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The majority of tank and silo weighing applications use traditional load cells as the measuring transducer to convert load to electrical output. However for many applications, especially those of high capacity and where the silo is already installed, load cells can be expensive and difficult (or even impossible) to fit. Even if load cells can be fitted, significant engineering is required to ensure system integrity. In these situations, the two alternatives for the user are level measurement or retrofit sensor systems. Although level measurement is typically cost effective and easy to fit, for certain applications the accuracy provided may not be satisfactory. Level control systems do not,for example, provide good accuracy for materials which do not sit level in the silo or where bulk densities change depending on the level.

Retrofit sensors provide a useful non intrusive alternative to level measurement and operate by converting the leg or suitable support member of a tank or silo into a large measuring sensor. Depending on their design, sensors may be bolted, welded, bonded, cast or press-fitted to the support member in order to measure the change in strain as the load on the member changes. Because large vessels are designed to be stiff structures, any legs or support structures usually provide very small changes in strain and therefore care is needed in fitting these type of sensors.

The performance of retrofit sensor systems is dependent on

  1. The magnitude of the strain change over the measuring range
  2. The repeatability of the deflection of the support member to which the sensor is fitted
  3. The effectiveness of the positioning and mounting of the sensors
  4. The effectiveness of the compensation for temperature effects
  5. The number of sensors used
  6. The method of calibration used

Recently, progress has been made in the use of sensors which can be fitted to concrete legs and this opens up new applications for retrofit sensors.

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