Preliminary results of new calibration approach and uncertainty assessment for PSICAM

Abstract

A rigorous view of the uncertainties in PSICAM (Point Source Integrated Cavity Absorption Meter) measurements are investigated. Multiple sources of error can greatly influence the accuracy of PSICAM absorption measurements: measurement noise, spectrometer stability, light source stability, calibration and external verification of the standard are a few of the sources of errors that were investigated. Measurement noise can be reduced through repetitive measurements. Using more than 300 averages for each measurement provided repeatability of measurements with a mean absolute difference less than 0.001 m -1 across the visible spectra. Like with other tube and cuvette style absorption meters, bubbles are a potent contaminant when determining absorption. The stability of the spectrometer with time and temperature are explored. Non-linearity and wavelength registration are also considered in reducing errors. Light source stability and output with time are also presented. Due to its small size the PSICAM is used both in the lab and at sea. Accurate absorption values require calibration to be conducted to account for the reflectivity (𝜌) of the sphere. Currently nigrosine dye is used to measure the reflectivity and requires a priori knowledge of the absorption of the dye. An examination of the stability of nigrosine dye is conducted for both filtered and non-filtered dye. A solid standard is proposed to improve the determination of reflectivity in the sphere.