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Analytical balances, the modern "pharmacist's balances"

An analytical balance is designed to weigh small quantities (0.1 mg) precisely and is therefore also known as a precision balance. The predecessors were automatic beam balances. Weights of 200 grams were suspended on both sides, and fractions of this mass were lifted off one side. To do this, a locking device on the beam was released. The decisive factor for the weighing process was the calibrated steel spring, via which a small force was applied to the beam. After that, the weight could be read easily. However, the beam scales had to be recalibrated or calibrated regularly. For alignment, like a spirit level, there was a bubble level. Cord plumb bobs sometimes replaced the spirit level. The measuring results were often sensitively disturbed by vibrations or temperature fluctuations. For this reason, the scales were placed on tables which had a special damping system for the vibrations. This mass-spring system had to withstand unusually low building vibrations. Weighing rooms had good insulation, were stably air-conditioned and had no windows.

Nowadays, the mass generates a torque which is then compensated by an electromagnetic force. The measurement is then very precise and fast. Nevertheless, struggled weighing tables are needed today. In order to obtain precise values in the laboratory, for example, it is necessary to protect the scales from contamination. The sample containers are completely dried in a so-called desiccator before the weighing process to ensure accurate results.

Applications in industry

With monolithic load cells, higher precision, speed as well as better reproducibility can be achieved. These are load cells that measure sensors made from a block of aluminum. Single-cell weighing systems with draft shields are becoming increasingly popular. Higher measuring speed, stable temperature behavior, more accurate readings, higher corner load stability and greater mechanical ruggedness characterize single-cell weighing systems. However, the following must be taken into account with this technology. Daily adjustment, especially in the event of temperature fluctuations. Here, the internal automatic calibration/adjustment feature would be recommended, as would the avoidance of air movements, the use of an ionizer to neutralize electrostatic charges on the sample. The use of a special weighing table makes the work easier. Some analytical balances have a pipette calibration program and an associated alibi memory.

In addition, the new semi-microbalances are true all-rounders among analytical balances. With semi-micro weighing ranges and a readability of 0.0001 g, there are no longer any limits to applications. Faster response times and more stable reproducibility are given under less than ideal conditions. The convenient keypad, interval timer, fully automatic data transfer, piece counting function are also among the other practical features. Both simple adjustment at the touch of a button and fully automatic adjustment are possible.

Analytical balances feature a heating chamber that can be heated up to 199 °C with halogen lamps so that moisture contents can also be measured. The sample is dried in this chamber and the moisture content is then determined from the difference in weight.

In some analytical balances used in laboratories, production or quality control, the lower part is made of an aluminum pressure housing and the upper part is made of plastic.

Some analytical balances have an animal weighing program, a recipe function and a counting function. The weighing units can be freely programmed. Step-by-step operation allows easy handling.

Anti-vibration tables can also be ordered. These consist of two tables placed one inside the other, one of which has a marble plate. The inertia of this plate prevents vibrations.

What should I bear in mind when weighing with the analytical balance?

For density determinations, the results are displayed with 4 or 5 decimal places. Air bubbles with a diameter of 1 mm cause a buoyancy of 0.5 mg. Therefore, no air bubbles should adhere to a solid or sink in a liquid. An oily surface causes air bubbles, which is why solid samples must be degreased. If a solid specimen is immersed in a liquid, the liquid will rise. Therefore, the suspension of the sieve pan is submerged. This causes the weight of the sample to become lighter. Samples that change in volume cannot be measured. Liquid absorbing samples cannot be weighed either. For the density determination of solids, auxiliary liquids are used. Therefore, temperature has an effect and density changes in the range of 0.1 to 1 per mille per °C occur. If the suspension of the dishes with the samples penetrates the liquid surface, the surface tension worsens the reproducibility. If a little detergent is added, this again increases reproducibility. Sinkers are used to determine the density of liquids. The larger this is, the greater the accuracy.

Drift of the balance display

The final result of the density determination is not influenced by the drift of the analytical balance. Very accurate readings are required when determining the density of liquids with a sinker. The balance must be readjusted when the ambient temperature changes or the location is changed. The density set must be removed and readjustment performed with the standard weighing plate. Standard weighing plates are made of stainless steel, which makes cleaning much easier.