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White dwarfs with helium-dominated atmospheres comprise approximately 20% ofall white dwarfs. Among the open questions are the total masses and the originof the hydrogen traces observed in a large number and the nature of the deficitof DBs in the range from 30000 - 45000K. We use the largest-ever sample (by afactor of 10) provided by the Sloan Digital Sky Survey (SDSS) to study thesequestions. The photometric and spectroscopic data of 1107 helium-rich objectsfrom the SDSS are analyzed using theoretical model atmospheres. Along with theeffective temperature and surface gravity, we also determine hydrogen andcalcium abundances or upper limits for all objects. The atmosphere models areextended with envelope calculations to determine the extent of the heliumconvection zones and thus the total amount of hydrogen and calcium present.When accounting for problems in determining surface gravities at low Teff, wefind an average mass for helium-dominated white dwarfs of 0.606+-0.004 Msun,which is very similar to the latest determinations for DAs. There are 32% ofthe sample with detected hydrogen, but this increases to 75% if only theobjects with the highest signal-to-noise ratios are considered. In addition,10-12% show traces of calcium, which must come from an external source. Theinterstellar medium (ISM) is ruled out by the fact that all polluted objectsshow a Ca/H ratio that is much larger than solar. We also present argumentsthat demonstrate that the hydrogen is very likely not accreted from the ISM butis the result of convective mixing of a residual thin hydrogen layer with thedeveloping helium convection zone. It is very important to carefully considerthe bias from observational selection effects when drawing these conclusions.

DB white dwarfs in the Sloan Digital Sky Survey Data Release 10 and 12