Calibration World Issue 1-2007 - Beamex - #15

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• Analogous scales have limited

readability. ^ There are random variations in the

indications as can be seen in the

Repeatability Test. ^ The weights are not in the exact

middle of the load receptor.

The values of uncertainty determined at each point of calibration are expressed as standard uncertainties (coverage probability: 68.27 %), which correspond to one standard deviation of a normally distributed variable. The combined standard uncertainty of the error at a certain point of calibration has a coverage probability of 68.27 % as well.

and 10.0039 kg 95.45 % of the time. However, the uncertainty of the results of later routine weighings is usually larger. Typical reasons for this are: •Routine weighing measurements

involve random loads, while

calibration is made at certain

calibration points. •Routine weighing measurements

are not repeated whereas indications

received through calibrations may

be averages of repeated weighing

measurements. •Finer resolution is often used in

calibration. •Loading/unloading cycles in

calibration and routine weighing

may be different. •A load may be situated eccentrically

in routine weighing. •Tare balancing device may be used

in routine weighing. •The temperature, barometric

pressure and relative humidity of

the air may vary. •The adjustment of the weighing

instrument may have changed.

Standard and expanded uncertainties of weighing results are calculated using technical data of the weighing instrument, its calibration results, knowledge of its typical behaviour and knowledge of the conditions of the location where the instrument is used. Defining the uncertainty of weighing results is highly recommended, at least once, for all typical applications and always for critical applications. Calculating the uncertainty of weighing results assists you in deciding whether or not the accuracy of the weighing instrument is sufficient and how often it should be calibrated. However, determining the uncertainty of weighing results is not part of calibration.

Expanded uncertainty in calibration U(E)

In practice, a coverage probability of 68.27 % is insufficient. Normally, it is extended to a level of 95.45 % by multiplying it with the coverage factor k = 2. If the distribution of the indicated error cannot be considered normal, or the reliability of the standard uncertainty value is insufficient, then a larger value should be used for the k-factor.

If you are able to use the k = 2 coverage factor, then the error and its extended uncertainty at the point of calibration are E = 2.5 g and U(E) = ±1.4 g. This means that the calculated error of the indication is 2.5 g and the actual error, with a coverage probability of 95.45 %, is between 1.1 g and 3.9 g.

Uncertainty of a weighing result

The purpose of calibration is to determine how accurate a weighing instrument is. As the above-mentioned case indicates, you know that if you repeat the calibration several times, the indication of weighing an object of 10 kilograms will be between 10.0011 kg

Calibrating and testing weighing instruments using CMX

CMX's scale calibration enables you to uniquely configure calibration and test each weighing instrument. Correspondingly, copying configurations from one scale to another is easy. Error limits can be set according to OIML or Handbook-44. Wide variation in user-specific limits is also possible.

CMX calculates combined standard uncertainty and expanded uncertainty at calibration of the weighing instrument. It allows you to enter additional, user-defined uncertainty components in addition to supported uncertainty components.

CMX's versatile calibration certificate and possibility to define a user specific certificate assure that you can fulfill requirements set for your calibration certificates.

L ±^u(E) -J

^ 68.27%"

L_ U(E) = 2u(E) _ I

I*- 95.45 % -*■

U(E) = 3u(E) K------------------ 99.73 % ------------------*■

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