The present invention provides methods for checking and calibrating one or more concentration sensors in an open or closed system. More specifically, in one embodiment of the present invention, the disclosed method allows for the checking and calibration of one or more concentration sensors in which removal of the liquid from the system is required. In two additional embodiments, the disclosed methods allow for the checking and calibration of one or more concentration sensors without having to remove the liquid from the closed system thereby minimizing contamination of the system while at the same time greatly reducing or eliminating contact of the user with the liquid.