The electronic precision balance (EPB), also called a top-loading balance, offers accurate weight measurements in a variety of environmental conditions as they are relatively unaffected by temperature or air fluctuations. EPBs are ideal choices for daily weighing in a wide variety of labs including biomedical labs, educational chemistry and biology labs, and research and quality control labs in pharmaceutical, chemical, food, and textile industries. EPBs are available in a range of maximum capacities to suit the particular needs of the user from >100g to >1000g with a readability of 0.001g (or 3 decimal places). Modern EPBs are equipped with sophisticated weighing pans that minimize the effects of environmental conditions and off-center, corner loading of the sample.
The electronic analytical balance (EAB) offers accurate weight measurements of small masses at high precision in a controlled environment (e.g., draft shield) since temperature, air, vibration, and humidity affect EABs. EABs are ideal choices for precise weighing in select labs involved in standardized or government-regulated product preparation, complicated formulation, differential weighing, density determination, and pipette testing. EABs are also available in a variety of maximum capacities from >50g to ~500g with a readability of 0.0001g (or 4 decimal places). EABs require frequent calibration and regular monitoring to assure accurate weight measurements. Modern EABs are also equipped with sophisticated weighing pans along with internal calibration weights.
Both EPBs and EABs require calibration to guarantee accurate weight measurements 1, 2. Calibration is a process that establishes how a balance behaves by comparing balance readings to known reference weights. An essential component of the calibration process is to establish the inherent measurement uncertainty (MU) of the balance. The MU is a quantitative indication of the quality of a weight measurement. For example, if a balance reads 100g and has a measurement uncertainty of 0.01g, then the weight measurement lies between 99.99g and 100.01g with 95% confidence. Factors that contribute to MU include the balance itself, weight itself, rounding errors of a no-load reading, rounding errors of a load reading, repeatability, eccentricity (off-center position of the sample), air buoyancy, drift over time, and convection. The MU of a balance can only be determined by a calibration using international standards by a qualified technician. After the balance has been calibrated and the MU established, the question of whether the balance behaves well enough for your lab requirements involves tolerances which may be legal tolerances, manufacturer tolerances, or process tolerances.
The % Tolerance is calculated as: the: acceptable variation ÷ target weight value X 100.
For example, if the acceptable variation =2g and the target weight value =100g, then the tolerance =2% and a weight measurement of 98g to 102g is acceptable. The MU of the balance should always be LESS THAN the tolerance. So, if the balance has a MU =0.01g and the tolerance = 2% (i.e., 2g), then the accuracy of your weight measurement will not be affected by balance issues since 0.01g <2g. A properly calibrated balance and periodic user testing (e.g., sensitivity and repeatability tests) between calibrations guarantee consistent accurate weight measurements in your lab.
References:
1. Ioana Popa-Burke et al., J of Biomolecular Screening, 18(3): 331-340, 2013.
2. Jens ET Andersen, J of AOAC International 101 (6): 1977-84, 2018.