The influence of variations in atmospheric pressure on the uncertainty budget of weighing results

The various assays in quantitative analytical chemistry always include mass determinations, along with other operations, and the same is true for the uncertainty budget of such a procedure. In order to obtain an accurate weig hing result it is necessary to consider air buoyancy, which depends on, bes ides other parameters, atmospheric pressure. In contrast to air temperature and humidity, it is not possible to control the atmospheric pressure in a conventional laboratory. Therefore, it can be necessary to consider the unc ertainty of pressure in uncertainty calculations of weighing operations, de pending on the mass and density of the weighed object. The theoretical rela tionship between pressure variations and obtained weight value are derived. Experimental confirmation was obtained on the one hand by the mass determi nation of five Pyrex glass bottles over a period of 6 months; these are obj ects with a lower density than the reference weights of the balance, thus s howing a negative dependence of weight value on atmospheric pressure. On th e other hand, a tungsten cylinder was weighed over several weeks; this high -density object has a positive dependence. In both cases the experimental s lopes agreed with the theoretical values. The consequences on the uncertain ty budget of a weighing operation are remarkable: the total uncertainty of the mass determination of a 90 g glass bottle is 0.3 mg whereas it is 4 mu g for a 4.6 g tungsten cylinder. The contributions originating from the pre ssure variation alone are 0.3 mg and 3.5 mu g, respectively, i.e., the pres sure uncertainty dominates and is much more important than the uncertainty contributions of balance repeatability and linearity.