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A diverse array of science goals require accurate flux calibration ofobservations with the Atacama Large Millimeter/Submillimeter array (ALMA),however, this goal remains challenging due to the stochastic time-variabilityof the ``grid'' quasars ALMA uses for calibration. In this work, we use 343.5GHz (Band 7) ALMA Atacama Compact Array observations of four bright and stableyoung stellar objects over 7 epochs to independently assess the accuracy of theALMA flux calibration and to refine the relative calibration across epochs. Theuse of these four extra calibrators allow us to achieve an unprecedentedrelative ALMA calibration accuracy of $\sim 3\%$. On the other hand, when theobservatory calibrator catalog is not up-to-date, the Band 7 data calibrated bythe ALMA pipeline may have a flux calibration poorer than the nominal 10\%,which can be exacerbated by weather-related phase decorrelation whenself-calibration of the science target is either not possible or not attempted.We also uncover a relative flux calibration uncertainty between spectralwindows of 0.8\%, implying that measuring spectral indices within a single ALMAband is likely highly uncertain. We thus recommend various methods for sciencegoals requiring high flux accuracy and robust calibration, in particular, theobservation of additional calibrators combined with a relative calibrationstrategy, and observation of solar system objects for high absolute accuracy.

On the accuracy of the ALMA flux calibration in the time domain and across spectral windows