The BD PhoenixSpec™ Nephelometer is a portable device designed to measure turbidity of microbial suspensions equivalent to McFarland standards 0.10 to 4.50. The instrument may be used for measurement of inoculum density for the BBL Crystal™ System and the BD Phoenix™ System. The instrument is battery operated or can be used with an AC adapter.

The visual comparison of organism suspensions to turbidity standards is an accepted method of estimating organism densities. The most widely accepted standard is the McFarland standard.¹ A McFarland standard is prepared by adding barium chloride to aqueous sulfuric acid. The density of the resulting barium sulfate precipitate can be used to approximate the colony count of a prepared suspension; e.g., McFarland 1 is the equivalent of approximately 3.0 x 10⁸ colony forming units (CFU)/mL of E. coli, ATCC™ 25922. Other standards have been used for density measurements, including titanium dioxide² and latex particle suspensions.³ The PhoenixSpec Nephelometer uses latex particle suspensions as calibration standards that do not require shaking before use.

The instrumented measurement of turbidity relies on the ability of particles to scatter light while in suspension. The measurement of this scattered light is referred to as nephelometry.⁴ In order to obtain an accurate density measurement, a reliable method of calibration must be used.