Dusty, abrasive and hot bulk solids destroy probes that touch them, yet the plant still needs a reliable high level or blocked chute alarm. A microwave beam across the vessel answers that need from outside the process, often through a simple plastic window.
When a probe cannot survive inside a silo or chute, a beam of microwaves across it can still sense the material. This guide explains how the microwave barrier switch works, how to install and align it, and how to avoid false alarms.
What Is a Microwave Barrier Switch?
A microwave barrier switch is a non contact point level and presence detector made of a microwave transmitter on one side of a vessel, chute or conveyor and a receiver on the opposite side. When material fills the space between them, the received signal falls and the switch changes state, much like a conventional level switch but without touching the product.
Because the beam can pass through non metallic walls and windows, the units can sit outside the process completely. This makes them popular for hot, abrasive, sticky or very dusty solids where mechanical switches wear out quickly.
The idea is similar to a light curtain, yet microwaves pass easily through dust, steam and light build up that would blind an optical beam. Engineers often choose it when a vibrating fork level switch or paddle cannot be installed or keeps failing.
Endress Hauser Soliwave barriers operate in the 24.05 to 24.25 GHz ISM band. At that frequency the wavelength in air is only about 12 mm, so the units stay compact.
How the Microwave Beam Detects Bulk Solids
The transmitter sends a continuous low power microwave signal straight across to the receiver. The receiver compares the signal strength with an adjustable threshold, and the output relay changes when the signal drops below it.
Endress Hauser describes this as an attenuation principle, where the FQR57 transmitter emits a signal that the FDR57 transceiver detects directly opposite. Its technical information also lists a bulk flow mode that uses the Doppler shift of reflected energy to tell moving from stopped material.
Materials with water or high dielectric constant absorb more energy, so they switch with a clear margin. Very light or dry products, such as foam beads, attenuate weakly and need careful threshold setting, similar to the limits described for radar level transmitters.
Range, Wavelength and Fresnel Zone
The beam is not a thin line but a wide zone, and objects near the path also weaken the signal. Radio engineers call the important region the first Fresnel zone, and keeping it clear gives a strong, stable signal in the empty state.
First Fresnel zone radius at midpoint: r = 0.5 × √(λ × L)
Example: f = 24.15 GHz, distance L = 5 m
λ = 299.79 ÷ 24.15 = 12.41 mm = 0.01241 m
λ × L = 0.01241 × 5 = 0.06207
√0.06207 = 0.2491
r = 0.5 × 0.2491 = 0.1246 m, about 124.6 mm
So across a 5 m wide bin, a zone about 250 mm in diameter at the centre should be free of beams, ladders or ledges in the empty condition. Across a small 600 mm chute the zone is only about 43 mm in radius, which is why short paths are easy to set up.
Microwave Barrier Level Switch Path Calculator
Use the result to check that internal structures stay outside this zone, and remember that build up on the walls also enters it. For tall silos where a continuous reading is wanted instead, compare options in non contact vs guided wave radar selection.
In the empty state, note the received signal strength during commissioning and write it on the loop sheet. A later fall in that value with the chute empty shows growing build up long before a false alarm appears.
Main Components of the System
Generates the microwave beam, mounted on one side.
Measures received signal and drives the relay.
Sets sensitivity, delay and fail safe mode.
Some microwave barrier switch models combine the receiver and switching electronics in one housing, while others use a separate controller. Endress Hauser notes that its FQR57 and FDR57 pair is configured entirely through the Nivotester FTR525, with no adjustment on the field units.
Outputs are usually relay contacts or a PNP signal wired to a PLC input, with a switching delay to ignore brief interruptions. Where the alarm protects people or equipment, review it with the logic used for conveyor safety switches.
Mounting Through Windows and Nozzles
Endress Hauser lists PTFE, polyethylene and polypropylene as suitable window materials, along with uncoloured alumina ceramic of at least 99.5 percent purity and uncoloured borosilicate glass. Metal walls block microwaves completely, so steel bins always need a window or an open nozzle.
The Soliwave FDR56 page gives a process temperature of minus 40 to 70 °C for installation within the process, rising to 450 °C with a high temperature adapter. Its high pressure adapter allows up to 21 bar absolute.
Hycontrol states that its MWS blocked chute detector can sense through windows, with a range of up to 40 m for the DC version and 100 m for the AC version. A high temperature variant handles up to 600 °C.
6 Proven Steps to Install a Microwave Barrier Switch
Endress Hauser warns that the transmitter and receiver must not be rotated relative to each other about their longitudinal axis, except by 180 degrees or for distances under 300 mm. For very short paths it recommends a 90 degree installation angle.
In dusty plants, the housings should carry a suitable dust zone rating, as explained in hazardous area classification zones. Coal, sugar and grain dust are all combustible and demand certified equipment.
Mount the window flush with the inside wall or slightly recessed, never forming a ledge. A ledge collects material that slowly shadows the beam and causes nuisance alarms.
Blocked Chute Detection in Practice
Transfer chutes between conveyors block when wet or sticky material bridges, and the upstream belt then spills tonnes of product. A microwave barrier switch across the chute, just above the expected choke point, trips the upstream drive within seconds.
