Laboratory balances - when smaller weights need to be determined accurately
The laboratory balance is offered in numerous different versions. They range from simple precision balances to analytical balances and microbalances. These balance types differ mainly in their weighing ranges, accuracy and readability. Often these balances are equipped with a data interface through which they can be connected to a computer.
Precision scales are also called fine scales. The weighing result is displayed in numerical increments between 1 mg and 100 mg. They are often used as gold scales when buying gold, silver, jewels or pieces of jewellery. If the scale can be switched from grams to carats, it is also called a carat scale. In any case the gold scale must be calibrated. However, the calibration is only valid for one specific place of installation. If the balance is to be used at different locations, it must have a built-in calibration weight. This allows it to be recalibrated at any location. Even when the precision balance is used as a dental balance, it must be calibrated. In pharmacies, laboratory balances are also used as legal-for-trade formulation balances. Their weighing range is between two and six kilograms, and their resolution is 0.01 or 0.1 grams.
The characteristics of an analytical balance
Analytical balances are laboratory balances with a resolution of 0.1 mg. They are available as verified or unverified balances. With this resolution, weighing is very sensitive to interference. Therefore, these balances must be placed on a damping surface and the weighing unit must be equipped with a draft shield. Analytical balances are mainly used for quantitative chemical analyses. They are suitable for weights of a few hundred grams maximum.
This characterizes a microbalance
The microbalance is even more accurate and sensitive than the analytical balance. Its resolution is 0.001 mg = 1 µg or less. They are intended for determining weights up to a maximum of 50 g. These highly sensitive balances are used for chemical analyses, also in basic research.
A transparent protective cover protects the sample from disturbances from the environment, such as drafts.
An ionizer is used to neutralize the electrostatic charge. This charge would falsify the measuring results in the mg or µg range. A special weighing table protects the balance from shocks and vibrations by absorbing them. Otherwise they would influence the measurement result.
A calibration weight equal to the minimum sample quantity. This is the smallest weight to be weighed at the respective desired accuracy.
For density determination of solids and liquids, corresponding density determination sets are offered as supplements to the balance.
An RS-232 data interface can be retrofitted to some models of the laboratory balance if the balance is not directly equipped with one.
Computers that are compatible with the laboratory balances are connected to the balance via an interface port.
Why are calibration weights required?
A company that wants to be certified according to ISO 9001, 9002 or 9003 must regularly check the balances used in its operations. For this purpose, they must be tested with calibration weights and, if necessary, readjusted. This ensures the operational readiness, the reliability and the measuring accuracy of the scales. These calibration weights are made of stainless steel, brass, nickel-plated brass or cast iron. They also differ in the accuracy class for which they are approved. This depends on the display accuracy and the weighing range. With the help of these calibration weights, an employee of the company who is technically competent in the field of test engineering for laboratory balances can perform the test himself. He must document this completely. Users must be aware, however, that such a calibration does not constitute a verification of the laboratory balance. Calibration is official confirmation that the balance complies with the error limits of the specified accuracy class.
For simple precision balances this is accuracy class II, for analytical balances accuracy class I. When calibrated balances are used, there is no need for calibration for ISO certification. However, a distinction must be made between verified and legal-for-trade weighing instruments. Verifiable laboratory balances are designed in such a way that verification is possible. However, it must first be carried out.