Accuracy is a prime if not the prime criterion for analytical balances used in industries such as researching and compounding pharmaceuticals. Analytical balance weighing accuracy is a measure of how close the displayed weight is to the actual weight of samples on the weighing pan.
Two terms apply here. The first is readout, which is the smallest difference in weight that can be read and displayed by the unit. For an analytical balance it is typically 0.1 milligram or 0.001 g. Micro and ultra microbalances can weigh lighter samples.
Accuracy is sometimes confused with precision but it is an incorrect assumption. Precise means that results are consistent. However they may be consistently wrong, and therefore not accurate.
Instead, the second term or criterion to look for is linearity. Linearity is the maximum ± deviation of the displayed sample weight vs. the actual sample weight over the weighing range of the instrument. This recognizes that no scale is perfectly accurate over its weighing range. An analytical balance with readout of 0.1 milligram may have a linearity of ±0.3 mg.
Are these specifications acceptable? It is up to you – and the regulatory agencies that relate to your work. What this post looks at are factors that can throw off the most accurate, precise analytical balance – factors that are generally under the control of the technician.
Operator-Influenced Analytical Balance Weighing Errors
Operator instructions for analytical balances such as the Kern ABT series available from Tovatech are quite explicit on setting up and using the equipment. Deviations from these recommendations will affect accuracy.
Locate the balance on a firm, level surface in an area with minimal exposure to changes in temperature, humidity and vibration. These ambient conditions will affect weighing accuracy, either impacting the balance or the sample. For example, sample weight can change due to absorption of or giving off moisture. Place samples in a closed container inside a desiccator if they must acclimate to the balance temperature. It’s also important to avoid drafts – which is why analytical balances are enclosed with shields.
Once the balance is in place it must be calibrated. This is because the effect of gravity on samples (acceleration) differs by geographic location. Calibration can be done on site either using a calibration weight or in the case of the ABT series, by activating the calibration program. Recalibration, either manual or auto, should be done if the analytical balance is moved, if there is an unusual change in ambient conditions, or on a regular basis governed by the lab’s standard operating procedures.
Note that the ABT series automatically recalibrates itself under four conditions: (1) a 5⁰C change in environmental temperature, (2) after 4 hours since the previous calibration, (3) when the balance is switched from standby status to weighing mode and condition (1) or (2) has been met and (4) if the balance was disconnected from the power source.
For more on this important topic see our post on calibration.
Electrostatic charge, especially when weighing dry non-conductive samples, can cause weighing errors. The electromagnetic field between the items and the balance can cause the weighing result to be distorted by up to a gram. A discharge ionizer positioned next to the balance solves the problem when weighing in milligrams or smaller increments. Note that an unstable power supply can cause similar weighing errors. This necessitates relocating the balance to a stable power supply.
Liquids entering the weighing mechanism could render the balance useless. First tare a container then carefully add the liquid to the container little by little while it is on the balance until the desired weight is reached. When cleaning the balance use a dry or barely damp cloth followed by a dry cloth. Avoid using solvents. Immediately remove spilled solids with a soft brush or gentle vacuuming.
These suggestions are part of an overall good practices regime that should be adopted by any organization involved in using analytical balances. A thorough reading and understanding of operation manuals that accompany the equipment is highly recommended.
Contact the Tovatech PhDs for help in selecting an analytical balance that matches your requirement or for assistance in developing your operating procedures.