James Clark Ross Cruise AMT14 CTD Problem Report
Problem Report
Beam attenuance
The transmissometers suffered from operational difficulties due to the high temperature; casts 38 to 54 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.
There were no air/dark readings supplied from the cruise and the values generated from the instrument voltage based on manufacturer's readings are frequently larger than would be expected. The absolute attenuation values are therefore questionable but the relative profile should be reliable except for profiles where hysteresis was a problem.
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. The attenuance data will need further offset correction relative to the profile minimum values 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. The absolute attenuation values are therefore questionable but the relative profile should be reliable except for profiles where hysteresis was a problem.
