours per leg when conditions are favorable — clean access, sound steel, and reasonable working height. Difficult access, surface prep, or weather extends this.
• No roof work, no downtime. Because sensors mount on the legs, there is no roof penetration and the silo can stay in service. Production typically continues during the work.
• Consistent placement. Sensors should sit at a comparable, well-defined location on each instrumented leg so the readings combine cleanly.
• Protection. Sensors are rated to IP66 (planned) for dust and washdown exposure typical of powder and bulk-handling sites.

For a multi-leg silo, total field time is roughly the per-leg figure multiplied by the number of legs instrumented, plus wiring and calibration.

Step 3: Wiring and choosing a communications path

After the sensors are bonded, route and terminate the cabling to the controller, then pick an output that matches your existing control system and data needs.

Available outputs and links:

• 4–20 mA analog. The simplest choice when you only need a continuous mass or level signal into an existing PLC or display. One value, one loop.
• RS485 (Modbus). Use this for digital, multi-variable communication into a PLC, SCADA, or local HMI when you want more than a single analog value.
• API. For sending readings to an ERP, MES, or inventory platform when stored mass needs to flow into business systems.
• Wireless. 4G, WiFi, and Bluetooth are available where running new cable is impractical or where remote, unattended silos need to report back without local infrastructure.

Pick the option that fits the plant. A single silo feeding one line often only needs 4–20 mA; a tank farm reporting into central inventory is a better fit for RS485 or an API feed, and remote silos lean on 4G.

Step 4: On-site calibration

Calibration is what turns raw strain into a number you can act on, and it is where the accuracy figure is actually earned. The stated ±0.5–3% FS is achievable after on-site calibration — not out of the box.

A workable calibration approach:

• Establish a baseline with the silo at a known reference condition.
• Apply known mass changes where possible — for example, recording metered additions or discharges against the system reading.
• Let the algorithm and filtering account for the structure’s behavior across the working range, rather than at a single point.
• Repeat reference checks across low, mid, and higher fill levels so the curve holds over the full operating range, not just one band.

Calibration quality tracks directly with structural symmetry and the care taken here. Rushing this step is the most common reason a retrofit lands at the high end of the error range instead of the low end.

Step 5: Commissioning verification

Before handing the system over, confirm it behaves correctly under real operation rather than just at the moment of calibration.

Commissioning checklist:

• Readings track in the correct direction during known fills and discharges.
• Output signal (4–20 mA, RS485, or API) is received and decoded correctly by the destination system.
• Values are stable and free of excessive noise once filtering is active.
• Readings stay consistent across the operating range, not only near the calibration point.
• Sensor terminations and enclosures are sealed and mechanically secure.
• Wireless links (if used) report reliably from the installed location.

Document the as-built configuration: which legs were instrumented, the chosen output, calibration references, and the verification results. That record is what makes future troubleshooting and recalibration straightforward.

When a retrofit is not the right move yet

A patch-mount retrofit is well suited to sound, legged silos, but it is not a fix for structural problems. Hold off and get an assessment first when you see:

• Foundation settlement, cracking, or uneven bearing.
• Corroded, deformed, or previously repaired legs at the intended sensor locations.
• An unclear or bypassed load path, where the silo does not bear cleanly on its legs.
• Highly asymmetric or heavily modified leg arrangements that make load distribution unpredictable.

In these cases the structural issue comes first. Once it is resolved, the silo can be reassessed for instrumentation. Treating a weighing retrofit as a substitute for structural repair only buys unreliable data on top of an unresolved problem.

Putting it together

For an existing legged silo, a patch-mount weighing retrofit follows a clear path: review the structure, bond external sensors to the legs in roughly 2–4 hours each, wire and select the right communications output, calibrate on site, and verify under real operation — all without opening the roof or stopping production. The result is inventory data grounded in measured load rather than estimates, on equipment you already own.

If you want help scoping a retrofit for a specific silo or silo group, including the structural review and the right output for your control system, contact a Volivue engineer with your leg count, capacity, and material.

Related reading: more field guides and measurement engineering notes are in our Technical Insights.