Strong emphasis is placed on the importance of calibrating analytical balances in laboratories, universities and other areas where accurate and precise weighing to 0.1 or 0.01 mg is required. Even though a balance has been calibrated by the manufacturer it must be recalibrated locally.
Why is this so crucial? The results displayed on the balance are impacted by gravity. If Earth were a perfect sphere of uniform density, then gravity would be consistent. But it’s not, which means gravity varies wherever you go. While a person’s weight may not noticeably vary whether in Japan, Germany, Australia or North America, when weighing in milligrams gravity does impact results. And that explains the title of this post.
A Calibration Point Proved by the Kern Gnome
Kern & Sohn, manufacturers of analytical balances and digital scales offered by Tovatech, conducted an interesting experiment a few years ago by shipping a little fellow called the Kern Gnome* to many places throughout the world and noting his change in weight on a scale that had been calibrated in Germany. Some examples: 309.82 g in Antarctica, 308.58 in the UK and 307.9 in Japan.
Other Factors Affecting Analytical Balance Accuracy
Location is the prime factor, which is why instruction manuals shipped with precision scales and analytical balances insist that the instruments should be calibrated on receipt once they are placed in their “permanent” location. Permanent is in quotes because realistically these instruments may be relocated in a lab or moved to another location in the company. In such instances they must be recalibrated.
Recalibration should also take place if there are environmental changes and as a practice periodically at regularly scheduled intervals.
Scale Calibration and Recalibration Options
A purchasing decision is to select an analytical balance that is either externally or internally calibrated. External scale calibration is performed using calibration test weights, themselves precision manufactured and generally selected based on the capacity and resolution of the instrument. For example, the externally calibrated Kern AES 100-4C analytical balance with a maximum weight range of 160 g should use a separately purchased 150 g test weight. The 220 g capacity AES 200-4C should use a 200 g test weight.
External calibration procedures are relatively easy to perform and directions are provided in the analytical balance instruction manual. For more on this topic see our post on test weights.
Internally calibrated analytical balances, which utilize a motorized weight inside the balance, are exemplified by the companion Kern AEJ models that allow weighing accuracy checks at any time simply by pressing the CAL key on the control panel. Automatic recalibration is also initiated
- If the balance is disconnected from its power supply
- If there is a temperature fluctuation > 0.8⁰C
- At a specified time interval
Setup and calibrating procedures are spelled out in the operations manual.
Externally calibrated analytical balances are less costly than internally calibrated models with comparable performance parameters. The test weights come at a cost depending on weight class defined by the International Organization of Legal Metrology accuracy classes E1 to M3 described in our linked post above.
In addition, manually calibrating scales, while relatively easy to perform, does divert staff time away from other duties. Outside services can be hired to do this, but also at a cost.
For help in deciding analytical balances best suited to your operations and for further discussions on scale calibration for accuracy please contact the scientists at Tovatech.
*Google the Kern Gnome for more on this experiment.