When using a colorimeter or photometer it is important to use clean cuvettes that are free from scratches. Any discoloration or scratches will interfere with any optically based measurement. The HI731315 are the replacement glass cuvettes that are supplied with the Checker® HC colorimeters. If you need extra cuvettes to test more samples or are looking to replace the cuvettes that came with your Checker, this set comes with two cuvettes with caps.