A measure of the concentration of microorganisms in a bioreactor. The cell dry weight is obtained by completely drying a small but defined volume of the culture medium and weighing the dried material remaining. The cell wet weight can be measured more quickly but not as precisely, and is around four or five times greater than the cell dry weight. Measurement of the optical density or absorbance of the culture medium is a fast and relatively easy process used to determine the cell density as a direct correlation which typically uses wavelengths between 548nm to 600nm. Most determinations are carried out using off-line analysis, although absorbance measurements can be carried out in situ.