Calibration World Issue 1-2007 - Beamex - #14

Calibrating weighing instume

Any scale is capable of producing incorrect results.

^reighing is a common form of measurement in commerce, industries and households. Weighing instruments are often highly accurate, but users, i.e. their customers and/or regulatory bodies, often need to know just how inaccurate a particular scale may be. Originally, this information was obtained by classifying and verifying the equipment for type approval. Subsequently, the equipment was tested or calibrated on a regular basis.

Typical calibration procedures

Calibrating scales involves several different procedures depending on national- and/or industry-specific guidelines or regulations, or on the potential consequences of erroneous

weighing results. One clear and thorough guide is the EA-10/18, Guidelines on the Calibration of Non-automatic Weighing Instruments, which was prepared by the European Co-operation for Accreditation, and published by the European Collaboration in Measurement and Standards (euromet).

Typical scale calibration involves weighing various standard weights in three separate tests:

•repeatability test

•eccentricity test

•"weighing test (test for errors of indication)

In the pharmaceutical industry in the United States, tests for determining minimum weighing capability are also performed.

Repeated weighing measurements provide different indications

Usually, the object being weighed is placed on the load receptor and the weighing result is read only once. If you weigh the object repeatedly, you will notice slight, random variation in the indications. The Repeatability Test involves weighing an obj ect several times to determine the repeatability of the scale used.

Center of gravity matters

Test for errors in indication

The Weighing Test examines the error of the indication on the scale for several predefined loads. This enables you to correct the errors and definitions for non-linearity and hysteresis.

If the scale's maximum load limit is extremely large, it may be impractical to use standard weights for calibrating the entire range. In such a case, suitable substitution mass is used instead. Substitution mass should also be used if the construction of the scale does not allow the use of standard weights.

A truck scale is unsuitable for weighing letters

The purpose of the Minimum Weight Test is to determine the minimum weight, which can be assuredly and accurately measured using the scale in question. This condition is met if the measurement error is less than 0.1% of the weight, with a probability of 99.73%.

Combined standard uncertainty of the error u(E)

Knowing the error of the scale indication at the point of each calibration is not sufficient. You must also know how certain you can be about the error found at each point of calibration. There are several sources of uncertainty of the error, e.g.:

^ The masses of the weights are only known with a certain uncertainty. ^ Air convection causes extra force

on the load receptor. ^ Air buoyancy around the weights varies according to barometric pressure, air temperature and humidity. ^ A substitute load is used in

calibrating the scale. ^ Digital scale indications are rounded to the resolution in use.

The Eccentricity Test involves placing the object being weighed in the middle of the load receptor as accurately as possible. This is sometimes difficult due to the shape or construction of the object being weighed. Typical calibration procedures include the Eccentricity Test. You can determine how much the eccentricity of the load will affect the indication on the scale by weighing the same weight at the corners of the load receptor.

CALIBRATION WORLD 01- 2007 www.beamex.com/calibrationworld