*** Documentation for CTD data collected on RRS Discovery Cruise 174 (May -                                                         
Jun  1988) 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.                                                                                                                
                                                                                                                                    
PRESSURE: The calibration was based on laboratory measurements and employed                                                         
the equation for the CTD lowering                                                                                                   
                                                                                                                                    
        P = 0.09966 * PRAW + 5.17E-7 * PRAW**2 - 7.0 - 0.39(Ts-10)                                                                  
                                                                                                                                    
A quadratic expression was found to give an improved fit to the calibration                                                         
data  over  the  linear  one  and the  last  term  expresses  the  measured                                                         
dependence  of  output on the sensor temperature.  The temperature  of  the                                                         
sensor (Ts) lags considerably behind any changes in ambient temperature and                                                         
a lagged temperature was constructed in the manner described for the oxygen                                                         
sensor in Saunders (1985). Based on laboratory observations of the response                                                         
of  the  sensor  to  thermal shock,  a time constant  of  400  seconds  was                                                         
employed. For the raising of the CTD a correction for the hysteresis of the                                                         
sensor  was  applied  for  the first time.  The  correction  was  based  on                                                         
laboratory  measurements of the difference between the sensor  output  when                                                         
the  applied  pressure  is  increasing and when  the  applied  pressure  is                                                         
decreasing  both for the same external pressure.  With the sensor  on  this                                                         
CTD,  hysteresis  effects  for a deep cast (> 3000 dbars) reach  a  maximum                                                         
value  of about 5 dbars at 500 dbar on the upcast and are only 20 per  cent                                                         
smaller for a 1500 dbar lowering.                                                                                                   
                                                                                                                                    
TEMPERATURE: The calibration based on pre-cruise measurements was:                                                                  
                                                                                                                                    
        T = 0.0049953 * TRAW + 0.026                                                                                                
                                                                                                                                    
This equation has remained virtually unchanged since the acquisition of the                                                         
platinum  resistance  thermometer in 1983.  However,  in February  1989  it                                                         
appeared  to  have fallen by approximately 3 milliK.  Because the  platinum                                                         
resistance  thermometer  has a time constant of about 0.25  seconds  a  lag                                                         
correction is made as described in Saunders (1985).  This procedure is used                                                         
to reduce spikes in the salinity values.                                                                                            
                                                                                                                                    
Comparisons  were made on this cruise with a pair of SIS digital  reversing                                                         
thermometers (serial numbers 156 and 180), the results of this are given in                                                         
the table below. The agreement is satisfactory.                                                                                     
                                                                                                                                    
         Intercomparison of CTD and digital reversing thermometers                                                                  
                                                                                                                                    
Comparison   No. of    Difference (milliK)              Range                                                                       
             samples    mean     standard     Temperature    Pressure                                                               
                                 deviation      (degC)       (dbars)                                                                
                                                                                                                                    
T180 - T156    18       -3.6         1.1       2.4 to 2.9   1500 to 4100                                                            
TCTD - T180    27       -0.3         3.3      -0.3 to 8.1    300 to 4100                                                            
TCTD - T156    28       -2.6         3.8      -0.3 to 9.6    100 to 4100                                                            
                                                                                                                                    
CONDUCTIVITY/SALINITY: Conductivity is not yet calibrated in the laboratory                                                         
at  IOSDL: the conductivity cell factor is carried forward from  cruise  to                                                         
cruise.  A value of 0.99982 was adopted for use on the ship.  For the first                                                         
time corrections were made for changes in the cell dimensions with pressure                                                         
and temperature. The equation used was (Crease et al, 1988)                                                                         
                                                                                                                                    
        C = 0.99982 * CRAW (1 - 6.5E-6 * (T-15) + 1.5E-8 * P)                                                                       
                                                                                                                                    
Approximately  1580 samples of salinity were analysed on the ship  using  a                                                         
Guildline  Bench Salinometer,  standardised against IAPSO Standard Seawater                                                         
(batch  P107,  K=0.99997).  These revealed that the  nominal  CTD  salinity                                                         
needed  to  be corrected by between 0.003 and 0.005 during the  cruise  and                                                         
these corrections were made.                                                                                                        
                                                                                                                                    
                Comparison of CTD and sample salinities                                                                             
                                                                                                                                    
          Stations     Number          Sample - CTD                                                                                 
                                    mean         standard deviation                                                                 
                                                                                                                                    
          11731-48       17        -0.001            0.004                                                                          
          11749-62       74         0.001            0.002                                                                          
          11763-91       82         0.001            0.005                                                                          
                                                                                                                                    
OXYGEN:  A brand new oxygen sensor was available for use on the cruise  and                                                         
some  150  samples  were drawn for the Winkler titration method.  Plots  of                                                         
oxygen  sample  values  versus potential  temperature  showed  considerable                                                         
scatter  and values approximately 10 per cent lower than Transient  Tracers                                                         
in  the  Ocean (TTO) values which had been measured (in 1981)  at  all  the                                                         
three  locations on this cruise.  After investigation it was found that the                                                         
sulphuric acid used to fix the sample at an early stage was well below  the                                                         
approved strength and reluctantly the sample measurements were discarded.                                                           
                                                                                                                                    
Using  the  TTO data and the observation that,  for the Iceland  Basin  and                                                         
Charlie  Gibbs  Fracture Zone sections,  near a potential temperature of  3                                                         
degC,  the dissolved oxygen concentration is 6.30 ml/l (or 274 micromol/kg)                                                         
and also there exists a slight over-saturation in the near surface  values,                                                         
the  various  coefficients in the oxygen algorithm for the CTD sensor  were                                                         
calculated. These were:                                                                                                             
                                                                                                                                    
         cell constant = 1.577E-3                                                                                                   
         alpha = -0.0461 per degC                                                                                                   
         beta = 0.000136 per dbar                                                                                                   
     and CTD temperature fraction 0.33  (see Saunders, 1985)                                                                        
                                                                                                                                    
Adjustments were made to yield the canonical value of 6.3 at the  potential                                                         
temperature value of 3 degC.  For the data for the Faroes region,  the sub-                                                         
zero  temperature water was assumed to have a dissolved oxygen value of 6.9                                                         
ml/l (300 micromol/kg).                                                                                                             
                                                                                                                                    
TRANSMITTANCE: Nominal transmittance values were calculated for this  data,                                                         
not potential transmittance as on earlier cruises.                                                                                  
                                                                                                                                    
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. 1985.  Collection, calibration and processing of CTD data at                                                         
IOS.  International Council for the Exploration of the Sea.  C.M.  1985/C:5                                                         
12pp.                                                                                                                               
                                                                                                                                    
Saunders, P.M. and Roussopoulos, M.  1989.  CTD data from the RRS Discovery                                                         
cruise 174, Faeroe Islands to the Charlie-Gibbs Fracture Zone. Institute of                                                         
Oceanographic Sciences Deacon Laboratory, Report No. 271, 58pp.                                                                     
                                                                                                                                    
Transient Tracers in the Ocean (TTO) 1986 North Atlantic Study -  Shipboard                                                         
Physical and Chemical Data Report. Scripps Institution of Oceanography, SIO                                                         
Reference Series 86-15, 720pp.                                                                                                      
