Analytical balances and microbalances are highly sensitive instruments essential for accurately measuring small masses. Analytical balances measure small masses ranging from 0.1mg to 200 g. Microbalances can measure masses as small as 0.1 µg and are used for measuring particulates, calibrating pipettes, and any other application requiring precise measurement of very small masses. Because of the sensitivity of these balances, they are susceptible to environmental disturbances that can affect the accuracy of measurements. There are several considerations to minimize these disturbances, including balance placement, user behaviors, and stabilizing equipment, such as mounting plates and weighing tables.
Balance Placement
Select a low-traffic area away from windows or other sources of air movement, heat, loud noise, or humidity. Additionally, avoid areas near other equipment that may cause vibrations, such as centrifuges, or temperature fluctuations, such as near lab ovens. Select a quiet environment for the most sensitive precision balances, as acoustic disturbances can also affect accuracy. When setting up the balance, make sure it is level. Most balances include a built-in level and leveling legs to achieve this.
Weighing Tables
In addition to balance placement, there are a few options to improve the accuracy and consistency of analytical and microbalances. Damping mounts, generally composed of marble or granite, can be used on top of laboratory benches to minimize disturbances. However, a dedicated weighing table will offer better vibration isolation. Weighing tables are equipped with shock absorbers and solid surfaces, often made of granite, to combat both ground vibrations and those caused by the user. Granite is also selected as it is resistant to rapid temperature changes that can impact balance accuracy. Weighing tables should be placed where the table is not touching any adjacent walls, equipment, or benches. The table should not be used for other lab equipment or activities, including another analytical balance or microbalance.
The measurement interval on a microbalance is less than a single microgram. So, what degree of resolution are we looking to achieve with a weighing table? Whether constructed of both a stone surface and frame or a steel frame with a solid surface, weighing tables utilize shock absorbers within the frame to absorb more than 90% of ground movement. The degree of accuracy needed will depend on the application, but in general, accuracy deviating no more than 0.1% of the mass is ideal. Weighing tables composed entirely of stone will be much heavier and offer superior vibration damping. A table constructed entirely of stone may be preferable over a steel-framed balance table for microbalances used for calibration purposes or other highly sensitive applications.
User Best Practices
The user should take care not to touch the table during measurement. Shaking, bumping, or leaning on the table or counter can dramatically impact measurement accuracy. Always wear clean gloves when measuring, as fingerprints are hygroscopic and will affect measurement accuracy. Dust and debris within or on the balance will also affect accuracy, so users should keep the balance clean. Also, be careful when measuring any liquids, as any moisture that enters the balance may impact measurements. Select a weighing container for your material that is not overly large as this may interfere with balance accuracy.
