WEIGHING SYSTEMS AND ERRORS: A DEEPER VIEW

As you know, every type of measurement is achieved thanks to interconnected devices with the specific task of recording measures. Each system has a specific measurement accuracy, which also includes total error, which in turn contains various types of errors generated both by devices and the installation characteristics of devices. In other words, the smallest total error is, the more accurate measurement will be. Let’s see in detail which errors can affect measurement systems.

 

ERRORS IN WEIGHING SYSTEMS

The first distinction is between systematic errors and random errors. While systematic errors are predictable, quantifiable and can be summed in an algebraic way, random errors are calculated in a statistical way. In order to minimize errors, it is necessary to establish the performances of the system defining two essential data:

 

SYSTEMATIC ERRORS

As previously said, systematic errors can be summed in an algebraic way in absolute terms creating total systematic error. In measurement devices, errors are expressed as a percentage related to the specific full scale, therefore in systems with many cells, percentage values don’t change.

Among systematic values, there are:

Error= (rated output tolerance / 100) * (barycentre shift % / 100) * full scale capacity

 

RANDOM ERRORS

Total random error is calculated through the square root extraction of the sum of the squares of each error. Due to their nature, random errors must always be considered in every application, because they can’t be compensated. The calculation refers to absolute values, and therefore it is related to total full scale.     

Among random errors:

 

ERRORS DUE TO THERMAL DRIFTS

As systematic errors, also errors due to thermal drifts are calculated by the algebraic sum of single errors. Environmental operating conditions vary from system to system, therefore the error must be calculated for the effective variation of temperature in relation to the expected temperature for calibration.

In the long term, room temperature variations can be compensated with seasonal calibrations, reducing the values that have to be used for the calculation of the error due to thermal drifts. Moreover, errors due to thermal drifts produce:

Devices that can generate errors due to thermal drift are:

  1. Load cells
  2. Cell signal booster
  3. A/D converter
  4. Cell power cables
  5. Thermal expansions of the structure (they have to be considered only in very heavy or long structures or with high differential temperatures)
  6. Cell supply voltage (in modern microprocessor systems the error is zero)
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