The SM85 from FIAMA is a straightforward membrane-operated level switch that detects the presence or absence of bulk solid materials against the silo or hopper wall. When material presses against the 82 mm rubber diaphragm, it deflects inward and trips an internal microswitch — providing a dry-contact signal to a PLC, relay, or indicator panel. When the material level drops and pressure on the membrane is released, the switch resets. This simple electromechanical principle requires no power supply, no calibration, and no electronics at the sensing point, making it a robust alternative to capacitive or rotary paddle sensors in applications where material build-up, humidity, or vibration cause false triggering of electronic sensors.

What materials and conditions is the SM85 suited for?

The standard SM85 with black rubber membrane is designed for non-sticky powders and free-flowing granular materials — rice, corn, plastic granules, coffee beans, sand, lime, feed pellets. Material must flow freely enough to exert consistent lateral pressure against the silo wall at the sensing point. For materials with flammable dust potential, the SM85.ATEX variant uses an anti-static silicone rubber membrane and carries II 1/2 D Ex ia IIIC T85°C Da IP65 certification, suitable for Zone 20 (inside the tank) and Zone 21 (outside). An external intrinsically safe barrier is required for the ATEX version — it is not built into the switch.

How is sensitivity adjusted for different material densities?

A mechanical adjustment screw accessible on the instrument body sets the membrane deflection force required to trip the microswitch. For light, fluffy materials like expanded polystyrene beads or bran, the screw is backed out for maximum sensitivity — as little as 50 g of force applied at the centre of the membrane or the equivalent of 20 mm water column pressure distributed across the surface. For denser materials like sand or cement, the screw is tightened to require greater deflection force, preventing nuisance triggering from momentary pressure pulses during filling. The switch is immune to humidity and temperature variations, and the membrane resists shock and vibration transmitted through the silo structure.