A fluorometer may be used to measure ultralow concentrations of fluorescing species, such as ultralow concentrations of fluorescent tracer passing through a reverse osmosis membrane into a permeate stream. In some examples, the fluorometer may be recalibrated by resetting some but not all of the calibration parameters used to determine the concentration of fluorescent tracer in the permeate based on the measured fluorescent response of the fluorometer. For example, an intercept of a calibration curve may be reset or recalibrated for the fluorometer in situ, potentially providing significant accuracy improvements even though the fluorometer has not undergone a full recalibration.