Аннотация:After the installation of the GUV-2511 instrument in February 2015 at Stara Zagora, the instrument is operating continuously and measures the sky Solar irradiance at different wavelengths in the spectral range from 305 nm to 395 nm. In the past years the total ozone content (TOC) was determined by Stamnes look up tables (LUT), in which were modelled the ratios of irradiances for the 313 nm and 340 nm spectral bands for the Stara Zagora location as function of the zenith angle, the cloud optical depths and a mean albedo of 0.03 for different TOC, using the Tropospheric Ultraviolet and Visible (TUV) model version 4.1. developed by Madronich (1993). The instrument was calibrated versus satellite OMI-Aura ozone values by a shift of the GUV 2511 313 nm filter central wavelength to 313.7 nm. For the calibration ozone time series from 2015/2016 was used minimizing the standard ozone deviation in comparison to the OMI-AURA ozone values. The main goal of the present study is to check the long-time stability of the original calibration based on the examination of the mean biases, their ratios and the regression coefficients in two control groups. One group from 2015 to 2018 and the other one from 2019 to 2022. It was found that the absolute bias increases slightly by 1.8 DU. The increase is significant at the level of 0.05. However, the parameters of the linear regressions are not statistically different. The results let us conclude that the stability in time is acceptable.