Silo Weighing System with WST703 Micro Strain Sensors

It measures the micro-strain in your silo support leg caused by the weight of the vessel and its contents. Four foil strain gauges bonded in a full Wheatstone bridge convert that deformation into a millivolt signal proportional to load, which the weighing indicator converts into kilograms. In effect it turns an existing leg-supported vessel into a weighing system without installing load cells underneath it.

Any leg-supported or skirt-supported vessel that has a measurable vertical load path in its support structure. Typical applications are cement and aggregate silos, powder and granulate storage tanks, weigh hoppers, bins and bunkers. Handled materials include cement, sand, aggregates, flour, grain, feed pellets, minerals, chemicals and plastics. Outdoor and indoor installations are both supported.

Yes. The WST703 is a general-purpose weldable strain transducer, and silo weighing is one of four duties it is built for. Beyond silo, tank and hopper weighing it is used for load and overload monitoring on cranes, presses and mining machinery, for long-term stress monitoring of bridges, buildings, towers and steel structures, and for indirect force measurement where a traditional load cell cannot physically be installed. The same body, the same 300 to 1000 microstrain range and the same 1.0 to 2.0 mV per volt output apply to all of these duties.

No. Because the sensors are fitted to the outside of the existing support legs, the vessel does not have to be lifted onto load cells, emptied, or removed from service. No foundation modification is required, and the vessel is not cut or drilled — only a small external mounting pad is welded to each leg face. A four-leg silo is normally fitted within a single working day, depending on site access, and calibration is done afterwards with an empty-to-full cycle.

Both, in sequence. A small steel mounting pad is welded to the outer face of the support leg, and the sensor body is then bolted onto that pad with two dedicated high-tensile bolts supplied with the sensor. This is why the product is known in the industry as a bolt-on strain sensor. The vessel itself is never cut, drilled or otherwise modified, and because the sensor is bolted rather than welded in place, a damaged unit can be replaced in minutes without any hot work on the structure.

One sensor per support leg. A three-leg silo uses three sensors, a four-leg silo uses four. Gauging all legs is strongly recommended, because measuring only half of the legs makes accuracy dependent on how evenly the material is distributed inside the vessel. All sensors connect to a single summing junction box and one weighing indicator.

The sensor itself is a passive strain-gauge bridge rated at 1.0 to 2.0 mV per volt, with an input resistance of 380 ohm plus or minus 30 ohm and an output resistance of 350 ohm plus or minus 5 ohm, so it works with standard weighing indicators, strain-gauge transmitters and DAQ systems. Recommended excitation is 10 V DC. With a suitable indicator you can get relay set points for refill or overfill alarms, plus analogue output or RS485 Modbus for connection to a PLC or SCADA system.

Several installation formats are available to match different structure geometries, because a silo leg, a lattice member and a machine frame do not present the same mounting face. The sensor can be welded-on or bolt-on, and the mounting pad is selected to suit the surface it is fixed to. Send us a drawing or a photograph of the member you intend to instrument and we will specify the correct format with your quotation.

The sensor itself is specified at 1 to 2 percent of full scale comprehensive precision; built into a system with four gauged legs it typically achieves plus or minus 1 to 3 percent of full scale, depending on how stiff and symmetric the support structure is. A calibrated compression or shear-beam load cell achieves roughly plus or minus 0.02 to 0.05 percent of full scale. The gap is a property of the measuring principle: a strain sensor measures leg deformation rather than the direct force passing through a cell, so it remains sensitive to the stiffness and symmetry of the support structure.

Choose strain sensors when you need continuous content or inventory monitoring and refill triggering, when the vessel cannot easily be lifted or emptied, and when installed cost matters more than measurement uncertainty. Choose load cells when you need trade-grade accuracy, custody transfer, or legally verifiable weighing. The two approaches solve different problems and are frequently combined on the same site.

Yes, and this is one of the most common use cases. Strain sensors are also used as a cheap secondary check on an existing weighing installation, and as a fast replacement for individual weigh modules that have failed or been damaged, since a single sensor can be swapped without taking the vessel out of production.

No, and you should not specify it for that purpose. Applications such as invoicing, buying and selling by weight, or any legally binding measurement require a certified load cell system approved to OIML R76 or the equivalent national regulation. The WST703 is intended for internal inventory control, production monitoring and refill triggering, where a plus or minus 1 to 3 percent indication of contents is sufficient.

Yes, and you should plan for it. Because the output depends on the section, material and orientation of the leg it is fitted to, the system must be calibrated on site after installation using an empty-to-full cycle against a known reference weight or a documented fill quantity. The calibration takes one fill cycle to establish and is normally repeated annually. We supply a written calibration and installation procedure with every order, and our engineers support the commissioning by remote guidance.

The sensor is temperature compensated from minus 20 to plus 65 degrees C, with a temperature effect on zero and on output of plus or minus 0.1 percent of full scale per 10 degrees C, and creep of plus or minus 0.05 percent of full scale per 30 minutes. Residual effects come mainly from the vessel itself: steel with a solar-heated outer face can shift the reading by one to two percent between sun and shade, and powder settles and compacts after a fill event, which can change the apparent mass by a few percent over several hours. A digital low-pass filter with a 60 to 120 second time constant removes both the settling effect and most mechanical vibration.

Sensors are manufactured under an ISO 9001 quality management system and 100 percent inspected before shipment. Each unit is supplied with its own test record so you can verify it against your own reference. Third-party inspection by SGS, Intertek or a buyer-appointed agency can be arranged on request — tell us your inspection requirement when you request the quote.

The sensor is rated in microstrain, not in tonnes, so the question is not how heavy your silo is but how much the leg deforms under that load. Work out the axial stress in one leg, force divided by cross-sectional area, and it should land between about 63 and 210 megapascals, which corresponds to 300 and 1000 microstrain in steel. Below that window the signal becomes small against noise; above it you are outside the rated range. Send us the section and material of your support legs with the empty and full load per leg and our engineers will confirm the fit within 12 hours.

Please confirm six items: vessel type and support arrangement with a drawing if available, number of support legs, the estimated load per leg with the vessel empty and full — or simply the section and material of the legs, and we will calculate the strain for you — the material being handled, indoor or outdoor location with the ambient temperature range, and the required output or control interface. With those details we return a quotation and a model selection within 12 hours.

Trial orders are welcome, so you can start with the quantity needed for a single vessel before scaling to a plant-wide rollout. Lead time is quoted against the confirmed specification and quantity at the time of quotation. Standard export payment terms are available and can be discussed with the quotation.