James Clark Ross Cruise AMT12 CTD Problem Report
Problem Report
Beam attenuance
The transmissometers suffered from operational difficulties due to the high temperature; casts 20 to 44 appeared to be affected, and users should take account of quality control flags. Where data are binned to 1 decibar, there will be large sections of these casts where the data are null, due to the absence of good quality data for each bin.
The transmissometer has been calibrated with pure water as the reference for 100% transmission and therefore beam attenuation values in clear water should be close to 0 m -1 . Chelsea Instruments advise that ALPHAtracka is calibrated at the factory at 20°C in distilled water with an electrical conductivity less than one µS cm -1 and filtered to better than 5 µm and that it is possible that the user will encounter water which is purer than that used during the calibration. Indeed the minimum attenuance values for the both sensors were lower then 0 m -1 , suggesting that the calibration procedure recommended by Sea-Bird and Chelsea Instruments may need adjusting to use deep clear oceanic water as the reference for 100% transmission. The data from both sensors will need further offset correction to bring them in line with recognised values. Whether this should be done for the dataset as a whole or on a cast by cast basis is for the user to decide based on their requirements.
