Standard Calibration Solution Set for Apera Instruments TN500 White Light Portable Turbidity Meter. The standards have a one-year shelf life even if you've opened the bottle and happened not to have recapped it tightly. The convenient submicron polymer suspensions are non-toxic primary calibration standards approved by the US EPA for drinking water analysis for turbidity. AMCO Clear is the unique, non-toxic low-level turbidity standard that can reliably read from 0. 02 NTU to 10, 000 NTU every time. *Store in room temperature and never freeze.