Pre- and post-cruise calibration of optical instruments is highly recommended to help establish the measurements uncertainty. For example, some optical instruments – in particular, the ac-9, ac-S and backscattering and transmissometers with 660nm red LEDs – are known to drift significantly throughout a single cruise. It is therefore recommended, if high quality DIW is available and conditions are adequate, to calibrate these instruments throughout the cruise. If absolute calibration is not possible, one can use a valve (either manual or automated) to periodically pre-filter measured seawater through a 0.2um filter, and by difference obtain “calibration independent” particulate optical properties (assuming the interpolation between the dissolved measurement provides a good estimate of the properties of the dissolved fraction when measurements of unfiltered water are made (which could be confirmed/assessed/interpolated using a CDOM fluorometer). The ability to obtain particulate measurements that are calibration independent can provide highly sensitive and high quality measurements of particulate optical properties. Sequoia Scientific Inc. has a product available for in-line systems which includes measurements of flow, necessary to evaluate when to change filters (FlowControl-Lab). Flow meters can also be purchased from WETLabs or independently built for a fraction of the cost. If the backscattering sensor is placed after the switch, one can make measurements of backscattering of 0.2um fraction in addition to the bulk water. Long term changes in the 0.2mm filtered water fraction could provides a diagnostic of drift due, for example, of accumulation of material on the instrument’s face, and those measurements can be used to correct for it.


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