Weight force measurements on stiff structures

The strain transducers are particularly suitable for retrofitting with sensors for weight measurement or level determination

Load measurement with strain sensors

Weight measurement using load cells has become established in silo weighing as an accurate and reliable measurement method for level determination. However, retrofitting existing silos involves a great deal of construction work, as the silo usually has to be raised for installation.

As an alternative, strain sensors are available which can simply be applied to the supporting silo feet. The strain transducers are screwed onto the silo feet in a force-fit manner. Depending on the required accuracy, one or two strain sensors can be mounted per foot.

Thus the strains, which are fully triggered by the weight change in the silo structure, are measured.

One strain transducer per silo foot is sufficient for simple weight determinations. For higher accuracy requirements and temperature compensation, it is recommended to install two extensometers per silo foot. The achievable accuracy depends on the silo structure and the existing strains. The measurement of the weight via strain transducers is particularly suitable for the following applications:

  • Limit value monitoring of the filling level
  • Truck preloading for bulk materials

The use of strain sensors for weight determination brings the following advantages:

  • Simple and cost-effective installation on existing plants, also during operation
  • Separation of the measurement technology from the silo interior in the case of corrosive media
  • Reliable detection of moisture differences, voids and bulk cones compared to microwave and ultrasound

However, due to the structural conditions of silos, it is important to keep an eye on the environmental conditions. For example, one-sided temperature irradiation can cause expansion of the silo structures and thus cause a measurement error. Wind conditions also have a direct effect on the stresses in the silo structure. However, both wind conditions and temperature influences due to solar radiation do not have the same effect on every silo leg, or strain transducer. Thus, these influences can be actively compensated by offsetting the individual sensor signals. Alternatively, passive compensation can be performed by mounting two strain transducers on each leg in a T-alignment.

The strain transducers can be applied in a T orientation to provide passive compensation for temperature. By connecting such a pair of strain transducers in parallel, a temperature change can be compensated almost completely, since it always affects both sensors equally. A change in weight, on the other hand, only affects the strain transducer in the longitudinal direction, which means that signal distortions can be reliably detected and compensated.