James Clark Ross Cruise AMT13 CTD Problem Report
Problem Report
Cast A13_03
The pressure values were negative throughout the profile and the data were not banked.
Cast A13_04
The primary temperature, salinity, dissolved oxygen and oxygen saturation channels were suspect for the whole profile of cast 04. Secondary temperature and sailinity were used to calculate potential temperature and sigma theta.
Cast A13_07
On 16/09/2003 the CTD wire snapped during initial winching of the CTD, so the early morning casts were cancelled and cast A13_07 corresponded to an on-board test. The data were not banked
Cast A13_45
The primary temperature and salinity channels were suspect for the >100 db profile of cast 45. The dissolved oxygen data for AMT13_45 were suspect. These data have been excluded from the final data set and the secondary salinity and temperature channels should be used instead.
Beam attenuance
The transmissometers suffered from operational difficulties due to the high temperature; casts 31 to 51 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 profiles 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 attenuance data 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. The absolute attenuation values are therefore questionable but the relative profile should be reliable except for profiles where hysteresis was a problem.
