Turbidity method: it can be used to measure the bacterial concentration. Its principle is that the fermentation liquid in the fermentation tank enters the flowing cuvette at a certain flow rate, and the optical density of the fermentation liquid is detected with a wavelength of 500-600nm. Then the fermentation liquid flows back into the fermentation tank. The measured OD value is proportional to the cell concentration·
It is also commonly used in immunoassay. The basic principle is that antigen antibody complexes can be rapidly formed in special buffer solution to make the reaction solution turbid. When excess antibody is kept in the reaction solution, the complex formed will increase with the increase of the amount of antigen, and the turbidity of the reaction solution will also increase. The content of the tested substance can be calculated by comparing with a series of standard substances.
immune Turbidimetry According to different instrument design, it can be divided into two types, namely, turbidimeter measurement and nephelometer measurement. The turbidimeter measurement is to measure the attenuation of incident light caused by reflection, absorption or scattering, and its reading is represented by absorbance A. A reflects the ratio of incident light to transmitted light (A=2-log10T, T represents the percentage of turbidity). Scattering turbidimeter measurement is to measure that the incident light meets a particle (complex) at a certain angle scattering The amount of light that is amplified and expressed as a scattering value. 
