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Instrument: OPC : Optical Plankton Counter
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The Optical Plankton Counter (OPC) is an operational instrument that detects, sizes, and counts individual particles based on measuring the attenuance, or diminution in intensity, of a collimated light beam intercepted by transiting particles. The OPC can determine the size of individual particles with effective diameters in the range 250 Ám-2 cm. Thus, nearly the full size spectrum of mesozooplankton can be registered, as well as euphausiids, fish eggs, other small organisms, marine snow, and seston, or small particulate matter in suspension. The nature of the particles being sensed and quantified must be determined by physical capture, a process that is sometimes referred to as calibration. Particle concentrations as high as 10,000 particles per cubic meter can be resolved for counting purposes. The OPC is often towed, at speeds up to 8 knots, but it can also be attached to moorings and other platforms, and may be used in the laboratory. Its ordinary depth rating is 1000 m, but that of a special model is 4000 m.

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