To assess that the in-line system does not bias the measurements, it is critical to make measurements on both in-line as well as surface waters and contrast the two. These may include different IOP measurements (see below) as well as biogeochemical measurements. In addition, we expect certain relationships between parameters measured by different instruments; Transmissometers should agree to within a difference consistent with differences in their design (e.g. acceptance angle), beam-attenuation, backscattering and chlorophyll are all related in the ocean and relationships between them have been derived. Significant deviations from these relationships may point out a problem in the data. Crossing of oceanic fronts is generally observed in both physical and optical variables. A general QA/QC manual for optical sensor is the QUARTOD manual.