File:McFarland Densitometer with its set of calibrators.jpg

English: The McFarland Densitometer is a device used in microbiology to standardize the turbidity of bacterial suspensions for various tests, such as the determination of bacterial growth rates, susceptibility testing, and the preparation of bacterial inocula. Turbidity refers to the cloudiness or haziness of a liquid caused by the presence of suspended particles. In microbiology, turbidity is often used as an indicator of bacterial growth.The McFarland Densitometer consists of a small instrument with a set of standardized plastic tubes, each containing a specific turbidity standard. These standards are created by mixing barium sulfate or other materials with sulfuric acid to generate suspensions of specific optical densities. The optical density is related to the concentration of the suspended particles and thus correlates with the turbidity of the bacterial suspension.The set of calibrators that comes with the McFarland Densitometer includes tubes with varying turbidity levels, usually designated as McFarland standards 0.5, 1, 2, 3, 4, and so on. These standards have defined turbidity levels based on the concentration of the suspended particles. They are used to compare the turbidity of bacterial cultures and ensure that they have the desired density for various tests.Here's how the McFarland Densitometer and its set of calibrators are typically used:A bacterial suspension is prepared and mixed thoroughly to ensure uniform distribution of bacteria.A McFarland standard tube with a known turbidity (e.g., McFarland standard 0.5) is placed in the densitometer's sample holder.The bacterial suspension is adjusted by adding more bacteria or diluting with broth until its turbidity matches that of the McFarland standard in the densitometer. Once the turbidity of the bacterial suspension matches the turbidity of the McFarland standard, the bacterial culture is ready for further testing. This method provides a standardized way to prepare bacterial suspensions of known density for various microbiological tests. It ensures consistency in testing conditions and improves the accuracy and reliability of test results across different laboratories and experiments.