Mining, cement, power and steel plants in India use this arrangement on coal, limestone, clinker and iron ore lines. Many sites pair it with paddle level switches in bins and belt sway switches on the conveyors for complete protection.
| Application | Typical Material | Why Microwave Works |
|---|---|---|
| Conveyor transfer chute | Coal, ore, limestone | No wear from impact and abrasion |
| Silo high level | Cement, fly ash, flour | Works through dust and from outside |
| Bin low level | Wood chips, biomass | Ignores light build up |
| Flow or no flow | Grain on conveyor | Doppler mode senses movement |
| Hot product chute | Clinker, sinter | Mount outside with heat adapter |
Advantages and Limitations of Microwave Barriers
- No contact with abrasive or hot product.
- Works through dust, steam and light build up.
- Can mount outside the vessel behind windows.
- Long paths up to about 100 m with some models.
- Gives a point signal, not continuous level.
- Needs a clear line of sight and good alignment.
- Low density or dry light products attenuate weakly.
- Metal walls need windows or open nozzles.
Commissioning a Microwave Barrier Switch
- Confirm both units face each other on one axis.
- Check windows are clean, uncoloured and of approved material.
- Record the empty path signal strength.
- Block the beam with real product and record the covered signal.
- Set the threshold midway between empty and covered values.
- Set a switching delay to ignore falling lumps.
- Test the fail safe output on power loss.
- Confirm the alarm at the PLC or DCS.
Choose fail safe action so that loss of power or a failed transmitter gives the same signal as material present. This simple rule follows the thinking behind safety integrity levels, even when the loop is not formally SIL rated.
For background on how different technologies compare in difficult solids, see how to select the right level sensor. The microwave barrier switch earns its place where nothing should touch the product.
Troubleshooting False Alarms
A microwave barrier switch that alarms with an empty chute usually suffers from build up on windows, misalignment after maintenance or an object inside the beam. Clean the windows, recheck alignment and compare the signal with the commissioning record.
A switch that fails to alarm with material present often has its threshold set too low or sees a product with very weak attenuation. Damp material helps, while dry plastic pellets or foam may need a shorter path or a different technology such as a capacitance level switch.
Nearby radar devices rarely interfere, but avoid aiming two microwave barriers at the same receiver. Compare also with nuclear level measurement, which handles thick steel walls but needs licensing and strict safety rules.
Soliwave Microwave Barrier Technical Information
Blocked Chute Detector Demonstration
Microwave Barrier Switch FAQ
It is a non contact point level and presence detector that uses a microwave transmitter and an opposite receiver. Material in the beam reduces the received signal and changes the output relay state.
It is used for silo levels, blocked chutes and flow or no flow checks on conveyors. The units can mount outside the process behind non metallic windows in most cases.
Many modern units operate in the 24 GHz ISM band, such as 24.05 to 24.25 GHz for Soliwave models. The wavelength at that frequency is about 12 mm in air.
This short wavelength keeps antennas and housings compact for field mounting. It also passes easily through dust while still being absorbed well by most bulk solids.
It works through plastic, glass and ceramic walls or windows of suitable material and thickness. Endress Hauser lists PTFE, PE, PP, uncoloured alumina ceramic and borosilicate glass as suitable choices.
Metal walls block microwaves completely, so steel silos need windows or open nozzles. Windows should be flush with the inner wall to avoid material build up.
Endress Hauser gives up to 100 m for level limit detection and 10 m for bulk flow detection with Soliwave units. Hycontrol quotes 40 m for its DC version and 100 m for its AC version.
Actual range depends on the product, the windows and any build up present. Short chutes give the largest signal margin and the easiest setup.
Mount the transmitter and receiver on one common axis, facing each other directly. Do not rotate one unit relative to the other about that axis, except by 180 degrees.
Then record the received signal in the empty state and with real product in the beam. Set the switching threshold between these two values for reliable alarms.
Common causes are build up on windows, misalignment after maintenance or a new object in the beam path. Large falling lumps can also interrupt the beam briefly during normal flow.
Clean the windows, recheck alignment and compare the signal with the commissioning record. A short switching delay filters brief interruptions from falling lumps of material.
Dry, low density products attenuate microwaves only weakly, so the signal margin becomes small. Endress Hauser specifies a minimum solid density of more than 10 g/l for the FDR56.
Test with the real product in a trial chute before final selection whenever possible. If margin stays poor, consider a shorter path or a capacitance or vibrating level switch.
Related Articles
- Level Switch Selection Guide
- Vibrating Fork Level Switch Working
- Paddle Type Level Switch
- Capacitance Level Switch Working
- Conveyor Safety Switches Types
External References
- Soliwave FQR57 and FDR57 Technical Information, Endress Hauser
- Microwave Barrier Transceiver Soliwave FDR56, Endress Hauser
- Level Sensor, Wikipedia
What We Learn Today
- A microwave barrier switch sends a 24 GHz beam from a transmitter to an opposite receiver and switches when bulk solids attenuate the signal.
- The units can mount outside the process behind PTFE, polyethylene, ceramic or glass windows, while metal walls always need windows or open nozzles.
- Alignment on one axis, a clear Fresnel zone and a threshold set between empty and covered signals give reliable blocked chute alarms.