ASTM D4148-82(2012)

1.1 This test method covers determining the density and taxonomic classification of phytoplankton. It is applicable both to relatively sparse or dense phytoplankton concentrations, provided the suspended-sediment concentration is low. The Sedgwick Rafter (S-R) method requires less costly apparatus than does the inverted microscope method but gives less accurate results. The inherent inaccuracy in the Sedgwick-Rafter method is due to the design of the counting chamber and cannot be circumvented by a different choice of optics. For this reason, the S-R method is limited to the use of objective lenses having a working distance of approximately 1.6 mm or more. With 10× oculars the maximum overall magnification is approximately 250×. High concentrations of suspended sediment can obscure the algal cells, and thus cause interference.