The sensitivity of the COS FUV detectors declines with time and is monitored with regular observations of spectrophotometric standard stars (white dwarfs). An increase in the slope of the Time Dependent Sensitivity (TDS) has been observed since October 2013. The slope of the TDS depends on grating, central wavelength, and detector segment, and currently varies between ~1% per year and ~5% per year, with higher slopes at longer wavelengths, and with the Segment B channel showing a shallower slope than Segment A (see http://www.stsci.edu/hst/cos/performance/sensitivity/). A new COS/FUV TDS reference file (ycj1846tl_tds.fits) that takes into account the new TDS slopes was delivered on December 19 2014.
Users concerned with the absolute or relative flux calibration of COS FUV spectra taken after October 2013 should re-retrieve their data from the Mikulski Archive for Space Telescopes.
The absolute flux accuracy is within 5% for all modes except for the G130M 1055, 1096, and 1222 CENWAVE settings and for G140L data on Segment B. The absolute flux accuracy for the 1055/1096/1222 settings is 20% while it is 40% for G140L/FUVB.