This Accuracy Check Kit is for use with the Taylor TTi Colorimeter Series. It features five secondary absorbance standards to verify instrument performance at all wavelengths. Taylor formulates its reagents to remain effective for at least one year, with only very few exceptions (molybdenum indicator in liquid form is one; after four months old it should be tested against a standard periodically). As a general precaution, replace all reagents more than one year old, or at the beginning of a new testing season.