The HBG® hemocytometer  is a counting-chamber device originally designed and usually used for counting blood cells, hemocytometer was invented by Louis-Charles Malassez and consists of a thick glass microscope slide with a rectangular indentation that creates a precision volume chamber.

This instrument is used for visual counting of the number of cells in a Blood sample or other fluid under a microscope.  The device is carefully crafted so that the area bounded by the lines is known, and the depth of the chamber is also known. By observing a defined area of the grid, it is, therefore, possible to count the number of cells or particles in a specific volume of fluid and thereby calculate the concentration of cells in the fluid overall. A well-used type of hemocytometer is the Neubauer counting chamber. Other types of hemocytometers with different rulings are in use for different applications. Fuchs-Rosenthal rulings, commonly used for spinal fluid counting, Howard Mold rulings used for mold on food and food packaging products, McMaster Egg Slide ruling used for counting microbial eggs in fecal material, Nageotte Chamber ruling for counting low levels of white cells in white cell-reduced platelet components, Palmer Nanoplankton ruling for counting smaller plankters. Petroff-Hausser counter using Improved Neubauer rulings is used for bacteria & sperm counts & is offered with varying chamber depths. The Sedgwick-Rafter Cell ruling in a hemocytometer is primarily designed for use in the microscopy of drinking water.

The counting chamber should be filled by capillary action after the chamber lid (special cover-slip with certified thickness and flatness) has been put in place. This avoids the risk that the cells could sediment/stick to the glass or some volume to evaporate before the cover-slip is placed on top and resulting in an overestimation of the cell concentration. Sedimentation is less of a problem with bacteria but evaporation, more prevalent in low-humidity air-conditioned laboratories, still has to be minimized.