**  Documentation  for CTD data collected on RRS Charles Darwin  Cruise  42                                                         
(Sep  -  Oct  1989)  by the Institute  of  Oceanographic  Sciences  (Deacon                                                         
Laboratory), Godalming, Surrey, UK, under the direction of P.M. Saunders **                                                         
                                                                                                                                    
The  instrument used was a Neil Brown Systems CTD which measured  pressure,                                                         
temperature and conductivity and was fitted with a Beckman dissolved oxygen                                                         
electrode.   The  CTD  was  used  alongside  a  General  Oceanics   Rosette                                                         
Multisampler with 12 water bottles,  a 10kHz pinger,  a bottom echo-sounder                                                         
and a SeaTech 1m path transmissometer.  Lowering and retrieval rates of 0.5                                                         
to  1.5m/s were employed and the sensors were flushed with distilled  water                                                         
on  recovery.  Bottle samples and reversing thermometer  measurements  were                                                         
made  on  ascent  and sea water samples were  analysed  using  a  Guildline                                                         
Autolab Salinometer.                                                                                                                
                                                                                                                                    
Throughout  the  cruise the CTD performed flawlessly except on  the  second                                                         
station  where  noisy  data was encountered.  The CTD sea  cable  was  then                                                         
reterminated and there were no further problems.                                                                                    
                                                                                                                                    
A  new  conductivity cell was fitted en route to the  first  station.  This                                                         
required  unsoldering  the temperature sensor and enlarging the seating  of                                                         
the  conductivity cell.  The rewiring caused a shift in the calibration  of                                                         
the  temperature  sensor.  A working calibration for both  temperature  and                                                         
salinity  was  derived  from digital reversing  thermometers  and  salinity                                                         
samples, the latter good to +/-0.002.  A complete temperature recalibration                                                         
was performed on return to the laboratory.                                                                                          
                                                                                                                                    
TEMPERATURE: On the cruise 6 SIS digital reversing thermometers (RTM  4002)                                                         
and  one  reversing  pressure  meter (RPM 6000)  were  available  and  were                                                         
installed  on  the multisampler frames.  After the first two casts  it  was                                                         
apparent  that  even after the manufacturer's calibration  corrections  had                                                         
been  applied there were differences of the order of 0.01 degC between  the                                                         
values  obtained by instruments on the same frame.  The age of the  digital                                                         
reversing  thermometers  varied  from about one year to 6  months  and  the                                                         
differences  encountered on the cruise suggested calibration drift rates of                                                         
the  order  of  0.01 degC/year.  The standard  deviation  of  the  observed                                                         
differences for pairs of digital reversing thermometers and for CTD/digital                                                         
reversing  thermometer measurements is higher than previously observed  but                                                         
the unsteady conditions of measurement were largely responsible for this.                                                           
                                                                                                                                    
PRESSURE: The pressure sensor (RPM 6000) agreed well with the CTD  pressure                                                         
for  the shallow casts (<850 dbars) but exhibited large differences for the                                                         
moderately deep stations of the Charlie Gibbs Fracture Zone.                                                                        
                                                                                                                                    
SALINITY:  Sea water samples drawn from 26 CTD casts were analysed  in  the                                                         
constant temperature laboratory on Charles Darwin using a Guildline Autosal                                                         
8400  (Serial Number 42508) and standardised using Standard Seawater  P111.                                                         
Salinity  determination from Guildline ration was performed on  the  cruise                                                         
using  Ocean  Scientific International's software package 'Salinity'.  This                                                         
was highly efficient.                                                                                                               
                                                                                                                                    
OXYGEN: Dissolved oxygen samples were taken on 16 of the 26 CTD casts.  Two                                                         
samples were drawn from each Niskin bottle,  giving a total of 250  samples                                                         
for  analysis by Winkler titration.  The difference between the  duplicates                                                         
was  generally better than 0.01 ml/l with standard deviation of 0.014  (for                                                         
338 samples, the remainder had much larger differences suggesting errors in                                                         
the  sampling or analysing procedure).  This indicated that the accuracy of                                                         
the  measurements  was  better than 0.2 per cent  compared  to  a  possible                                                         
accuracy of 0.1 per cent for the Winkler titration procedure.                                                                       
                                                                                                                                    
TRANSMITTANCE:   Potential  transmittance,  which  takes  account  of   the                                                         
increasing  mass of clear water in the 1 metre path of the instrument  with                                                         
increasing pressure, was calculated.                                                                                                
                                                                                                                                    
DATA  PROCESSING:  Original values were averaged over an  interval  of  one                                                         
second  and  calibration  coefficients   and  correction  factors  applied.                                                         
Differences between successive values of each parameter were examined;  the                                                         
mean  difference and its standard deviation calculated and  values  greater                                                         
than  several  standard deviations from the mean difference  were  checked.                                                         
Only a limited amount of editing of the data was required. Data were sorted                                                         
on  pressure,  averaged at 2 dbars and missing  values  were  interpolated.                                                         
Derived  quantities were computed from algorithms published by Fofonoff and                                                         
Millard (1983).                                                                                                                     
                                                                                                                                    
                                                                                                                                    
References:                                                                                                                         
                                                                                                                                    
Crease, J. et. al.  1988.  The acquisition, calibration and analysis of CTD                                                         
data. UNESCO Technical Papers in Marine Science. No. 54, 96pp.                                                                      
                                                                                                                                    
Fofonoff,  N.P.  and Millard,  R.C.  1983.  Algorithms for  computation  of                                                         
fundamental  properties  of  seawater.  UNESCO Technical Papers  in  Marine                                                         
Science. No. 44, 53pp.                                                                                                              
                                                                                                                                    
Saunders, P.M., et. al. 1989. RRS Charles Darwin Cruise 42, 22 Sep - 16 Oct                                                         
1989:  Overflow  studies in the Charlie-Gibbs Fracture Zone  and  near  the                                                         
Faeroe  Islands.  Institute  of Oceanographic Sciences  Deacon  Laboratory,                                                         
Cruise Report No. 209, 34pp.                                                                                                        
