Weighing is one of the most commonly performed tasks in a laboratory. There is increasing attention on good weighing practices in labs. Therefore, focus on the accuracy and suitability of core testing procedures, equipment used, and the various elements that influence the weighing process, is crucial. Due attention to the following factors can help ensure good weighing practices:
- Weighing Equipment and Processes: Use a good quality balance. Leading lab equipment manufacturers supply advanced micro, semi-micro, analytical and precision balances that provide a high degree of accuracy and need no special weighing environment. Electronic balances have simplified the weighing operation and greatly reduced weighing times. Take Ohaus, for instance. They offer a comprehensive range of precision, portable and analytical balances that can meet any laboratory weighing requirement. Applications include pill counting, formulation, differential weighing, density determination, pipette calibration, high point weighing, moisture determination, totalization, dynamic weighing, below balance weighing, check weighing, percent weighing, and more.
- Handling Weights: The manner in which standard objects weighed are handled can have a significant impact on their masses. Always wear gloves or use forceps when handling small weights. If the objects to be weighed are touched by human hands, it can leave grease or oily films that will affect the mass and result in incorrect results when weighed.
Care must be taken when weighing objects or magnets with electrostatic charges as the presence of the magnetic field tend to influence the results. This can be prevented by placing the magnetic material to be weighed in a magnetically shielded container.
- Operation: As analytical balances are high precision instruments, they should be operated carefully. Careful operation will provide consistency in repeated measurements. All the weights should be placed gently in the center of the weighing pan/platform and the dials of the balance should be turned cautiously. Care should be taken not to shock load the balance.
- Comparison Weighing/Mass Calibrations: To eliminate the errors of the built-in weights, the unknown object needs to be compared to a known mass standard. Calibrating a balance is a regulatory requirement for laboratories looking for compliance with Good Laboratory Practices/Good Manufacturing Practices (GLP/GMP) at the national and international level. In comparison weighing on a full electronic balance, balance indications are used to calculate the mass difference between the standard and the unknown object. However, as there are no built-in weights in the full electronic balance, the full range of the digital indications can be taken into account for “optical scale” of the balance.
Factors that can Influence Weighing Results
- Vibration: Install the balance in a stable place as any kind of vibration can affect the results
- Temperature: Keep the temperature of the room as stable as possible. Ideally, relative atmospheric humidity should be between 45% and 60%
- Sunlight: See that the balance is not exposed to direct sunlight