The HI731335 are the replacement caps that screw on the glass cuvettes used with the HI96 series portable photometers. Each cap has a locking indexing tab that ensures the cuvette is inserted into the photometer in the same position. The caps are also used to block any light that would interfere with the optically based measurement. The HI731335 includes 4 caps. Locking index tab for proper alignment in the photometer Light blocking to prevent stray light from entering When using a photometer it is important to insert the cuvette in the same position when performing a blank measurement (zero) and a read with reacted sample in order to maintain a consistent path length in accordance with the Beer-Lambert Law (see below).  The HI731335 have a locking index tab in the cap that fits with the HI96 series photometers to ensure that the cuvette is inserted in the same orientation each time.   The quantity of radiation absorbed is given by the Beer-Lambert Law: A = log Io/I   Absorbance is also given by: A= ελ • C • l where: A is a dimensionless number ελ the proportionality constant, is called the molar extinction coežcient or molar absorptivity; it is a constant for a given substance, provided the temperature and wavelength are constant [L/(mol•cm)] C is the concentration of the substance (mol/liter) l is theoptical distance light travels through sample (cm) The HI731335 are the replacement caps for glass cuvettes that are supplied with all HI96 series portable photometers. The are also designed to keep any stray light from entering the sample cuvette holder of the optically based system.      More Information SKU HI731335 Product Name Caps for Glass Cuvette Used with HI96 Series Portable Photometers (4 pcs) - HI731335 Quote Required Yes Notes Package Quantity 4 read more