Accuracy and pourability of gypsum preweighed packages

The purpose of this study was to determine the amount of gypsum provided by the manufacturers in their preweighed packages and to determine the amount of gypsum dispensed from the packages as a function of the method used.

One hundred twenty unopened packages from 8 manufacturers were weighed. Ten packages from each manufacturer were opened with the contents completely removed by tapping the packages, and the contents were weighed (thorough method). The other 5 packages from each manufacturer were opened, quickly poured, and the contents weighed (quick method). All packages were then slit open on all sides and completely cleaned. The cleaned packages were then weighed. Calculations were made to determine (1) the amount of gypsum that was "supplied" by the manufacturer and (2) the amount removed from the packages using the "quick" and "thorough" methods. The raw data was normalized to percentage of stated weight. A Welch analysis of variance was used to evaluate the heteroscedastic data at a 95% confidence level. Thereafter, a post hoc Student t test was used to determine if the percentages of stated weight for (1) supplied, (2) thorough, and (3) quick were significantly different from 100%.

A statistically significant difference was found for the amount of gypsum supplied by the manufacturers (p <.001) and the amount of material removed from the packages via the 2 different methods used (p <.001). Seven of the 8 manufacturers' preweighed packages were remarkably close to their stated weight for amounts "supplied" and the "thorough" method. High standard deviations from the mean were found for the "quick" method of gypsum removal.

Within the limits of this study, results indicate that the use of a "thorough" method to remove gypsum from preweighed packages was important. Ideally, gypsum supplied in preweighed packages should be weighed initially to ensure an optimal water/powder ratio.