Dobson spectrophotometer 83 was established in 1962 as a standard for total ozone measurements in the United States. In 1980 the instrument was designated by the World Meterological Organization (WMO) as the primary standard Dobson spectrophotometer for the world. Since the early 1960s, virtually all (~90) Dobson instruments in the global Dobson instrument network have been calibrated several times, either directly or indirectly, relative to instrument 83. Calibrations of instrument 83 by the Langley method were performed in 1962 at Sterling, Virginia, and during 1972--1987 at Mauna Loa Observatory (MLO), Hawaii. A detailed analysis of these calibration data, as well as calibration data based on standard lamp measurements made since 1962, indicates that the long-term (25-year) ozone measurement precision for the instrument is known to within a uncertainty of ¿0.5%. On an absolute scale, the ozone measurements made at MLO with instrument 83 are estimated to be too low by about 2.4%, as a result of errors in the A and D wavelength ozone absorption coefficients used with the instrument and their temperature dependence. Cumulative other biases most likely do not exceed ¿1%. This documentation of the calibration history of instrument 83 lends credence to the accuracy with which other Dobson instruments have been calibrated in the past and to Dobson and satellite instrument ozone measurement comparisons, thereby increasing confidence in ozone trends determined by these ozone measurement systems in recent years. ¿ American Geophysical Union 1989