Cartesian Diver as a Density Comparator

A hydrostat ic weighing system is described t hat provicies inciepenci (' nt valu ('s of apparent mass with a standard deviat ion of a bou t 0.2 microgra m. The ciefi ni t ion of " inciep(, IHI<'n t" used here includes the requirement t hat t he sample under observat ion be removed from Lhe water, dried, and recleaned between measurements. This precis ion is between one a nd two orders better than ex ist ing hydros tatic balances, a nd permits high-quali ty density measLU(,ments without r e course to large sample s izes . The system is used as a co mparator , hen ce t he absolute acc uracy of results can be no better t han t hat of t he standards used . Dntn are prese nted which were taken during experiments on two 2-gram pieces of s ingle cry s tal s ilicon, a nd the s tandard deviat ion ill dens ity of a s ingle determina tion was computed to be 4.4 X IO7 g/cm3 (0.22 ppm). The system is particularly well suited to detection of slight dens ity changes in small s ize sampl e'S. In this ser v ice, it is not subject to t he r eq uirement for high acc uracy standards. Data arc reportC'd on dens ity cha nges of abou t 50 ppm that occlllTed in a 250 m illi gram ceramic crysLal. Two independe nt determinat ions on t his change differed by only 7 percent.