Testing Your Laboratory Balance (Page 6 of 6) in pdf

Span

Span refers to the adjustment of the sensitivity of the instrument across the full weighing

range

Span differs from the previous performance parameters in that it is readily adjustable,

whereas cornerload, linearity, and repeatability generally are not. Span adjustment of

instruments is different from instrument to instrument. Generally, the adjustment procedure is

described in the user's manual which comes with the instrument. Follow the instructions in

your user's guide.

Many (not all) instruments now include internal calibration weights, so calibration is as easy

as pushing a single button. The user may reasonably ask, "How do I know the internal

calibration weight is correct ?" The answer is that the only way to know is to have an external

standard for comparison. The advantage of an external standard, whether it's used to adjust

span, or just to confirm the internal weight, is that it is completely external to the instrument,

and can therefore be compared to external standards. By that process, the overall calibration

of the instrument is matched to international standards and all other weighing instruments.

Standardized test weights are made to various levels of accuracy. ASTM class 1 is the most

accurate weight class commonly available. Most weights in ASTM class 1 are accurate to one

part in 400,000. Since many lab balances are considerably more accurate (resolution of

greater than one part in a million), one might reasonably wonder how a standard weight can

be used to test or adjust analytical balances. The answer would be that the standard weight can

itself be calibrated to a level of accuracy that exceeds even the most accurate lab balances.

The characterization of the weight will determine the actual value of the weight to a much

higher degree of precision than required by the ASTM class 1 standard. Such a weight can be

used to verify the accuracy of an internal calibration weight. Simple "traceability" of an

external weight is insufficient. A calibration certificate for a class 1 weight must specify the

actual weight value with deviation from nominal. A calibration service which supplies such a

certificate should be registered (via A2LA, or similar) to provide that level of accuracy. The

registration confirms via third party testing that the calibration certificate issuer is capable of

such accuracy and is audited to be delivering that accuracy reliably.

After placing a test weight on an operating instrument and finding that the displayed weight

value does not exactly match the nominal value of the test weight, many users have concluded

that the instrument is miscalibrated. However, that conclusion is by no means certain unless

the test weight has been calibrated and its correction from nominal value is known. The

calibration weights internal to high quality lab balances are more accurate than commonly

available test weights. In the absence of a calibration certificate for a specific weight, users

should presume that internal weights are more accurate than external test weights.

IES Corporation

Repair of Laboratory Balances

(800) 541-0852

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Testing Your Laboratory Balance Page 6