Discovery Cruise D284 AMT15 CTD Problem Report
Problem Report
Beam attenuance
The transmissometer appeared to suffer from slight pressure or temperature hysteresis at depth. The pattern showed a minimum attenuance in the range 150-200 db and then a slight increase in value as the profile went deeper. Cast 52 appeared to be obviously affected, and users should take account of quality control flags. For cast 52 where data are binned to 1 decibar, there will be large sections of the cast 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 from the stainless steel rig mounted tranmissometer 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 from each transmissometer 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 at depth.
