Level Measurement in Dusty, High-Temperature Silos

Technical guide by Arvin, METRAVON Instruments · Reviewed September 2026

Short answer

For cement, fly ash, mineral powder and similar tall silos, evaluate dust, process-connection temperature, filling impact, buildup and internal obstructions before choosing a radar. A narrow-beam non-contact radar is often the first technology to assess because it can avoid more wall and structural echoes, but frequency alone does not guarantee a reliable result. Mounting geometry, antenna condition, echo quality and dynamic commissioning remain decisive.

Why these silos are difficult

During pneumatic or mechanical filling, airborne dust can be far denser than it appears after the process stops. The material may form a steep peak below the inlet, a funnel above the outlet, or persistent wall buildup. Roof beams, ladders, stiffeners, fill pipes and the cone can all produce fixed echoes. At the same time, high process temperature can be very different from the actual temperature at the instrument flange.

The measurement objective also matters. A level trend for process control is not the same as a mass value for inventory. Converting level to tonnes requires a verified vessel strapping table and a representative bulk-density range. When density and surface shape vary, the calculated mass must be treated as an estimate with a stated uncertainty.

Technology and design comparison

Mounting location

Plot the radar position, filling points, vessel wall and internal structures on a roof drawing. Check the beam envelope over the complete measuring range rather than only at the antenna. Keep the antenna away from the direct filling stream and from locations where a steep material peak would place the surface at a severe angle to the beam.

The geometric center is not automatically the best position. It may align with a filling pipe or a symmetrical vessel feature that creates repeated reflections. A slightly offset location with a clean view can be better, provided it still measures a representative part of the surface.

Temperature and process connection

Confirm continuous and peak medium temperature, the expected temperature at the process connection, ambient temperature and exposure duration. Review the final model's data sheet for antenna, seal and electronic limits. Insulation, a thermal extension or ventilation may be required, but these measures must be engineered for the selected instrument and installation.

A long or narrow nozzle can shield the antenna and create strong near-field echoes. Verify nozzle diameter, length, antenna position and any minimum projection requirement. The flange should be aligned so the beam points toward the intended surface, not the wall.

Dust, buildup and purge air

• Choose a position that minimizes direct material impact and persistent deposition.
• Provide safe access for inspection and cleaning when buildup is expected.
• Evaluate purge air only when the antenna design and site air quality support it.
• Use clean, dry air at an appropriate pressure; uncontrolled purging can introduce condensation or contamination.
• Set the maintenance interval from observed signal and buildup trends, not an arbitrary calendar alone.

Commissioning and acceptance

1. Record the empty-vessel echo profile and identify fixed structures before creating false-echo suppression.
2. Check the configured reference point, empty distance, measuring range, blocking distance, output scaling and fault current.
3. Observe echo strength and tracking during the dustiest part of filling, not only when the silo is quiet.
4. Compare low, medium and high levels with independent references where practicable.
5. Review trends during filling, discharge and rest for jumps, lost echoes or implausible rates of change.
6. Test high-level alarms, loss-of-echo handling and communication failure at the control system.

Information required for selection

Provide the material, particle size and buildup tendency, silo height and diameter, roof and cone drawings, filling and discharge locations, measuring range, process and flange temperatures, pressure, dust severity, nozzle dimensions, internal obstructions, hazardous-area classification, available purge air, power, output or protocol, and the intended use of the inventory value.

Frequently asked questions

Why is a narrow beam useful in a silo?

It can make it easier to avoid the wall, roof structures and fill pipes. It does not eliminate attenuation, antenna buildup or poor mounting geometry, so the complete application still needs review.

Should the radar always be installed in the center?

No. Choose the point from the beam path, inlet position, expected surface shape and internal structures. The center is suitable only when those conditions support it.

Is an empty-silo test sufficient?

No. It is useful for checking reference distance and fixed echoes, but acceptance should also cover dusty filling, normal discharge and levels near important alarm points.

Can level be reported as tonnes?

Yes, as an estimated value when the strapping table and bulk density are validated. State the conversion method and uncertainty, especially when moisture, aeration or compaction changes density.

Build the temperature profile, not one maximum value

The temperature at the instrument connection can differ greatly from the bulk material or gas temperature. Record normal, cleaning, startup and upset values, duration of peaks, ambient temperature and heat transfer through the nozzle. Hot dust and vapour may rise into the antenna even when the average silo temperature is lower. Confirm limits for the process seal, antenna, electronics and cable entry separately.

Where thermal isolation or cooling is required, design it as part of the measurement assembly. An extension can protect electronics but may create a long nozzle that blocks the beam or collects deposits. Purge air must be clean, dry, available and permitted by the process. Monitor loss of cooling if exceeding the temperature limit could damage the device.

Dust, buildup and echo margin

Describe dust intensity during the worst filling method, particle conductivity, stickiness and likelihood of condensation. Dry airborne dust may be manageable for radar, while a moist conductive layer on the antenna can become the dominant problem. Select antenna geometry and mounting that limit accumulation, and provide safe inspection access. Do not rely on frequent manual cleaning where access requires production shutdown or entry into a hazardous area.

Review the echo curve at empty, during heavy filling and near the operating limits. Record wanted-echo strength, false echoes and available margin. A narrow beam helps avoid walls and structures but does not remove the need for correct alignment. False-echo suppression should cover confirmed fixed objects only; aggressive masking can hide the real surface after product conditions change.

Mechanical design for severe service

Locate the instrument outside direct impact and away from vibration transmitted by feeders or cleaning equipment. Verify nozzle diameter, length, roof angle and structural strength. Protect the housing and cable from hot surfaces, falling material and washdown without enclosing the electronics beyond their ambient rating. Use process materials and gaskets compatible with abrasion, chemistry and thermal cycling.

For high-high protection, use an independently assessed switch at a point that preserves shutdown volume. Its sensing element also needs protection from impact and buildup. Define the fail-safe output and proof-test method. A continuous radar alarm derived from the same device may support operations but should not silently replace independent protection where the risk assessment requires it.

Dynamic commissioning procedure

Before filling, verify reference distance, empty echo, output scale, fault current and communication. During filling, capture echo curves at increasing dust load and observe whether the selected echo follows the material. During discharge, check for funnel formation and wall retention. Compare with a safe independent reference at multiple points and document the process state for each result.

Simulate signal loss, power failure and communication timeout. Confirm that the control system marks bad or stale data and that protective functions take the intended state. Save the accepted configuration and diagnostic baseline. Repeat critical checks after the first thermal cycle because expansion, condensation and deposits may not appear during a cold test.

Risk-based inspection and troubleshooting

Set inspection frequency from temperature margin, deposition rate, process criticality and safe access. Trend signal strength and false-echo behaviour rather than waiting for a lost reading. When performance changes, first compare process and echo data with the baseline; do not immediately increase gain or expand masking. Inspect alignment, antenna, seals, purge and nozzle before changing parameters.

Review the application after a new material, higher fill rate, process-temperature increase or structural modification. These changes can invalidate the original assumptions while the instrument remains electrically healthy. Record as-found condition and corrective action so repeated buildup or thermal problems drive a design improvement rather than recurring cleaning.

Keep approved drawings and baseline echo records accessible to maintenance personnel.

Engineering note: Dust explosion protection, high-temperature limits and installation requirements must be verified for the exact model, certificate, hazardous area and applicable site standards.